Little effects on soil organic matter chemistry of density fractions after seven years of forest soil warming

Soil conditioners promote the formation of Fe-bound organic carbon and its stability

The close association of soil organic carbon (SOC) with Fe oxides is an important stabilization mechanism for soil organic matter (SOM) against biodegradation. Soil conditioners are of great importance in improving soil quality and soil health. Yet it remains unclear how different conditioners would affect the fractionation of SOC, particularly the Fe-bound organic carbon (Fe-OC). Field-based experiments were conducted in farmland to explore the fractionation of organic carbon (OC) and Fe oxides under the effects of three different soil conditioners (mineral, organic, and microbial conditioners). The results showed that all soil conditioners increased the total OC and Fe-OC contents, with the contribution of Fe-OC to total OC increasing from 1.57% to 2.99%. The low OC/Fe molar ratio indicated that surface adsorption played a crucial role in soil Fe-OC accumulation. Nuclear magnetic resonance (NMR) results suggested that soil conditioner altered the composition of SOM, accelerating O-alkyl C degradation and increasing recalcitrant alkyl C and aromatic C sequestration. Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) analysis indicated that all conditioners promoted the association of OC and Fe oxides. Furthermore, comprehensive analysis of 13C isotope and synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy showed that the mineral conditioner enhanced the association of microbial-derived OC and Fe oxides, whereas the organic conditioner increased the association of plant-derived OC with Fe oxides. These findings provide important insights into the potential mechanisms through which soil conditioners regulate the stability of OC and guide agricultural management.

Interactions between organic matter and Fe oxides at soil micro-interfaces: Quantification, associations, and influencing factors

Iron (Fe) oxides are widely recognized to prevent the degradation of organic matter (OM) in environments, thereby promoting the persistence of organic carbon (OC) in soils. Thus, discerning the association mechanisms of Fe oxides and OC interactions is key to effectively influencing the dynamics and extent of organic C cycling in soils. Previous studies have focused on i) quantifying Fe oxide-bound organic carbon (Fe-OC) in individual environments, ii) investigating the distribution and adsorption capacity of Fe-OC, and iii) assessing the redox cycling and transformation of Fe-OC. Furthermore, the widespread application of high-tech instrumentation and methods has greatly contributed to a better understanding of the mechanism of organic mineral assemblages in the past few decades. However, few literature reviews have comprehensively summarized Fe-OC distributions, associations, and characteristics in soil-plant systems. Here, studies investigating the Fe-OC contents among different environments are reviewed. In addition, the mechanisms and processes related to OM transformation dynamics occurring at mineral-organic interfaces are also described. Recent studies have highlighted that diverse interactions occur between Fe oxides and OC, with organic compounds adhering to Fe oxides due to their huge specific surfaces area and active reaction sites. Moreover, we also review methods for understanding Fe-OC interactions at micro-interfaces. Lastly, developmental prospects for understanding coupled Fe-OC geochemical processes in soil environments at molecular- and nano-scales are outlined. The summary suggests that combined advanced techniques and methods should be used in future research to explore micro-interfaces and in situ descriptions of organic mineral assemblages. This review also suggests that future studies need to consider the functional and spatial complexity that is typical of soil/sediment environments where Fe-OC interactions occur.

Clarifying the evidence for microbial- and plant-derived soil organic matter, and the path toward a more quantitative understanding

Predicting and mitigating changes in soil carbon (C) stocks under global change requires a coherent understanding of the factors regulating soil organic matter (SOM) formation and persistence, including knowledge of the direct sources of SOM (plants vs. microbes). In recent years, conceptual models of SOM formation have emphasized the primacy of microbial-derived organic matter inputs, proposing that microbial physiological traits (e.g., growth efficiency) are dominant controls on SOM quantity. However, recent quantitative studies have challenged this view, suggesting that plants make larger direct contributions to SOM than is currently recognized by this paradigm. In this review, we attempt to reconcile these perspectives by highlighting that variation across estimates of plant- versus microbial-derived SOM may arise in part from methodological limitations. We show that all major methods used to estimate plant versus microbial contributions to SOM have substantial shortcomings, highlighting the uncertainty in our current quantitative estimates. We demonstrate that there is significant overlap in the chemical signatures of compounds produced by microbes, plant roots, and through the extracellular decomposition of plant litter, which introduces uncertainty into the use of common biomarkers for parsing plant- and microbial-derived SOM, especially in the mineral-associated organic matter (MAOM) fraction. Although the studies that we review have contributed to a deeper understanding of microbial contributions to SOM, limitations with current methods constrain quantitative estimates. In light of recent advances, we suggest that now is a critical time to re-evaluate long-standing methods, clearly define their limitations, and develop a strategic plan for improving the quantification of plant- and microbial-derived SOM. From our synthesis, we outline key questions and challenges for future research on the mechanisms of SOM formation and stabilization from plant and microbial pathways.

Cover crop functional types differentially alter the content and composition of soil organic carbon in particulate and mineral-associated fractions

Cover crops (CCs) can increase soil organic carbon (SOC) sequestration by providing additional OC residues, recruiting beneficial soil microbiota, and improving soil aggregation and structure. The various CC species that belong to distinct plant functional types (PFTs) may differentially impact SOC formation and stabilization. Biogeochemical theory suggests that selection of PFTs with distinct litter quality (C:N ratio) should influence the pathways and magnitude of SOC sequestration. Yet, we lack knowledge on the effect of CCs from different PFTs on the quantity and composition of physiochemical pools of SOC. We sampled soils under monocultures of three CC PFTs (legume [crimson clover]; grass [triticale]; and brassica [canola]) and a mixture of these three species, from a long-term CC experiment in Pennsylvania, USA. We measured C content in bulk soil and C content and composition in contrasting physical fractions: particulate organic matter, POM; and mineral-associated organic matter, MAOM. The bulk SOC content was higher in all CC treatments compared to the fallow. Compared to the legume, monocultures of grass and brassica with lower litter quality (wider C:N) had higher proportion of plant-derived C in POM, indicating selective preservation of complex structural plant compounds. In contrast, soils under legumes had greater accumulation of microbial-derived C in MAOM. Our results for the first time, revealed that the mixture contributed to a higher concentration of plant-derived compounds in POM relative to the legume, and a greater accumulation of microbial-derived C in MAOM compared to monocultures of grass and brassica. Mixtures with all three PFTs can thus increase the short- and long-term SOC persistence balancing the contrasting effects on the chemistries in POM and MAOM imposed by monoculture CC PFTs. Thus, despite different cumulative C inputs in CC treatments from different PFTs, the total SOC stocks did not vary between CC PFTs, rather PFTs impacted whether C accumulated in POM or MAOM fractions. This highlights that CCs of different PFTs may shift the dominant SOC formation pathways (POM vs. MAOM), subsequently impacting short- and long-term SOC stabilization and stocks. Our work provides a strong applied field test of biogeochemical theory linking litter quality to pathways of C accrual in soil.

Biocrusts Modulate Climate Change Effects on Soil Organic Carbon Pools: Insights From a 9-Year Experiment

Accumulating evidence suggests that warming associated with climate change is decreasing the total amount of soil organic carbon (SOC) in drylands, although scientific research has not given enough emphasis to particulate (POC) and mineral-associated organic carbon (MAOC) pools. Biocrusts are a major biotic feature of drylands and have large impacts on the C cycle, yet it is largely unknown whether they modulate the responses of POC and MAOC to climate change. Here, we assessed the effects of simulated climate change (control, reduced rainfall (RE), warming (WA), and RE + WA) and initial biocrust cover (low (< 20%) versus high (> 50%)) on the mineral protection of soil C and soil organic matter quality in a dryland ecosystem in central Spain for 9 years. At low initial biocrust cover levels, both WA and RE + WA increased SOC, especially POC but also MAOC, and promoted a higher contribution of carbohydrates, relative to aromatic compounds, to the POC fraction. These results suggest that the accumulation of soil C under warming treatments may be transitory in soils with low initial biocrust cover. In soils with high initial biocrust cover, climate change treatments did not affect SOC, neither POC nor MAOC fraction. Overall, our results indicate that biocrust communities modulate the negative effect of climate change on SOC, because no losses of soil C were observed with the climate manipulations under biocrusts. Future work should focus on determining the long-term persistence of the observed buffering effect by biocrust-forming lichens, as they are known to be negatively affected by warming.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10021-022-00779-0.

Plant and microbial regulations of soil carbon dynamics under warming in two alpine swamp meadow ecosystems on the Tibetan Plateau

Increasing temperature plays important roles in affecting plant and soil microbial communities as well as ecological processes and functions in terrestrial ecosystems. However, mechanisms of warming influencing soil carbon dynamics associated with plant-microbe interactions remain unclear. In this study, open-top chambers (OTCs) experiments were carried out to detect the responses of plants, soil microbes, and SOC contents, physical fractions (by particle-size fractionation) and chemical composition (by solid-state ¹³C NMR spectroscopy) to warming in two alpine swamp meadows (Kobresia humilis vs K. tibetica) on the Tibetan Plateau. Our results showed that four years of warming had significant influences on plant belowground biomass, microbial community and SOC contents in the K. humilis swamp meadow, but had much weaker or minor effects in the K. tibetica swamp meadow with water-logged status and lower level of warming. In the K. humilis swamp meadow, warming increased microbial biomass, C-hydrolysis gene abundance and N-acetylglucosaminidase enzyme activity. These positive effects of warming on microbial biomass and functions further increased soil dissolved inorganic nitrogen and alleviated the nitrogen limitation for plant growth, potentially leading to higher plant biomass. Therefore, increases in SOC and particulate organic carbon (POC) under warming were likely attributed to the higher C input with promoted plant biomass overweighting the simultaneous higher C degradation and release in the K. humilis swamp meadow. Conversely, warming marginally reduced soil alkyl C, which was likely associated with enhanced decomposition by fungi and gram-positive bacteria. Overall, the increases in unprotected POC and decreases in recalcitrant alkyl C demonstrate the sensitivity of SOC physical fractions as well as chemical composition to climate warming in the K. humilis alpine swamp meadow, and suggest that the overall stability of SOC might be lower despite the gain in the content of SOC after climate warming in this alpine swamp meadow.

Effects of long-term fertilization on calcium-associated soil organic carbon: Implications for C sequestration in agricultural soils

Although the contribution of calcium ion (Ca²⁺) to stabilizing organic carbon (OC) in soils has been known for years, we still have a limited understanding of the quantity and molecular composition of Ca²⁺ bound SOC (Ca-OC) evolution in response to long-term fertilization. Here we report the role of Ca²⁺ in the accumulation of OC in the topsoil (0-20 cm) from two long-term (25-37 years) fertilization experiment sites. Approximately 4.54-19.27% and 9.00-25.15% of SOC was bound with Ca²⁺ in the Ferric Acrisol and Fluvic Cambisol, respectively. The application of NPK mineral fertilizers (NPK) decreased (p < 0.05) the Ca-OC stocks from 3.40 t ha^-1 to 0.96 t ha^-1 and from 2.03 t ha^-1 to 1.17 t ha^-1 in the Ferric Acrisol and Fluvic Cambisol, respectively. Swine manure (M) addition did not change (p > 0.05) the Ca-OC stock in Ferric Acrisol, but enhanced (p < 0.05) that from 2.03 t ha^-1 to 9.75 t ha^-1 in Fluvic Cambisol. Fourier transform infrared and carbon (1s)-near X-ray absorption spectroscopies showed that Ca²⁺ was mainly bound with aromatic carbon and carboxylic carbon. Long-term M fertilization facilitated the binding of Ca²⁺ with O-alkyl C, suggesting an increment of Ca-linked polysaccharide. Calcium ion was preferentially associated with ¹³C enriched organic matter (OM). Mineral fertilization promoted the ¹³C-enriched organic compounds in the Ca-OC, while organic fertilization facilitated the binding of ¹³C-depleted organic C with Ca²⁺. This study suggests that Ca-OC may be a potentially vital and stable OC pool in arable soils, and provides direct evidence for the preferential association of OC with Ca²⁺ in edaphic environments.

Comparison of Soil Properties, Understory Vegetation Species Diversities and Soil Microbial Diversities between Chinese Fir Plantation and Close-to-Natural Forest

With the continuous increase in plantation areas, the reduction of natural forest areas, and the unreasonable management of plantations by human beings, the service function of forest ecosystems has gradually reduced. Therefore, close-to-natural forestry has become important for the sustainable development of modern forestry. However, the differences in soil properties, enzyme activities, microbial diversities, and undergrowth vegetation species diversities have not been systematically explained for Chinese fir (Cunninghamia lanceolata). How do these indicators interact? The purpose of this paper was to study the difference in soil properties and biodiversity in different aged Chinese fir plantations and close-to-natural forests to explore their interactions and to provide direction for close-to-nature management. The results showed that the above indicators were significantly different in different aged Chinese fir plantations, soil pH, organic matter, total nitrogen, total phosphorus, total potassium, alkali-hydrolyzed nitrogen, available phosphorus concentrations, and urease activities in close-to-natural forests were significantly higher than plantations. It is worth mentioning that the richness of undergrowth vegetation species diversity (especially shrubs) and soil microbial diversities (especially fungi) in the close-to-natural forests were obviously higher than those in plantations. The correlation analysis results showed that the diversity of shrub species with respect to soil properties presented a higher correlation than herb species, the diversity of fungi with respect to soil properties presented a higher correlation than that observed for bacteria, and the diversity of fungi were significantly correlated with the diversity of undergrowth vegetation species diversities, but the correlation between bacteria and undergrowth vegetation species diversities was not significant. Our results suggest that the above factors are fundamental factors for the transformation of Chinese fir plantations to close-to-natural forests. To realize close-to-natural forestry, we must change the tree structure, combine the difference of those factors in different aged plantations, restore undergrowth vegetation species diversity, and thus improve microbial diversity and increase decomposition, transformation, and improvement of soil properties.

Soil biochemical properties and stabilisation of soil organic matter in relation to deadwood of different species

Despite the increasing number of studies on deadwood, we still have limited knowledge of its dynamics. The aim of this study was to examine the effect of deadwood on the biochemical properties of soil and stabilisation of soil organic matter (SOM). The investigation was carried out in the Czarna Rózga Reserve in Central Poland. The logs of four tree species at different stages of decomposition (III, IV and V) were selected for the analysis. Three replicate logs were sampled for each combination of decay classes, and the soil samples were collected from directly under the logs and from 1 m away from the logs. In this way, changes to the chemical and biochemical properties of the wood were determined. The SOM was physically fractioned. As the rate of deadwood decomposition increases, its biochemical activity increases and its chemical properties change. The biochemical activity, especially the soil's enzyme activity, was stimulated under highly decayed deadwood. The effects of deadwood are visible in SOM fractions, particularly in the content of the light fraction of SOM.

Soil carbon loss with warming: New evidence from carbon-degrading enzymes

Climate warming affects soil carbon (C) dynamics, with possible serious consequences for soil C stocks and atmospheric CO2 concentrations. However, the mechanisms underlying changes in soil C storage are not well understood, hampering long-term predictions of climate C-feedbacks. The activity of the extracellular enzymes ligninase and cellulase can be used to track changes in the predominant C sources of soil microbes and can thus provide mechanistic insights into soil C loss pathways. Here we show, using meta-analysis, that reductions in soil C stocks with warming are associated with increased ratios of ligninase to cellulase activity. Furthermore, whereas long-term (≥5 years) warming reduced the soil recalcitrant C pool by 14%, short-term warming had no significant effect. Together, these results suggest that warming stimulates microbial utilization of recalcitrant C pools, possibly exacerbating long-term climate-C feedbacks.

Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century

Managing soil organic matter (SOM) stocks to address global change challenges requires well-substantiated knowledge of SOM behavior that can be clearly communicated between scientists, management practitioners, and policy makers. However, SOM is incredibly complex and requires separation into multiple components with contrasting behavior in order to study and predict its dynamics. Numerous diverse SOM separation schemes are currently used, making cross-study comparisons difficult and hindering broad-scale generalizations. Here, we recommend separating SOM into particulate (POM) and mineral-associated (MAOM) forms, two SOM components that are fundamentally different in terms of their formation, persistence, and functioning. We provide evidence of their highly contrasting physical and chemical properties, mean residence times in soil, and responses to land use change, plant litter inputs, warming, CO₂ enrichment, and N fertilization. Conceptualizing SOM into POM versus MAOM is a feasible, well-supported, and useful framework that will allow scientists to move beyond studies of bulk SOM, but also use a consistent separation scheme across studies. Ultimately, we propose the POM versus MAOM framework as the best way forward to understand and predict broad-scale SOM dynamics in the context of global change challenges and provide necessary recommendations to managers and policy makers.

Changing rainfall frequency affects soil organic carbon concentrations by altering non-labile soil organic carbon concentrations in a tropical monsoon forest

Soil stores a substantial proportion of carbon (C), making it the greatest terrestrial C pool and pivotal to stabilizing the global climate system. Rainfall amounts and regimes have been changing in many places, but effects of precipitation changes on soil organic C (SOC) stabilization are not completely understood. Considerable attention has been focused on the consequences of changes in rainfall amounts, with rainfall regimes having been less studied. This study was conducted in a tropical climax forest to clarify the effects of rainfall changes on SOC fractions, with permanganate oxidation and density fractionations employed to divide the labile and non-labile SOC fractions. Two rainfall manipulation treatments, i.e., increased rainfall frequency with the total rainfall amount unchanged (IRF) and decreased rainfall amount by 50% with rainfall frequency unaltered (DRA), were conducted for two years, with ambient rainfall (AR) as the control. As a result, the IRF treatment increased the SOC concentration that mainly originated from increases in the non-labile SOC content. Relative to the AR control, the DRA treatment did not change the total SOC concentration although the labile SOC concentration increased. This typically is due to a small proportion of the labile fraction to the total SOC content. Our results suggest that this water-rich mature forest is resistant to rainfall amount changes to a great extent (e.g., decrease of 50% as in the present study) from the SOC stabilization perspective, while changes in rainfall frequency could exert more notable effects.

Evaluating the mycostimulation potential of select carbon amendments for the degradation of a model PAH by an ascomycete strain enriched from a superfund site

Although ecological flexibility has been well documented in fungi, it remains unclear how this flexibility can be exploited for pollutant degradation, especially in the Ascomycota phylum. In this work, we assess three mycostimulation amendments for their ability to induce degradation in Trichoderma harzanium, a model fungus previously isolated from a Superfund site contaminated with polycyclic aromatic hydrocarbons. The amendments used in the present study were selected based on the documented ecological roles of ascomycetes. Chitin was selected to simulate the parasitic ecological role while cellulose and wood were selected to mimic bulk soil and wood saprobic conditions, respectively. Each amendment was tested in liquid basal medium in 0.1 and 1% (w/v) suspensions. Both chitin and cellulose amendments were shown to promote anthracene degradation in T. harzanium with the 0.1% chitin amendment resulting in over 90% removal of anthracene. None of the targets monitored for gene expression were found to be upregulated suggesting alternate pathways may be used in T. harzanium. Overall, our data suggest that mycostimulation amendments can be improved by understanding the ecological roles of indigenous fungi. However, further research is needed to better estimate specific amendment requirements for a broader group of target fungi and follow up studies are needed to determine whether the trends observed herein translate to more realistic soil systems.

The origin of soil organic matter controls its composition and bioreactivity across a mesic boreal forest latitudinal gradient

Warmer climates have been associated with reduced bioreactivity of soil organic matter (SOM) typically attributed to increased diagenesis; the combined biological and physiochemical transformation of SOM. In addition, cross-site studies have indicated that ecosystem regime shifts, associated with long-term climate warming, can affect SOM properties through changes in vegetation and plant litter production thereby altering the composition of soil inputs. The relative importance of these two controls, diagenesis and inputs, on SOM properties as ecosystems experience climate warming, however, remains poorly understood. To address this issue we characterized the elemental, chemical (nuclear magnetic resonance spectroscopy and total hydrolysable amino acids analysis), and isotopic composition of plant litter and SOM across a well-constrained mesic boreal forest latitudinal transect in Atlantic Canada. Results across forest sites within each of three climate regions indicated that (1) climate history and diagenesis affect distinct parameters of SOM chemistry, (2) increases in SOM bioreactivity with latitude were associated with elevated proportions of carbohydrates relative to plant waxes and lignin, and (3) despite the common forest type across regions, differences in SOM chemistry by climate region were associated with chemically distinct litter inputs and not different degrees of diagenesis. The observed climate effects on vascular plant litter chemistry, however, explained only part of the regional differences in SOM chemistry, most notably the higher protein content of SOM from warmer regions. Greater proportions of lignin and aliphatic compounds and smaller proportions of carbohydrates in warmer sites' soils were explained by the higher proportion of vascular plant relative to moss litter in the warmer relative to cooler forests. These results indicate that climate change induced decreases in the proportion of moss inputs not only impacts SOM chemistry but also increases the resistance of SOM to decomposition, thus significantly altering SOM cycling in these boreal forest soils.