Elevated moisture stimulates carbon loss from mineral soils by releasing protected organic matter

Iron‑carbon complex types and bonding forms jointly control organic carbon mineralization in paddy soils

The crucial role of iron (Fe) oxides in stabilizing soil organic carbon (SOC) is well recognized, but their effects on SOC mineralization remain poorly understood. To address this knowledge gap, we evaluated the effects of four typical Fe-bound OC (Fe-OC) complexes including adsorbed ferrihydrite (Fh)- and goethite (Goe)- 13C, coprecipitated Fh/Goe-13C and 13C-glucose as the control, on OC mineralization during an 80-day anaerobic incubation in a paddy soil. 13C-tracing indicated that Fe-13C complexes significantly stimulated CO2 emissions from both the input 13C and SOC compared with glucose alone. In contrast, the addition of Fh- and Goe-C complexes consistently inhibited CH4 emissions by 72-91 % and 21-61 % compared with glucose addition, respectively. Fe-OC complexes reduced the CO2 equivalent by 62-71 % and 17-41 % in soils with Fh-C and Goe-C complexes, respectively. We concluded that Fe crystallinity and its bonding forms with organic carbon jointly control SOC mineralization. The coprecipitated Goe-C complexes had the lowest OC mineralization rate and highest OC residence time among four Fe-OC complexes. These findings highlighted that promoting the formation of coprecipitated well-ordered minerals would increase SOC sequestration by reducing OC mineralization and mitigating the global warming effect in paddy management.

Chromium(VI) Adsorption and Reduction in Soils under Anoxic Conditions: The Relative Roles of Iron (oxyhr)oxides, Iron(II), Organic Matters, and Microbes

Chromium (Cr) transformation in soils mediated by iron (Fe) (oxyhr)oxides, Fe(II), organic matter (OM), and microbes is largely unexplored. Here, their coupling processes and mechanisms were investigated during anoxic incubation experiments of four Cr(VI) spiked soil samples with distinct physicochemical properties from the tropical and subtropical regions of China. It demonstrates that easily oxidizable organic carbon (EOC, 55-84%) and microbes (16-48%) drive Cr(VI) reduction in soils enriched with goethite and/or hematite, among which in dryland soils microbial sulfate reduction may also be involved. In contrast, EOC (38 ± 1%), microbes (33 ± 1%), and exchangeable and poorly crystalline Fe (oxyhr)oxide-associated Fe(II) (29 ± 3%) contribute to Cr(VI) reduction in paddy soils enriched with ferrihydrite. Additionally, exogenous Fe(II) and microbes significantly enhance Cr(VI) reduction in ferrihydrite- and goethite-rich soils, and Fe(II) greatly promotes but microbes slightly inhibit Cr passivation. Both Fe(II) and microbes, especially the latter, promote OM mineralization and result in the most substantial OM loss in ferrihydrite-rich paddy soils. During the incubation, part of the ferrihydrite converts to goethite but microbes may hinder the transformation. These results provide deep insights into the geochemical fates of redox-sensitive heavy metals mediated by the complicated effects of Fe, OM, and microbes in natural and engineered environments.

Increased rainfall frequency enhances dry seasonal soil mineral-associated organic carbon in a subtropical forest

Climate models predict longer and more severe droughts, and alterations in the frequency and intensity of rainfall events. However, how changing precipitation patterns affect soil organic carbon (SOC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) remains unclear. Here, we conducted a three-year rainfall manipulation experiment with ambient rainfall as the control, removal of half the total rainfall amount with unaltered frequency (DRA), and increased rainfall frequency with the total amounts unchanged (IRF) in a subtropical forest. The results showed that the rainfall treatments did not change SOC content or fractions during the wet season. In the dry season, DRA significantly increased topsoil POC but did not change SOC or MAOC. IRF significantly increased POC and MAOC levels. The increased MAOC was associated with the newly formed binary complexes of Fe3+, Al3+, and Ca2+, and the adsorption/precipitation of reduced short-range-ordered Fe oxides, likely derived from the enhanced reductive dissolution in the IRF treatment. Our results suggest that changes in rainfall frequency affect organo-mineral interactions more profoundly relative to the rainfall amount and that these effects are season-dependent. Therefore, moisture-sensitive geochemical processes play an essential role in SOC stabilization in subtropical forests.

Active role of iron-dependent AOM in paddy fields under different long-term fertilizer management schemes

Iron-dependent anaerobic oxidation of methane (iron-AOM) has recently been reported to occur in paddy soils, but its actual role and regulation remain unclear. Here, we confirmed the occurrence of iron-AOM at different layers of paddy soils (0-10 cm, 10-20 cm, and 30-40 cm) across tillering, elongation, flowering, and ripening periods under three long-term fertilizer management schemes (CK-unamended, CF-chemical fertilizer, and CFS-chemical fertilizer with straw). The iron-AOM activity contributed 41 % to total AOM, which was greater than that of nitrite- (32 %) and nitrate-AOM (27 %). The iron-AOM activity varied significantly with soil layers, growth periods and fertilizer types, with layer being the most important variable. Soil moisture content and organic carbon content were most significant influencing factors on the AOM activity, and a Candidatus Methanoperedens ferrireducens-like lineage potentially catalyzed iron-AOM. Our results suggest that iron-AOM has an important potential for mitigating methane emissions from rice paddies.

Emerging investigator series: Coprecipitation with glucuronic acid limits reductive dissolution and transformation of ferrihydrite in an anoxic soil

Ferrihydrite, a poorly crystalline Fe(III)-oxyhydroxide, is abundant in soils and is often found associated with organic matter. Model studies consistently show that in the presence of aqueous Fe(II), organic carbon (OC)-associated ferrihydrite undergoes less transformation than OC-free ferrihydrite. Yet, these findings contrast microbial reductive dissolution studies in which the OC promotes the reductive dissolution of Fe(III) in ferrihydrite and leads to the release of associated OC. To shed light on these complex processes, we quantified the extent of reductive dissolution and transformation of native Fe minerals and added ferrihydrite in anoxic soil incubations where pure 57Fe-ferrihydrite (57Fh), pure 57Fe-ferrihydrite plus dissolved glucuronic acid (57Fh + GluCaq), a 57Fe-ferrihydrite-13C-glucuronic acid coprecipitate (57Fh13GluC), or only dissolved glucuronic acid (13GluCaq) were added. By tracking the transformation of the 57Fe-ferrihydrite in the solid phase with Mössbauer spectroscopy together with analysis of the iron isotope composition of the aqueous phase and chemical extractions with inductively coupled plasma-mass spectrometry, we show that the pure 57Fe-ferrihydrite underwent more reductive dissolution and transformation than the coprecipitated 57Fe-ferrihydrite when identical amounts of glucuronic acid were provided (57Fh + GluCaqversus57Fh13GluC treatments). In the absence of glucuronic acid, the pure 57Fe-ferrihydrite underwent the least reductive dissolution and transformation (57Fh). Comparing all treatments, the overall extent of Fe(III) reduction, including the added and native Fe minerals, determined with X-ray absorption spectroscopy, was highest in the 57Fh + GluCaq treatment. Collectively, our results suggest that the limited bioavailability of the coprecipitated OC restricts not only the reductive dissolution of the coprecipitated mineral, but it also limits the enhanced reduction of native soil Fe(III) minerals.

Comparative analysis of soil organic carbon and soil properties in landscapes of Kerala: insights from the Western Ghats of India

Soil organic carbon (SOC) is known to vary among different ecosystems and soilscapes, yet the degree of variation remains uncertain. Comparing SOC levels in undisturbed ecosystems like forests with those in gradually altered ecosystems can provide valuable insights into the impact of land use on carbon dynamics. This study aimed to evaluate the effects of different land uses on soil fertility parameters in the tropical region of Kerala, focusing on forests as well as cultivated agricultural landscape such as coconut, pepper, tapioca, acacia plantations, and mixed home garden cropping systems. Significant variations were observed among different crops and land use systems in terms of soil fertility. Forests exhibited the highest SOC content at 3.78 g kg-1, while acacia plantations showed the lowest at 0.76 g kg-1. Additionally, various soil properties such as different carbon fractions (e.g., humic acid, fulvic acid), total nitrogen, carbon, available nutrients, physical properties, aggregate size fractions, microbial biomass carbon, and spectral signatures differed significantly across the different land uses. These findings suggest a decline in soil fertility in altered ecosystems compared to adjacent forest soils, highlighting the vital role of forests in conserving natural resources and maintaining soil health. In addition, among the different landscapes studied, mixed cropping systems of home gardens sustained soil fertility better than monocropping systems. The observed variations in soil physicochemical properties among different land use types indicate a threat to sustainable crop production. Effective management practices aimed at improving soil fertility and sustaining crop production in these altered ecosystems are essential. This study highlights the importance of adopting appropriate management strategies to conserve soil health and ensure sustainable crop production in tropical landscapes like Kerala. The holistic approach adopted in this study, encompassing a wide range of soil fertility parameters across various land uses, along with its implications for sustainable land management, adds significant novelty and relevance to the existing literature on soil dynamics in tropical regions like Kerala.

Trade-off between Pore-Throat Structure and Mineral Composition in Modulating the Stability of Soil Organic Carbon

The preservation of soil organic carbon (OC) is an effective way to decelerate the emission of CO2 emission. However, the coregulation of pore structure and mineral composition in OC stabilization remains elusive. We employed the in situ nondestructive oxidation of OC by low-temperature ashing (LTA) combined with near edge X-ray absorption fine structure (NEXAFS), high-resolution microtomography (μ-CT), field emission electron probe microanalysis (FE-EPMA) with C-free embedding, and novel Cosine similarity measurement to investigate the C retention in different aggregate fractions of contrasting soils. Pore structure and minerals contributed equally (ca. 50%) to OC accumulation in macroaggregates, while chemical protection played a leading role in C retention with 53.4%-59.2% of residual C associated with minerals in microaggregates. Phyllosilicates were discovered to be more prominent than Fe (hydr)oxides in C stabilization. The proportion of phyllosilicates-associated C (52.0%-61.9%) was higher than that bound with Fe (hydr)oxides (45.6%-55.3%) in all aggregate fractions tested. This study disentangled quantitatively for the first time a trade-off between physical and chemical protection of OC varying with aggregate size and the different contributions of minerals to OC preservation. Incorporating pore structure and mineral composition into C modeling would optimize the C models and improve the soil C content prediction.

Warming Leads to Changes in Soil Organic Carbon Molecules Due to Decreased Mineral Protection

Climate change affects the content and composition of soil organic carbon (SOC). However, warming-induced changes in the SOC compounds remain unknown. Using nuclear magnetic resonance spectroscopy, molecular mixing models, and Fourier transform ion cyclotron resonance mass spectrometry, we analyzed the variations and relationships in molecular compounds in Mollisol with 10-56 g C kg-1 soil-1 by translocating soils under six climate regimes. We found that increased temperature and precipitation were negatively correlated with carbohydrate versus lipid and lignin versus protein. The former was consistent across soils with varying SOC contents, but the latter decreased as the SOC content increased. The carbohydrate-lipid correlations were related to dithionite-citrate-extractable Fe, while the lignin-protein correlations were linked to changes in moisture and pyrophosphate-extractable Fe/Al. Our findings indicate that the reduction in the mineral protection of SOC is associated with molecular alterations in SOC under warming conditions.

Long-term organic amendments increase the vulnerability of microbial respiration to environmental changes: Evidence from field and laboratory studies

Organic amendments can improve soil fertility and microbial diversity, making agroecosystems more resilient to stress. However, it is uncertain whether organic amendments will enhance the functional capacity of soil microbial communities, thereby mitigating fluctuations in microbial respiration caused by environmental changes. Here, we examined the impacts of long-term organic amendments on the dynamics of microbial catabolic capacity (characterized by enzyme activities and carbon source utilization) and microbial respiration, as well as their interrelationships during a period with fluctuating temperature and rainfall in the field. We then subjected the field soil samples to laboratory heating disturbances to further evaluate the importance of microbial catabolic capacity in explaining patterns of microbial respiration. In both field and laboratory experiments, organic amendments tended to increase the stability of microbial catabolic capacity, but significantly increased the vulnerability of microbial respiration to environmental changes. However, the direction and driving factors of microbial respiration affected by environmental changes differed between the field and laboratory experiments. Environmental changes in the field suppressed the promotional effects of organic amendments on microbial respiration mainly through reducing microbial catabolic capacity, while laboratory heating further enhanced microbial respiration mainly due to increased soil resource availability. Together, these findings suggest that increased microbial respiration variations under organic amendments may potentially increase the uncertainty in predicting soil carbon emissions in the scenario of ongoing climate/anthropogenic changes, and highlight the necessity of linking laboratory studies on environmental changes to field conditions.

Response of C:N:P stoichiometry to long-term drainage of peatlands: Evidence from plant, soil, and enzyme

Drought induced by climate warming and human activities regulates carbon (C) cycling of peatlands by changing plant community composition and soil properties. Estimating the responses of peatlands C cycling to environmental changes requires further study of C: nitrogen (N): phosphorus (P) stoichiometric ratios of soil, plants, and enzyme activities. However, systematic studies on the stoichiometry of above-ground and below-ground ecosystems of peatlands post drainage remain scarce. This study compared stoichimetric ratios of plant and soil and stoichimetric ratios of enzyme activities with different functions in two different parts of a minerotrophic peatland, a natural undisturbed part and a part that had been drained for almost 50 years, in Northern China. For the shrub plants, the average C:N:P ratios of leaf in natural and drained peatland were 448:17:1 and 393:15:1, respectively. This indicated that the growth rate of shrub plants is higher in the drained peatland than in the natural peatland, which makes P element more concentrated in the photosynthetic site. However, from the perspective of the dominant plant, the mean C:N:P ratio of Carex leaf was 650:25:1 in the natural peatland, but was 1028:50:1 for Dasiphora fruticosa in drained peatland. This indicated that the intensification of P-limitation of plant growth after drainage. Soil C:N:P ratios of above water table depth (AWT) were 238:15:1 and 277:12:1, but were 383:17:1 and 404:19:1 for below water table depth (BWT) in the natural and the drained peatland, respectively. Soil C:P ratios were greater than the threshold elemental ratio of C:P (174:1), but the soil C:N ratios were less than the threshold elemental ratio of C:N (23:1), which suggested that P was the most limiting nutrient of soil. The soil microbial activities were co-limited by C&P in Baijianghe peatlands. However, the microbial metabolic P limitation was intensified, but the C limitation was weakened for the above water table depth soil after long-term drainage. There are connection between plant-microbe P limitation in peatlands. The P limitation of microbial metabolism was significant positively correlated with soil C:N but negatively with soil moisture. The increase in the lignocelluloses index suggested considerable decomposition of soil organic matter after peatland drainage. These results of stoichiometric ratios from above- to below ground could provide scientific base for the C cycling of peatland undergone climate change.

Soil moisture dynamics regulates the release rates and lability of copper in contaminated paddy soils

The climate changes have caused more extreme precipitation and drought events in the field and have exacerbated the severity of wet-dry events in soils, which will inevitably lead to severe fluctuations in soil moisture content. Soil moisture content has been recognized to influence the distribution of heavy metals, but how temporal changes of soil moisture dynamics affect the release rates and lability of heavy metals is still poorly understood, which precludes accurate prediction of environmental behavior and environmental risk of heavy metals in the field. In this study, we combined experimental and modeling approaches to quantify copper release rates and labile copper fractions in two paddy soils from southern China under different moisture conditions. Our kinetic data and models showed that the release rates and lability of copper were highly associated with the soil moisture contents, in which, surprisingly, high soil moisture contents effectively reduced the release rates of copper even with little changes in the reactive portions of copper in soils. A suite of comprehensive characterization on soil solid and solution components along the incubation suggested that soil microbes may regulate soil copper lability through forming microbially derived organic matter that sequestered copper and by increasing soil particle aggregation for protecting copper from release. This study highlights the importance of incorporating soil moisture dynamics into future environmental models. The experimental and modeling approaches in this study have provided basis for further developing predictive models applicable to paddy soils with varying soil moisture under the impact of climate change.

The effect of bulk-biochar and nano-biochar amendment on the removal of antibiotic resistance genes in microplastic contaminated soil

Biochar amendment has significant benefits in removing antibiotic resistance genes (ARGs) in the soil. Nevertheless, there is little information on ARGs removal in microplastic contaminated soil. Herein, a 42-day soil microcosm experiment were carried out to study how two coconut shell biochars (bulk- and nano-size) eliminate soil ARGs with/without microplastic presence. The results showed that microplastic increased significantly the numbers and abundances of ARGs in soil at 14d of cultivation. And, two biochars amendment effectively inhibited soil ARGs spread whether or not microplastic was present, especially for nano-biochar which had more effective removal compared to bulk-biochar. However, microplastic weakened soil ARGs removal after applying same biochar. Two biochars removed ARGs through decreasing horizontal gene transfer (HGT) of ARGs, potential host-bacteria abundances, some bacteria crowding the eco-niche of hosts and promoting soil properties. The adverse effect of microplastic on ARGs removal was mainly caused by weakening mobile genetic elements (MGEs) removal, and by changing soil properties. Structural equation modeling (SEM) analysis indicated that biochar's effect on ARGs profile was changed by its size and microplastic presence through altering MGEs abundances. These results highlight that biochar amendment is still an effective method for ARGs removal in microplastic contaminated soil.

Climate and mineral accretion as drivers of mineral-associated and particulate organic matter accumulation in tidal wetland soils

Tidal wetlands sequester vast amounts of organic carbon (OC) and enhance soil accretion. The conservation and restoration of these ecosystems is becoming increasingly geared toward "blue" carbon sequestration while obtaining additional benefits, such as buffering sea-level rise and enhancing biodiversity. However, the assessments of blue carbon sequestration focus primarily on bulk SOC inventories and often neglect OC fractions and their drivers; this limits our understanding of the mechanisms controlling OC storage and opportunities to enhance blue carbon sinks. Here, we determined mineral-associated and particulate organic matter (MAOM and POM, respectively) in 99 surface soils and 40 soil cores collected from Chinese mangrove and saltmarsh habitats across a broad range of climates and accretion rates and showed how previously unrecognized mechanisms of climate and mineral accretion regulated MAOM and POM accumulation in tidal wetlands. MAOM concentrations (8.0 ± 5.7 g C kg-1 ) (±standard deviation) were significantly higher than POM concentrations (4.2 ± 5.7 g C kg-1 ) across the different soil depths and habitats. MAOM contributed over 51.6 ± 24.9% and 78.9 ± 19.0% to OC in mangrove and saltmarsh soils, respectively; both exhibited lower autochthonous contributions but higher contributions from terrestrial or marine sources than POM, which was derived primarily from autochthonous sources. Increased input of plant-derived organic matter along the increased temperature and precipitation gradients significantly enriched the POM concentrations. In contrast, the MAOM concentrations depended on climate, which controlled the mineral reactivity and mineral-OC interactions, and on regional sedimentary processes that could redistribute the reactive minerals. Mineral accretion diluted the POM concentrations and potentially enhanced the MAOM concentrations depending on mineral composition and whether the mineral accretion benefited plant productivity. Therefore, management strategies should comprehensively consider regional climate while regulating sediment supply and mineral abundance with engineering solutions to tap the OC sink potential of tidal wetlands.

Temporal dynamics of soil dissolved organic carbon in temperate forest managed by prescribed burning in Northeast China

Dissolved organic carbon (DOC) is an important function of soil organic carbon and sensitive to environmental disturbance. Few studies have explored the variations in soil DOC dynamics and effects on soil physicochemical properties following prescribed burnings. In this study, Pinus koraiensis plantation forests in Northeast China were selected and subjected to prescribed burning in early November 2018. Soil DOC and different soil physicochemical and biological properties in the 0-10 cm and 10-20 cm soil layers were sampled six times within two years after a prescribed burning. In this study, some soil physicochemical (SOC, TN, and ST) and microbial biomass properties (MBC) recovered within two years after a prescribed burning. Compared to the unburned control stands, the post-fire soil DOC concentrations in the upper and lower soil layers increased by 16% and 12%, respectively. Soil DOC concentrations varied with sampling time, and peaked one year after the prescribed burning. Our results showed that soil chemical properties (NH4+-N and pH) rather than biological properties (microbial biomass) were the main driving factors for changes in post-fire soil DOC concentrations. Current study provides an important reference for post-fire and seasonal soil C cycling in plantation forests of Northeast China.

Biodegradable mulch films improve yield of winter potatoes through effects on soil properties and nutrients

Biodegradable mulch films are recognized as a promising substitute of polyethylene (PE) films to alleviate the "white pollution". Biodegradable mulch films with optimum degradation rates increase crop yield even compared to PE films. However, the mechanisms underlying this yield-increasing effect remains elusive. In this study, three biodegradable film treatments (BFM1, BFM2 and BFM3) and one PE film treatment (PFM) were used to evaluate their effects on soil and winter potatoes, and a partial least squares path model (PLS-PM) was constructed to investigate their relationships. The degradation rates of films under different treatments were ranked as BFM3 > BFM2 >BFM1 > PFM, and presented distinctive effects on soil properties and nutrients, structure of soil bacterial community, and yield traits of winter potatoes. The PLS-PM showed that mulch treatments affected potato yield through effects on soil properties (soil water and temperature) and soil nutrients (TOC, DOC, TN and NO3--N). The disintegration of the biodegradable films decreased soil water content and temperature, and reduced the loss of soil nutrients in the topsoil at the later growth stage of winter potatoes compared to PE films. Additionally, the elevated content of soil TN and NO3--N under treatment BFM1 may play a key role in its yield-increasing effect on potatoes compared to treatments PFM and BFM2. Thus, biodegradable mulch films with proper degradation rates regulate soil TN and NO3--N through their effects on soil water and temperature, and subsequently improve the yield of winter potatoes compared to PE mulch films.

Consider the Anoxic Microsite: Acknowledging and Appreciating Spatiotemporal Redox Heterogeneity in Soils and Sediments

Reduction-oxidation (redox) reactions underlie essentially all biogeochemical cycles. Like most soil properties and processes, redox is spatiotemporally heterogeneous. However, unlike other soil features, redox heterogeneity has yet to be incorporated into mainstream conceptualizations of soil biogeochemistry. Anoxic microsites, the defining feature of redox heterogeneity in bulk oxic soils and sediments, are zones of oxygen depletion in otherwise oxic environments. In this review, we suggest that anoxic microsites represent a critical component of soil function and that appreciating anoxic microsites promises to advance our understanding of soil and sediment biogeochemistry. In sections 1 and 2, we define anoxic microsites and highlight their dynamic properties, specifically anoxic microsite distribution, redox gradient magnitude, and temporality. In section 3, we describe the influence of anoxic microsites on several key elemental cycles, organic carbon, nitrogen, iron, manganese, and sulfur. In section 4, we evaluate methods for identifying and characterizing anoxic microsites, and in section 5, we highlight past and current approaches to modeling anoxic microsites. Finally, in section 6, we suggest steps for incorporating anoxic microsites and redox heterogeneities more broadly into our understanding of soils and sediments.

The influence of precipitation timing and amount on soil microbial community in a temperate desert ecosystem

Introduction

Global climate change may lead to changes in precipitation patterns. This may have a significant impact on the microbial communities present in the soil. However, the way these communities respond to seasonal variations in precipitation, particularly in the context of increased precipitation amounts, is not yet well understood.

Methods

To explore this issue, a five-year (2012-2016) field study was conducted at the northeast boundary of the Ulan Buh Desert, examining the effects of increased precipitation during different periods of the growing season on both bacterial and fungal communities. The study included five precipitation pattern treatments: a control group (C), as well as groups receiving 50 and 100% of the local mean annual precipitation amount (145 mm) during either the early growing season (E50 and E100) or the late growing season (L50 and L100). The taxonomic composition of the soil bacterial and fungal communities was analyzed using Illumina sequencing.

Results

After 5 years, the bacterial community composition had significantly changed in all treatment groups, with soil bacteria proving to be more sensitive to changes in precipitation timing than to increased precipitation amounts within the desert ecosystem. Specifically, the alpha diversity of bacterial communities in the late growing season plots (L50 and L100) decreased significantly, while no significant changes were observed in the early growing season plots (E50 and E100). In contrast, fungal community composition remained relatively stable in response to changes in precipitation patterns. Predictions of bacterial community function suggested that the potential functional taxa in the bacterial community associated with the cycling of carbon and nitrogen were significantly altered in the late growing season (L50 and L100).

Discussion

These findings emphasize the importance of precipitation timing in regulating microbial communities and ecosystem functions in arid regions experiencing increased precipitation amounts.

Electron Transfer, Atom Exchange, and Transformation of Iron Minerals in Soils: The Influence of Soil Organic Matter

Despite substantial experimental evidence of electron transfer, atom exchange, and mineralogical transformation during the reaction of Fe(II)_aq with synthetic Fe(III) minerals, these processes are rarely investigated in natural soils. Here, we used an enriched Fe isotope approach and Mössbauer spectroscopy to evaluate how soil organic matter (OM) influences Fe(II)/Fe(III) electron transfer and atom exchange in surface soils collected from Luquillo and Calhoun Experimental Forests and how this reaction might affect Fe mineral composition. Following the reaction of ⁵⁷Fe-enriched Fe(II)_aq with soils for 33 days, Mössbauer spectra demonstrated marked electron transfer between sorbed Fe(II) and the underlying Fe(III) oxides in soils. Comparing the untreated and OM-removed soils indicates that soil OM largely attenuated Fe(II)/Fe(III) electron transfer in goethite, whereas electron transfer to ferrihydrite was unaffected. Soil OM also reduced the extent of Fe atom exchange. Following reaction with Fe(II)_aq for 33 days, no measurable mineralogical changes were found for the Calhoun soils enriched with high-crystallinity goethite, while Fe(II) did drive an increase in Fe oxide crystallinity in OM-removed LCZO soils having low-crystallinity ferrihydrite and goethite. However, the presence of soil OM largely inhibited Fe(II)-catalyzed increases in Fe mineral crystallinity in the LCZO soil. Fe atom exchange appears to be commonplace in soils exposed to anoxic conditions, but its resulting Fe(II)-induced recrystallization and mineral transformation depend strongly on soil OM content and the existing soil Fe phases.

Coprecipitation with Ferrihydrite Inhibits Mineralization of Glucuronic Acid in Anoxic Soils

It is known that the association of soil organic matter (SOM) with iron minerals limits carbon mobilization and degradation in aerobic soils and sediments. However, the efficacy of iron mineral protection mechanisms under reducing soil conditions, where Fe(III)-bearing minerals may be used as terminal electron acceptors, is poorly understood. Here, we quantified the extent to which iron mineral protection inhibits mineralization of organic carbon in reduced soils by adding dissolved ¹³C-glucuronic acid, a ⁵⁷Fe-ferrihydrite-¹³C-glucuronic acid coprecipitate, or pure ⁵⁷Fe-ferrihydrite to anoxic soil slurries. In tracking the re-partitioning and transformation of ¹³C-glucuronic acid and native SOM, we find that coprecipitation suppresses mineralization of ¹³C-glucuronic acid by 56% after 2 weeks (at 25 °C) and decreases to 27% after 6 weeks, owing to ongoing reductive dissolution of the coprecipitated ⁵⁷Fe-ferrihydrite. Addition of both dissolved and coprecipitated ¹³C-glucuronic acid resulted in increased native SOM mineralization, but the reduced bioavailability of the coprecipitated versus dissolved ¹³C-glucuronic acid decreased the priming effect by 35%. In contrast, the addition of pure ⁵⁷Fe-ferrihydrite resulted in negligible changes in native SOM mineralization. Our results show that iron mineral protection mechanisms are relevant for understanding the mobilization and degradation of SOM under reducing soil conditions.

Groundwater pollutants characterization by geochemometric technique and geochemical modeling in tropical savanna watershed

Multiple interactions of geogenic and anthropogenic activities can trigger groundwater pollution in the tropical savanna watershed. These interactions and resultant contamination have been studied using applied geochemical modeling, conventional hydrochemical plots, and multivariate geochemometric methods, and the results are presented in this paper. The high alkalinity values recorded for the studied groundwater samples might emanate from the leaching of carbonate soil derived from limestone coupled with low rainfall and high temperature in the area. The principal component analysis (PCA) unveils three components with an eigenvalue > 1 and a total dataset variance of 67.37%; this implies that the temporary hardness of the groundwater and water-rock interaction with evaporite minerals (gypsum, halite, calcite, and trona) is the dominant factor affecting groundwater geochemistry. Likewise, the PCA revealed anthropogenic contamination by discharging [Formula: see text] [Formula: see text][Formula: see text] and [Formula: see text] from agricultural activities and probable sewage leakages. Hierarchical cluster analysis (HCA) also revealed three clusters; cluster I reflects the dissolution of gypsum and halite with a high elevated load of [Formula: see text] released by anthropogenic activities. However, cluster II exhibited high [Formula: see text] and [Formula: see text] loading in the groundwater from weathering of bicarbonate and sylvite minerals. Sulfate ([Formula: see text]) dominated cluster III mineralogy resulting from weathering of anhydrite. The three clusters in the Maiganga watershed indicated anhydrite, gypsum, and halite undersaturation. These results suggest that combined anthropogenic and natural processes in the study area are linked with saturation indexes that regulate the modification of groundwater quality.

Iron plaque formation and its effect on key elements cycling in constructed wetlands: Functions and outlooks

Ecological restoration of wetland plants has emerged as an environmentally-friendly and less carbon footprint method for treating secondary effluent wastewater. Root iron plaque (IP) is located at the important ecological niches in constructed wetlands (CWs) ecosystem and is the critical micro-zone for pollutants migration and transformation. Root IP can affect the chemical behaviors and bioavailability of key elements (C, N, P) since its formation/dissolution is a dynamic equilibrium process jointly influenced by rhizosphere habitats. However, as an efficient approach to further explore the mechanism of pollutant removal in CWs, the dynamic formation of root IP and its function have not been fully studied, especially in substrate-enhanced CWs. This article concentrates on the biogeochemical processes between Fe cycling involved in root IP with carbon turnover, nitrogen transformation, and phosphorus availability in CWs rhizosphere. As IP has the potential to enhance pollutant removal by being regulated and managed, we summarized the critical factors affecting the IP formation from the perspective of wetland design and operation, as well as emphasizing the heterogeneity of rhizosphere redox and the role of key microbes in nutrient cycling. Subsequently, interactions between redox-controlled root IP and biogeochemical elements (C, N, P) are emphatically discussed. Additionally, the effects of IP on emerging contaminants and heavy metals in CWs rhizosphere are assessed. Finally, major challenges and outlooks for future research in regards to root IP are proposed. It is expected that this review can provide a new perspective for the efficient removal of target pollutants in CWs.

Carbon sequestration in paddy soils: Contribution and mechanisms of mineral-associated SOC formation

In this work, comparative study of paddy and upland soils were carried out to unravel mechanisms of enhanced soil organic carbon (SOC) sequestration in paddy soils using fractionation methods, ¹³C NMR and Nano-SIMS analysis, as well as organic layer thickness calculations (Core-Shell model). The results showed that although there is a strong increase in particulate SOC in paddy soils compared to that in the upland soils, the increase in mineral-associated SOC is more important, explaining 60-75% of SOC increase in the paddy soils. In the wet and dry alternate cycles of paddy soil, iron (hydr)oxides adsorb relatively small and soluble organic molecules (fulvic acid-like), promote catalytic oxidation and polymerization, thus accelerating formation of larger organic molecules. Upon reductive iron dissolution, these molecules are released and incorporated into existing less soluble organic compounds (humic acid or humin-like), which are coagulated and associated with clay minerals, becoming part of the mineral-associated SOC. The functioning of this "iron wheel" process stimulates accumulation of relatively young SOC into mineral-associated organic carbon pool, and reduces the difference in chemical structure between oxides-bound and clay-bound SOC. Further, the faster turnover of oxides and soil aggregates in paddy soil also facilities interaction between SOC and minerals. The formation of mineral-associated SOC may delay degradation of organic matter during both wet and dry period in the paddy field, therefore enhancing carbon sequestration in paddy soils.

Transformations of Ferrihydrite-Extracellular Polymeric Substance Coprecipitates Driven by Dissolved Sulfide: Interrelated Effects of Carbon and Sulfur Loadings

The association of poorly crystalline iron (hydr)oxides with organic matter (OM), such as extracellular polymeric substances (EPS), exerts a profound effect on Fe and C cycles in soils and sediments, and their behaviors under sulfate-reducing conditions involve complicated mineralogical transformations. However, how different loadings and types of EPS and water chemistry conditions affect the sulfidation still lacks quantitative and systematic investigation. We here synthesized a set of ferrihydrite-organic matter (Fh-OM) coprecipitates with various model compounds for plant and microbial exopolysaccharides (polygalacturonic acids, alginic acid, and xanthan gum) and bacteriogenic EPS (extracted from Bacillus subtilis). Combining wet chemical analysis, X-ray diffraction, and X-ray absorption spectroscopic techniques, we systematically studied the impacts of C and S loadings by tracing the temporal evolution of Fe mineralogy and speciation in aqueous and solid phases. Our results showed that the effect of added OM on sulfidation of Fh-OM coprecipitates is interrelated with the amount of loaded sulfide. Under low sulfide loadings (S(-II)/Fe < 0.5), transformation to goethite and lepidocrocite was the main pathway of ferrihydrite sulfidation, which occurs more strongly at pH 6 compared to that at pH 7.5, and it was promoted and inhibited at low and high C/Fe ratios, respectively. While under high sulfide loadings (S(-II)/Fe > 0.5), the formation of secondary Fe-S minerals such as mackinawite and pyrite dominated ferrihydrite sulfidation, and it was inhibited with increasing C/Fe ratios. Furthermore, all three synthetic EPS proxies unanimously inhibited mineral transformation, while the microbiogenic EPS has a more potent inhibitory effect than synthetic EPS proxies compared at equivalent C/Fe loadings. Collectively, our results suggest that the quantity and chemical characteristics of the associated OM have a strong and nonlinear influence on the extent and pathways of mineralogical transformations of Fh-OM sulfidation.

Composting Processes for Agricultural Waste Management: A Comprehensive Review

Composting is the most adaptable and fruitful method for managing biodegradable solid wastes; it is a crucial agricultural practice that contributes to recycling farm and agricultural wastes. Composting is profitable for various plant, animal, and synthetic wastes, from residential bins to large corporations. Composting and agricultural waste management (AWM) practices flourish in developing countries, especially Pakistan. Composting has advantages over other AWM practices, such as landfilling agricultural waste, which increases the potential for pollution of groundwater by leachate, while composting reduces water contamination. Furthermore, waste is burned, open-dumped on land surfaces, and disposed of into bodies of water, leading to environmental and global warming concerns. Among AWM practices, composting is an environment-friendly and cost-effective practice for agricultural waste disposal. This review investigates improved AWM via various conventional and emerging composting processes and stages: composting, underlying mechanisms, and factors that influence composting of discrete crop residue, municipal solid waste (MSW), and biomedical waste (BMW). Additionally, this review describes and compares conventional and emerging composting. In the conclusion, current trends and future composting possibilities are summarized and reviewed. Recent developments in composting for AWM are highlighted in this critical review; various recommendations are developed to aid its technological growth, recognize its advantages, and increase research interest in composting processes.

A critical review of the central role of microbial regulation in the nitrogen biogeochemical process: New insights for controlling groundwater nitrogen contamination

With the increase of nitrogen (N) input in vadose zones-groundwater systems, N contamination in groundwater has become a global environmental and geological issue that has a profound impact on the ecological environment and human health. N migration in the vadose zone is the most significant means of contaminating the groundwater aquifer. However, the current research on the control of groundwater N contamination focuses solely on the content change of certain indicators and is unable to comprehend the cause and subsequent development of groundwater N contamination. These factors pose significant environmental management challenges in areas where groundwater is contaminated with nitrate. In recent years, research on the migration and transformation behavior of various N forms in vadose zones-groundwater systems has yielded some breakthroughs but also encountered some roadblocks. The biogeochemical behavior of nitrogen consists of a series of intricate chain reaction cycles (called N-cycle). The crucial role of microorganisms in the N biogeochemical process has attracted the interest of soil carbon- and N-cycle researchers and become a hot topic of study. Nonetheless, the role of microbial regulation in groundwater systems has been largely neglected and needs to be summarized immediately. Consequently, this review summarizes recent advancements, mechanisms, and challenges, and proposes a dynamic perspective on microbial regulation. On the basis of these findings, we propose a dynamic and comprehensive groundwater N system centered on microbial regulation. In addition, we critically summarized the migration and transformation behavior of the most recent N indicators, the impact of global environmental change on each N component, and the non-negligible effects of these factors on the control of groundwater N contamination. Future research must focus on the migration and transformation behavior of nitrogen in the deep vadose zone, based on the dynamic regulation of microorganisms, and complete the missing pieces of the developed N-cycle index system. These are essential for providing scientific guidance for global N management and effectively mitigating N contamination in groundwater.

Nitrate as an alternative electron acceptor destabilizes the mineral associated organic carbon in moisturized deep soil depths

Numerous studies have investigated the effects of nitrogen (N) addition on soil organic carbon (SOC) decomposition. However, most studies have focused on the shallow top soils <0.2 m (surface soil), with a few studies also examining the deeper soil depths of 0.5-1.0 m (subsoil). Studies investigating the effects of N addition on SOC decomposition in soil >1.0 m deep (deep soil) are rare. Here, we investigated the effects and the underlying mechanisms of nitrate addition on SOC stability in soil depths deeper than 1.0 m. The results showed that nitrate addition promoted deep soil respiration if the stoichiometric mole ratio of nitrate to O₂ exceeded the threshold of 6:1, at which nitrate can be used as an alternative acceptor to O₂ for microbial respiration. In addition, the mole ratio of the produced CO₂ to N₂O was 2.57:1, which is close to the theoretical ratio of 2:1 expected when nitrate is used as an electron acceptor for microbial respiration. These results demonstrated that nitrate, as an alternative acceptor to O₂, promoted microbial carbon decomposition in deep soil. Furthermore, our results showed that nitrate addition increased the abundance of SOC decomposers and the expressions of their functional genes, and concurrently decreased MAOC, and the ratio of MAOC/SOC decreased from 20% before incubation to 4% at the end of incubation. Thus, nitrate can destabilize the MAOC in deep soils by stimulating microbial utilization of MAOC. Our results imply a new mechanism on how above-ground anthropogenic N inputs affect MAOC stability in deep soil. Mitigation of nitrate leaching is expected to benefit the conservation of MAOC in deep soil depths.

Moisture-dependent response of soil carbon mineralization to temperature increases in a karst wetland on the Yunnan-Guizhou Plateau

Wetlands are facing gradual drying, leading to large carbon loss due to the transformation from anaerobic to aerobic conditions, but the temperature and drought effects from the temperature and moisture fluctuation on soil organic carbon (SOC) mineralization remain uncertain. An incubation study with three moisture levels (100%, 60%, and 40% WHC, marked as W100, W60, and W40, respectively) and four temperature levels (5, 10, 15, 20 °C, marked as T5, T10, T15, and T20, respectively) was conducted to determine the effect of temperature and moisture interactions on SOC mineralization in the karst wetland of the Yunnan-Guizhou Plateau. Compared with T5, the cumulative mineralization CO₂ in T20 increased by 83.18% (W40), 154.63% (W60), and 148.16% (W100), respectively. The mineralized CO₂ at W60 and W40 significantly decreased compared to that at W100 at the four temperature levels. Temperature, moisture and their interactions had significant positive effects on SOC mineralization rates and cumulative mineralized CO₂. The temperature sensitivity of SOC mineralization rates (Q₁₀) under W40 and W60 increased by 22.03% and 24.52%, respectively, compared to that under W100. The cumulative mineralized CO₂ was positively related to soil urease activity and negatively related to soil pH, N-NH₄⁺, SOM, and catalase activity. Temperature and moisture fluctuation and soil properties explained 85.40% of the variation in SOC mineralization, among which temperature and moisture fluctuation, soil properties, and their interactions explained 19.71%, 4.81%, and 60.88%, respectively. Our results indicated that SOC mineralization is influenced by the joint effect of temperature and drought, as well as their induced changes in soil properties, in which higher temperatures can increase soil CO₂ emissions by enhancing the SOC mineralization rate, but the positive effect may be weakened from the drying wetland.

Pristine and sulfidized zinc oxide nanoparticles alter bacterial communities and metabolite profiles in soybean rhizocompartments

A better understanding of bacterial communities and metabolomic responses to pristine zinc oxide manufacture nanoparticles (ZnO MNPs) and its sulfidized product (s-ZnO MNPs), as well as their corresponding Zn ions in rhizocompartments, critical in the plant-microbe interactions, could contribute to the sustainable development of nano-enabled agriculture. In this study, soybean (Glycine max) were cultivated in soils amended with three Zn forms, namely ZnSO₄·7H₂O, ZnO MNPs and s-ZnO MNPs at 0, 100 and 500 mg·kg^-1 for 70 days. Three Zn forms exposures profoundly decreased the bacterial alpha diversity in roots and nodules. High dose (500 mg·kg^-1) groups had a stronger impact on the bacterial beta diversity than low dose (100 mg·kg^-1) groups. In the rhizosphere soil and roots, 500 mg·kg^-1 of ZnSO₄ and s-ZnO MNPs treatments showed the largest shifts in bacterial community structure, respectively. In addition, several significant changed bacterial taxa and metabolites were found at the high dose groups, which were associated with carbon and nitrogen metabolism. PLS-DA plot showed good discrimination in metabolomic profiles of rhizosphere soil and roots between three Zn forms treatments and control. Most metabolic pathways perturbed were closely linked to oxidative stress. Overall, our study indicates either dissolved or nano-particulate Zn exposure at high dose can drastically affected bacterial communities and metabolite profiles in soybean rhizocompartments.

Microbial autotrophy explains large-scale soil CO2 fixation

Microbial communities play critical roles in fixing carbon from the atmosphere and fixing it in the soils. However, the large-scale variations and drivers of these microbial communities remain poorly understood. Here, we conducted a large-scale survey across China and found that soil autotrophic organisms are critical for explaining CO₂ fluxes from the atmosphere to soils. In particular, we showed that large-scale variations in CO₂ fixation rates are highly correlated to those in autotrophic bacteria and phototrophic protists. Paddy soils, supporting a larger proportion of obligate bacterial and protist autotrophs, display four-fold of CO₂ fixation rates over upland and forest soils. Precipitation and pH, together with key ecological clusters of autotrophic microbes, also played important roles in controlling CO₂ fixation. Our work provides a novel quantification on the contribution of terrestrial autotrophic microbes to soil CO₂ fixation processes at a large scale, with implications for global carbon regulation under climate change.

Divergent responses of cropland soil organic carbon to warming across the Sichuan Basin of China

Cropland soils are considered to have the potential to sequester carbon (C). Warming can increase soil organic C (SOC) by enhancing primary production, but it can also cause carbon release from soils. However, the role of warming in governing cropland SOC dynamics over broad geographic scales remains poorly understood. Using over 4000 soil samples collected in the 1980s and 2010s across the Sichuan Basin of China, this study assessed the warming-induced cropland SOC change and the correlations with precipitation, cropland type and soil type. Results showed mean SOC content increased from 11.10 to 13.85 g C kg^-1. Larger SOC increments were observed under drier conditions (precipitation < 1050 mm, dryland and paddy-dryland rotation cropland), which were 1.67-2.23 times higher than under wetter conditions (precipitation > 1050 mm and paddy fields). Despite the significant associations of SOC increment with crop productivity, precipitation, fertilization, cropland type and soil type, warming also acted as one of major contributors to cropland SOC change. The SOC increment changed parabolically with the rise in temperature increase rate under relatively drier conditions, while temperature increase had no impact on cropland SOC increment under wetter conditions. Meanwhile, the patterns of the parabolical relationship varied with soil types in drylands, where the threshold of temperature increase rate, the point at which the SOC increment switched from increasing to decreasing with warming, was lower for clayey soils (Ali-Perudic Argosols) than for sandy soils (Purpli-Udic Cambosols). These results illustrate divergent responses of cropland SOC to warming under different environments, which were contingent on water conditions and soil types. Our findings emphasize the importance of formulating appropriate field water management for sustainable C sequestration and the necessity of incorporating environment-specific mechanisms in Earth system models for better understanding of the soil C-climate feedback in complex environments.

Rejuvenation of iron oxides enhances carbon sequestration by the 'iron gate' and 'enzyme latch' mechanisms in a rice-wheat cropping system

The 'enzyme latch' theory believes that oxygen constraints on phenol oxidase can restrain the activity of hydrolytic enzymes responsible for decomposition, while the 'iron (Fe) gate' theory suggests that Fe oxidation can decrease phenol oxidase activity and enhance Fe-lignin complexation under oxygen exposure. The objective of this study was to explore the roles of the 'enzyme latch' and 'Fe gate' mechanisms in regulating soil organic carbon (SOC) sequestration in a rice-wheat cropping system subjected to six fertilization treatments: control (CT), chemical fertilizer (CF), CF plus manure (CFM), CF plus straw (CFS), CF plus manure and straw (CFMS), and CF plus organic-inorganic compound fertilizer (OICF). Soil samples were collected after the rice and wheat harvests and wet sieved into large macroaggregates, small macroaggregates, microaggregates, and silt and clay particles. Variations in amorphous and free Fe oxides, Fe-bound organic carbon and phenol oxidase activity were examined. After nine years, compared with the initial soil, the activation degree of free Fe oxides increased by 1.3- to 1.6-fold and the topsoil SOC stock increased by 13-61% across all treatments. Amorphous Fe oxide content, phenol oxidase activity and aggregate mean-weight diameter were higher after the wheat harvest than after the rice harvest. Amorphous Fe oxide content was positively correlated with Fe-bound organic carbon content (P < 0.001) but negatively correlated with phenol oxidase activity (P < 0.001). Therefore, seasonal alternation of wetting and drying can progressively drive the rejuvenation of Fe oxides and simultaneously affect the activity of phenol oxidase. Oxidative precipitation of amorphous Fe oxides promoted the formation of organo-Fe complexes and macroaggregates, while flooding of the paddies decreased the activity of phenol oxidase, thereby resulting in year-round hindered decomposition. Organic fertilization strengthened the roles of the 'Fe gate' and 'enzyme latch' mechanisms, and thus accelerated SOC sequestration in the rice-wheat cropping system.

Divergent changes in particulate and mineral-associated organic carbon upon permafrost thaw

Permafrost thaw can stimulate microbial decomposition and induce soil carbon (C) loss, potentially triggering a positive C-climate feedback. However, earlier observations have concentrated on bulk soil C dynamics upon permafrost thaw, with limited evidence involving soil C fractions. Here, we explore how the functionally distinct fractions, including particulate and mineral-associated organic C (POC and MAOC) as well as iron-bound organic C (OC-Fe), respond to permafrost thaw using systematic measurements derived from one permafrost thaw sequence and five additional thermokarst-impacted sites on the Tibetan Plateau. We find that topsoil POC content substantially decreases, while MAOC content remains stable and OC-Fe accumulates due to the enriched Fe oxides after permafrost thaw. Moreover, the proportion of MAOC and OC-Fe increases along the thaw sequence and at most of the thermokarst-impacted sites. The relatively enriched stable soil C fractions would alleviate microbial decomposition and weaken its feedback to climate warming over long-term thermokarst development.

Based on observations from thermokarst-impacted sites on the Tibetan Plateau, the authors find substantial particulate organic carbon loss but stable mineral-associated organic carbon and enriched iron-bound organic carbon upon permafrost thaw.

Mapping of Soil Organic Carbon Stocks Based on Aerial Photography in a Fragmented Desertification Landscape

Northern China’s agropastoral ecotone has been a key area of desertification control for decades, and digital maps of its soil organic carbon (SOC) stocks are needed to reveal the gaps between the actual SOC levels and baseline to support land degradation neutrality (LDN) under the Sustainable Development Goals. However, reliable soil information is scarce, and accurate prediction is hindered by the fragmented landscape, which is a dominant characteristic of desertified land. To improve the patchiness identification and accuracy of SOC prediction, we conducted field surveys and collected low-altitude aerial images along the desertification degrees (severe and extremely severe, moderate, slight) in the Horqin Sandy Land. Linear regressions were performed on the relationships between the normalized difference vegetation index and the fractional vegetation cover (FVC) extracted from aerial images, and regression kriging was applied to predict SOC stocks based on the soil-forming factors (vegetation, climate, and topography). Our prediction and cross-validation showed that the fragmented structure and prediction accuracy of SOC stocks were both greatly improved for desertified land. The FVC (R2c = 0.94) and evapotranspiration (R2c = 0.86) had significant positive effects on SOC stocks, respectively, with indirect and direct causal relationships. Our results could provide soil information with better patchiness and accuracy to help policymakers determine the future LDN status in this fragmented desertification landscape. As drone technology becomes more available, it will fully support digital mapping of soil properties.

Shared Microbial Taxa Respond Predictably to Cyclic Time-Varying Oxygen Limitation in Two Disparate Soils

Periodic oxygen (O₂) limitation in humid terrestrial soils likely influences microbial composition, but whether communities share similar responses in disparate environments remains unclear. To test if specific microbial taxa share consistent responses to anoxia in radically different soils, we incubated a rainforest Oxisol and cropland Mollisol under cyclic, time-varying anoxic/oxic cycles in the laboratory. Both soils are known to experience anoxic periods of days to weeks under field conditions; our incubation treatments consisted of anoxic periods of 0, 2, 4, 8, or 12 d followed by 4 d of oxic conditions, repeated for a total of 384 d. Taxa measured by 16S rRNA gene sequences after 48 d and 384 d of experimental treatments varied strongly with increasing anoxic period duration, and responses to anoxia often differed between soils at multiple taxonomic levels. Only 19% of the 30,356 operational taxonomic units (OTUs) occurred in both soils, and most OTUs did not respond consistently to O₂ treatments. However, the OTUs present in both soils were disproportionally abundant, comprising 50% of sequences, and they often had a similar response to anoxic period duration in both soils (p < 0.0001). Overall, 67 OTUs, 36 families, 15 orders, 10 classes, and two phyla had significant and directionally consistent (positive or negative) responses to anoxic period duration in both soils. Prominent OTUs and taxonomic groups increasing with anoxic period duration in both soils included actinomycetes (Micromonosporaceae), numerous Ruminococcaceae, possible metal reducers (Anaeromyxobacter) or oxidizers (Candidatus Koribacter), methanogens (Methanomicrobia), and methanotrophs (Methylocystaceae). OTUs decreasing with anoxic duration in both soils included nitrifiers (Nitrospira) and ubiquitous unidentified Bradyrhizobiaceae and Micromonosporaceae. Even within the same genus, different OTUs occasionally showed strong positive or negative responses to anoxic duration (e.g., Dactylosporangium in the Actinobacteria), highlighting a potential for adaptation or niche partitioning in variable-O₂ environments. Overall, brief anoxic periods impacted the abundance of certain microbial taxa in predictable ways, suggesting that microbial community data may partially reflect and integrate spatiotemporal differences in O₂ availability within and among soils.

Nitrogen Dynamics and Sweet Potato Production under Indigenous Soil Moisture Conservation Practices in the Leeward Kohala Field System, Hawai’i Island

Intensive cultivation of ‘uala (sweet potato) in the Leeward Kohala field system on Hawai’i Island supported substantial populations of Native Hawaiians prior to its abandonment in the 19th century. Productivity is influenced by the heterogeneity of the climate and biogeochemical soil characteristics across the substantial ecological gradient. Agricultural infrastructure and associated practices were developed to manage crop production eventuating from the variation in inter- and intra-annual rainfall. Mechanisms of sustaining soil nitrogen (N) are still unclear; however, a pronounced source of N is soil organic matter decomposition. This study investigated in situ the effects of indigenous Hawaiian mulching practices against two control treatments on soil moisture and temperature dynamics to facilitate N mineralization in soil mounds cultivated with ‘uala. Field experiments were set in two agricultural restoration plots with distinct climatic and soil characteristics. Data included soil moisture and temperature, soil and plant N, growth and development of ‘uala, and real-time weather data. Concurrently, N mineralization was also investigated under controlled conditions. All indigenous mulching treatments were found to significantly increase soil moisture, regulate temperature variation, and improve N availability compared to control. Differences in soil properties between treatments translated to significant differences in above-ground biomass. The data suggest that these differences would extend to tuber production, but the use of a long-gestation variety limited tuber production in the study. Increased temperatures in the controlled experiments were observed to increase inorganic N significantly, but less substantially than soil moisture. Indigenous practices in the region could have greatly mitigated plant stress due to moisture, temperature, and N availability, increasing productivity and reducing the variability of the Leeward Kohala Field System.

Warming and humidification mediated changes of DOM composition in an Alfisol

Dissolved organic matter (DOM) represents the most mobile and reactive pool of soil organic matter (SOM). Climate changes, such as global warming and altered precipitation exert considerable influences on the quality and quantity of soil DOM. However, rare reports have focused on the interactive effects of soil warming and increased precipitation. In the present study, we conducted a 90-day incubation experiment to investigate how the concentration, source and chemical composition of DOM from an Alfisol respond to the variations of temperatures (15, 30 and 45 °C) and moistures (40%, 60%, and 80% of saturated soil water content). Four DOM components were identified through fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC). Increased temperature alone aggravated the decomposition of plant-derived aromatic components (C2 and C4) but promoted the accumulation of microbial-derived aliphatic carbon (C1) and tryptophan-like component (C3). Increased fungi/bacteria ratio with warming was responsible for the decomposition of plant-derived components. Warming-induced disassociation of Ca-bearing mineral to colloidal Ca facilitated the accrual of microbial-derived aliphatic DOM. Humidification alone and humidification + warming significantly increased the concentration of DOM and the percentage of plant-derived aromatic carbon (C2, C4), which was attributed to the release of Fe-bearing mineral-OC. Based on the above findings along with the results of two-way ANOVA and Variation partition analysis, we infer that moisture will play a dominant role in regulating the chemical composition of DOM in Alfisols under both warming and humidification which in turn impact global C cycling and the ultimate climate.

Characterizing the post-monsoon CO2, CH4, N2O, and H2O vapor fluxes from a tropical wetland in the Himalayan foothill

Wetlands are emitters of greenhouse gases. However, many of the wetlands remain understudied (like temperate, boreal, and high-altitude wetlands), which constrains the global budgets. Himalayan foothill is one such data-deficient area. The present study reported (for the first time) the greenhouse gas fluxes (CO₂, CH₄, N₂O, and H₂O vapor) from the soils of the Nakraunda wetland of Uttarakhand in India during the post-monsoon season (October 2020 to January 2021). The sampling points covered six different types of soil within the wetlands. CO₂, CH₄, N₂O, and H₂O vapor emissions ranged from 82.89 to 1052.13 mg m^-2 h^-1, 0.56 to 2.25 mg m^-2 h^-1, 0.18 to 0.40 mg m^-2 h^-1, and 557.96 to 29,397.18 mg m^-2 h^-1, respectively, during the study period. Except for CO₂, the other three greenhouse gas effluxes did not show any spatial variability. Soils close to "swamp proper" emitted substantially higher CO₂ than the vegetated soils. Soil temperature exhibited exponential relationships with all the greenhouse gas fluxes, except for H₂O vapor. The Q₁₀ values for CO₂, CH₄, and N₂O varied from 3.42 to 4.90, 1.66 to 2.20, and 1.20 to 1.30, respectively. Soil moisture showed positive relationships with all the greenhouse gas fluxes, except for N₂O. The fluxes observed from Nakraunda were in parity with global observations. However, this study showed that wetlands experiencing lower temperature regime are also capable of emitting a substantial amount of greenhouse gases and thus, requires more study. Considering the seasonality of greenhouse gas fluxes should improve global wetland emission budgets.

Climate Effects on Subsoil Carbon Loss Mediated by Soil Chemistry

Subsoils store at least 50% of soil organic carbon (SOC) globally, but climate change may accelerate subsoil SOC (SOC_sub) decomposition and amplify SOC-climate feedbacks. The climate sensitivity of SOC_sub decomposition varies across systems, but we lack the mechanistic links needed to predict system-specific SOC_sub vulnerability as a function of measurable properties at larger scales. Here, we show that soil chemical properties exert significant control over SOC_sub decomposition under elevated temperature and moisture in subsoils collected across terrestrial National Ecological Observatory Network sites. Compared to a suite of soil and site-level variables, a divalent base cation-to-reactive metal gradient, linked to dominant mechanisms of SOC_sub mineral protection, was the best predictor of the climate sensitivity of SOC decomposition. The response was "U"-shaped, showing higher sensitivity to temperature and moisture when either extractable base cations or reactive metals were highest. However, SOC_sub in base cation-dominated subsoils was more sensitive to moisture than temperature, with the opposite relationship demonstrated in reactive metal-dominated subsoils. These observations highlight the importance of system-specific mechanisms of mineral stabilization in the prediction of SOC_sub vulnerability to climate drivers. Our observations also form the basis for a spatially explicit, scalable, and mechanistically grounded tool for improved prediction of SOC_sub response to climate change.

High carbon losses from oxygen-limited soils challenge biogeochemical theory and model assumptions

Oxygen (O₂ ) limitation contributes to persistence of large carbon (C) stocks in saturated soils. However, many soils experience spatiotemporal O₂  fluctuations impacted by climate and land-use change, and O₂ -mediated climate feedbacks from soil greenhouse gas emissions remain poorly constrained. Current theory and models posit that anoxia uniformly suppresses carbon (C) decomposition. Here we show that periodic anoxia may sustain or even stimulate decomposition over weeks to months in two disparate soils by increasing turnover and/or size of fast-cycling C pools relative to static oxic conditions, and by sustaining decomposition of reduced organic molecules. Cumulative C losses did not decrease consistently as cumulative O₂ exposure decreased. After >1 year, soils anoxic for 75% of the time had similar C losses as the oxic control but nearly threefold greater climate impact on a CO₂ -equivalent basis (20-year timescale) due to high methane (CH₄ ) emission. A mechanistic model incorporating current theory closely reproduced oxic control results but systematically underestimated C losses under O₂  fluctuations. Using a model-experiment integration (ModEx) approach, we found that models were improved by varying microbial maintenance respiration and the fraction of CH₄ production in total C mineralization as a function of O₂ availability. Consistent with thermodynamic expectations, the calibrated models predicted lower microbial C-use efficiency with increasing anoxic duration in one soil; in the other soil, dynamic organo-mineral interactions implied by our empirical data but not represented in the model may have obscured this relationship. In both soils, the updated model was better able to capture transient spikes in C mineralization that occurred following anoxic-oxic transitions, where decomposition from the fluctuating-O₂ treatments greatly exceeded the control. Overall, our data-model comparison indicates that incorporating emergent biogeochemical properties of soil O₂ variability will be critical for effectively modeling C-climate feedbacks in humid ecosystems.

Methane oxidation activity inhibition via high amount aged biochar application in paddy soil

Fresh biochar application has been recommended as an effective strategy to mitigate methane emissions from paddy soil due to its role in promoting potential methane oxidation (PMO). However, little is known about the effect of biochar on PMO after long-term aging. Here, a laboratory incubation experiment was performed to gain insight into the combined effects of biochar characteristics on soil PMO during the biochar aging process. Biochar samples aged with an HNO₃/H₂SO₄ mixture (1:3) were prepared and added at 22.5 and 2.8 t ha^-1 to paddy soil. Mixed acid with a dilution of 10- 40% indicated the aging process in paddy field properly. Smaller biochar particles, smoother surface, higher surface area and porosity were observed with the increase of mixed acid ratio. Carboxylic functional groups (COOR) on biochar surface increased accompanied with biochar aging process. Aged biochar application at 22.5 t ha^-1 inhibited paddy soil PMO, which mainly caused from the increased COOR groups induced NH₄⁺-N inhibition for PMO. Increased soil NH₄⁺-N inhibited PMO by limiting the growth of methanotrophs and nitrifiers due to decreased soil pH. In contrast, 2.8 t aged biochar ha^-1 application showed no significant difference on PMO compared with control. Our results indicated that aged biochar exerted in paddy soil have limited effect on mitigating methane emissions from paddy soil in the long term.

Contrasting pathways of carbon sequestration in paddy and upland soils

Paddy soils make up the largest anthropogenic wetlands on earth, and are characterized by a prominent potential for organic carbon (C) sequestration. By quantifying the plant- and microbial-derived C in soils across four climate zones, we identified that organic C accrual is achieved via contrasting pathways in paddy and upland soils. Paddies are 39%-127% more efficient in soil organic C (SOC) sequestration than their adjacent upland counterparts, with greater differences in warmer than cooler climates. Upland soils are more replenished by microbial-derived C, whereas paddy soils are enriched with a greater proportion of plant-derived C, because of the retarded microbial decomposition under anaerobic conditions induced by the flooding of paddies. Under both land-use types, the maximal contribution of plant residues to SOC is at intermediate mean annual temperature (15-20°C), neutral soil (pH~7.3), and low clay/sand ratio. By contrast, high temperature (~24°C), low soil pH (~5), and large clay/sand ratio are favorable for strengthening the contribution of microbial necromass. The greater contribution of microbial necromass to SOC in waterlogged paddies in warmer climates is likely due to the fast anabolism from bacteria, whereas fungi are unlikely to be involved as they are aerobic. In the scenario of land-use conversion from paddy to upland, a total of 504 Tg C may be lost as CO2 from paddy soils (0-15 cm) solely in eastern China, with 90% released from the less protected plant-derived C. Hence, preserving paddy systems and other anthropogenic wetlands and increasing their C storage through sustainable management are critical for maintaining global soil C stock and mitigating climate change.

Reductive release of Fe mineral-associated organic matter accelerated by oxalic acid

The properties and composition of soil dissolved organic matter (DOM) are highly affected by the adsorption and desorption of organic matter (OM) on soil minerals and heterotrophic microbial respiration. Organic acids (e.g., oxalic acid), components of root exudates, have been revealed to liberate organic matter (OM) by the dissolution of protective mineral phases and stimulate microbial degradation of OM. However, the effects of organic acids on the properties and composition of soil DOM molecules and the related mechanisms are still poorly understood. In this study, we conducted microcosm incubation experiments with and without oxalic acid addition, and aimed to elucidate the variations of DOM properties and composition, employing a combination of Fourier transform ion cyclotron resonance mass spectrometry, optical spectroscopy, and bacterial community composition analysis. Our results indicated that the released OM from the direct dissolution of protective mineral phases by oxalic acid further stimulated the microbial reductive release of Fe mineral-associated OM under anoxic conditions. Furthermore, the addition of oxalic acid enhanced the degradation of aliphatic compounds and lignins with low O/C ratios, and increased the accumulation of lignins with high O/C ratios, tannins, and condensed aromatics. Linking the bacterial community composition to DOM molecular properties and composition further suggested that the enhanced reductive release of Fe mineral-associated OM was highly related to the increased abundances of Proteobacteria and Actinobacteria. Overall, oxalic acid induced long-lasting impacts on soil DOM properties and composition under anoxic soil conditions in our study. We expect that our results will contribute to understanding the dynamics of soil DOM molecules in the environment.

Iron-reducing bacteria decompose lignin by electron transfer from soil organic matter

Iron-reducing bacteria (IRB) are crucial for electron transfer in anaerobic soil microsites. The utilization of the energy gathered by this mechanism by decomposers of organic matter is a challenging and fascinating issue. We hypothesized that bacteria reducing Fe(III) (oxyhydr)oxides to soluble Fe(II) obtain electrons from reduced soil organic matter (SOM_r) involving lignin oxidation. Iron-reducing bacteria were isolated from topsoils of various climates (humid temperate, cold temperate, subpolar), vegetation types (mostly grasslands and forests), and derived from various parent materials treatments assigned as Granitic, Volcanic-allophanic, Fluvio-glacial, Basaltic-Antarctic and Metamorphic. After the screening of IRB by phospholipid fatty acid (PLFA) analysis and PCR identification (full-length 16S rDNA), the IRB were inoculated to 20 samples (five soils and 4 replicates) and a broad range of parallel processes were traced. Geobacter metallireducens and Geobacter lovleyi were the main Geobacteraceae-strains present in all soils and strongly increased the activity of ligninolytic enzymes: lignin peroxidase and manganese peroxidase. Carbon dioxide (CO₂) released from IRB-inoculated soils was 140% higher than that produced by Fenton reactions (induced by H₂O₂ and Fe(II) addition) but 40% lower than in non-sterile soils. CO₂ release was closely correlated with the produced Fe (II) and H₂O₂ consumption. The highest CO₂ was released from Basaltic-Antarctic soils with the highest Fe content and was closely correlated with lignin depolymerization (detection by fluorescence images). All IRB oxidized the lignin contained in the SOM within a wide pH range and in soils from all parent materials. We present a conceptual model showing electron shuttling from SOM containing lignin (as a C and energy source) to IRB to produce energy and promote Fe(III) (oxyhydr)oxides reduction was proposed and discussed.

Contribution of the Fenton reaction and ligninolytic enzymes to soil organic matter mineralisation under anoxic conditions

Mechanisms of carbon dioxide (CO₂) release from soil in the absence of oxygen were studied considering the Fenton process, which encompasses the reaction of H₂O₂ with Fe(II) yielding a hydroxyl radical (OH), in combination with manganese peroxidase (MnP) and lignin peroxidase (LiP). This study aimed to explain the high rate of soil organic matter (SOM) mineralisation and CO₂ release from humid temperate rainforest soils under oxygen-limited conditions. The investigated mechanisms challenge the traditional view that SOM mineralisation in rainforest is slow due to anaerobic (micro)environments under high precipitation and explain intensive CO₂ release even under oxygen limitation. We hypothesised that the Fenton reaction (FR) greatly contributes to the CO₂ released from SOM mineralised under anaerobic conditions especially in the presence of ligninolytic enzymes. We used a novel technique that combines labelled H₂¹⁸O₂ and Fe(II) to induce the FR and measured CO¹⁸O, Fe(II) solubilisation, and peroxide consumption in a closed gas circulation system for 6 h. Maximal CO₂ amount was released when the FR was induced in combination with LiP addition. The CO₂ efflux with LiP was 10-fold that of abiotic FR reactions without enzymes, or in soils amended with MnP. This was consistent with i) the contribution of ¹⁸O from peroxide to CO₂ release, ii) peroxide consumption, and iii) Fe(II) solubilisation by FR. The amount of consumed peroxide was closely correlated with the CO¹⁸O derived from soil without enzyme addition or with LiP addition. Concluding, abiotic Fenton Reaction coupled with oxidative enzymes, such as LiP, are crucial for SOM oxidation under anaerobic conditions, e.g. in temperate rainforest soils.

Effects of litter and root manipulations on soil carbon and nitrogen in a Schrenk's spruce (Picea schrenkiana) forest

Plant detritus represents the major source of soil carbon (C) and nitrogen (N), and changes in its quantity can influence below-ground biogeochemical processes in forests. However, we lack a mechanistic understanding of how above- and belowground detrital inputs affect soil C and N in mountain forests in an arid land. Here, we explored the effects of litter and root manipulations (control (CK), doubled litter input (DL), removal of litter (NL), root exclusion (NR), and a combination of litter removal and root exclusion (NI)) on soil C and N concentrations, enzyme activity and microbial biomass during a 2-year field experiment. We found that DL had no significant effect on soil total organic carbon (SOC) and total nitrogen (TN) but significantly increased soil dissolved organic carbon (DOC), microbial biomass C, N and inorganic N as well as soil cellulase, phosphatase and peroxidase activities. Conversely, NL and NR reduced soil C and N concentrations and enzyme activities. We also found an increase in the biomass of soil bacteria, fungi and actinomycetes in the DL treatment, while NL reduced the biomass of gram-positive bacteria, gram-negative bacteria and fungi by 5.15%, 17.50% and 14.17%, respectively. The NR decreased the biomass of these three taxonomic groups by 8.97%, 22.11% and 21.36%, respectively. Correlation analysis showed that soil biotic factors (enzyme activity and microbial biomass) and abiotic factors (soil moisture content) significantly controlled the change in soil C and N concentrations (P < 0.01). In brief, we found that the short-term input of plant detritus could markedly affect the concentrations and biological characteristics of the C and N fractions in soil. The removal experiment indicated that the contribution of roots to soil nutrients is greater than that of the litter.

Interactive global change factors mitigate soil aggregation and carbon change in a semi-arid grassland

The ongoing global change is multi-faceted, but the interactive effects of multiple drivers on the persistence of soil carbon (C) are poorly understood. We examined the effects of warming, reactive nitrogen (N) inputs (12 g N m^-2  year^-1 ) and altered precipitation (+ or - 30% ambient) on soil aggregates and mineral-associated C in a 4 year manipulation experiment with a semi-arid grassland on China's Loess Plateau. Our results showed that in the absence of N inputs, precipitation additions significantly enhanced soil aggregation and promoted the coupling between aggregation and both soil fungal biomass and exchangeable Mg²⁺ . However, N inputs negated the promotional effects of increased precipitation, mainly through suppressing fungal growth and altering soil pH and clay-Mg²⁺ -OC bridging. Warming increased C content in the mineral-associated fraction, likely by increasing inputs of root-derived C, and reducing turnover of existing mineral-associated C due to suppression of fungal growth and soil respiration. Together, our results provide new insights into the potential mechanisms through which multiple global change factors control soil C persistence in arid and semi-arid grasslands. These findings suggest that the interactive effects among global change factors should be incorporated to predict the soil C dynamics under future global change scenarios.

Patterns and trends of topsoil carbon in the UK: Complex interactions of land use change, climate and pollution

The UK Countryside Survey (CS) is a national long-term survey of soils and vegetation that spans three decades (1978-2007). Past studies using CS data have identified clear contrasting trends in topsoil organic carbon (tSOC) concentrations (0-15 cm) related to differences between habitat types. Here we firstly examine changes in tSOC resulting from land use change, and secondly construct mixed models to describe the impact of indirect drivers where land use has been constant. Where it occurs, land use change is a strong driver of SOC change, with largest changes in tSOC for transitions involving SOC-rich soils in upland and bog systems. Afforestation did not always increase tSOC, and the effect of transitions involving woodland was dependent on the other vegetation type. The overall national spatial pattern of tSOC concentration where land use has been constant is most strongly related to vegetation type and topsoil pH, with contributions from climate variables, deposition and geology. Comparisons of models for tSOC across time periods suggest that declining SO₄ deposition has allowed recovery of topsoils from acidification, but that this has not resulted in the increased decomposition rates and loss of tSOC which might be expected. As a result, the relationship between pH and tSOC in UK topsoils has changed significantly between 1978 and 2007. The contributions of other indirect drivers in the models suggest negative relationships to seasonal temperature metrics and positive relationships to seasonal precipitation at the dry end of the scale. The results suggest that the CS approach of long-term collection of co-located vegetation and soil biophysical data provides essential tools both for identifying trends in tSOC at national and habitat levels, and for identifying areas of risk or areas with opportunities for managing topsoil SOC and vegetation change.

Synergy effect of peroxidase enzymes and Fenton reactions greatly increase the anaerobic oxidation of soil organic matter

In temperate rainforest soils of southern Chile (38 °S), there are high rates of soil organic carbon (SOC) mineralization under oxygen (O₂) limitation. We study the combined effects of Fenton reactions and the activity of two enzymes manganese peroxidase (MnP) and lignin peroxidase (LiP), which was hypothesised potentiate SOC mineralization under anoxic conditions leading to carbon dioxide (CO₂) release. Both mechanisms produce free radicals when competing for SOC oxidation in the absence of microorganisms. We quantify the CO₂ release by induced Fenton reactions in combination with MnP and LiP under aerobic and anaerobic conditions (20 °C, 36 h) in temperate rainforest soils. CO₂ levels released by Fenton reactions and enzyme activity were eight times higher than those released by Fenton reaction and peroxidase enzymes in individual treatment. Approximately 31% of the CO₂ released under aerobic soil incubation was found to be abiotic (sterilized), while 69% was biotic (non-sterilized soils), and respective values of 17% and 83% were recorded under anaerobic conditions. The relative fluorescence intensity clearly shows ·OH radicals production from Fenton reactions. In conclusion, levels of MnP and LiP coupled with Fenton reactions strongly increase SOC mineralization under long periods of O₂ limitation in temperate rainforest soils.