Tree species affect cation exchange capacity (CEC) and cation binding properties of organic matter in acid forest soils

Soil organic matter and total nitrogen as key driving factors promoting the assessment of acid-base buffering characteristics in a tea (Camellia sinensis) plantation habitat

The issue of soil acidification in tea plantations has become a critical concern due to its potential impact on tea quality and plant health. Understanding the factors contributing to soil acidification is essential for implementing effective soil management strategies in tea-growing regions. In this study, a field study was conducted to investigate the effects of tea plantations on soil acidification and the associated acid-base buffering capacity (pHBC). We assessed acidification, pHBC, nutrient concentrations, and cation contents in the top 0-20 cm layer of soil across forty tea gardens of varying stand ages (0-5, 5-10, 10-20, and 20-40 years old) in Anji County, Zhejiang Province, China. The results revealed evident soil acidification due to tea plantation activities, with the lowest soil pH observed in tea gardens aged 10-20 and 20-40 years. Higher levels of soil organic matter (SOM), total nitrogen (TN), Olsen phosphorus (Olsen-P), available iron (Fe), and exchangeable hydrogen (H+) were notably recorded in 10-20 and 20-40 years old tea garden soils, suggesting an increased risk of soil acidification with prolonged tea cultivation. Furthermore, prolonged tea cultivation correlated with increased pHBC, which amplified with tea stand ages. The investigation of the relationship between soil pHBC and various parameters highlighted significant influences from soil pH, SOM, cation exchange capacity, TN, available potassium, Olsen-P, exchangeable acids (including H+ and aluminum), available Fe, and available zinc. Consequently, these findings underscore a substantial risk of soil acidification in tea gardens within the monitored region, with SOM and TN content being key driving factors influencing pHBC.

Recent advances in environmental and agricultural applications of hydrochars: A review

Hydrochar is a carbonaceous material that is generated through the process of hydrothermal carbonization (HTC) from biomass, which has garnered considerable attention in recent years owing to its potential applications in a diverse range of fields, such as environmental remediation and agriculture. Hydrochar is produced from a diverse range of biomass waste materials and retains exceptional properties, including high carbon content, stability, and surface area, making it an optimal candidate for various enviro-agricultural applications. Moreover, it delves into the production process of hydrochar, with explicit emphasis on the optimization of certain properties during the production of hydrochar from bio-waste. Furthermore, the potential of hydrochar as an adsorbent and catalyst support for heavy metals and dyes was extensively explored, along with a soil remediation potential that can improve the physical, chemical and biological properties of soil. This comprehensive review aims to provide a thorough overview of hydrochar with a particular focus on its production, properties, and prospective applications. The significance of hydrochar is accentuated and the growing need for alternative sources of energy and materials that are environmentally sustainable is highlighted in this paper. Besides, the consequence of hydrochar on soil properties such as water-holding capacity, nutrient retention, and total soil porosity, as well as its influence on soil chemical properties such as cation exchange capacity, electrical conductivity, and surface functionality is scrutinized.

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.

Phosphate-solubilizing fungi enhances the growth of Brassica chinensis L. and reduces arsenic uptake by reshaping the rhizosphere microbial community

High concentrations of arsenic in soil and plant systems are a threat to human health and ecosystems. The levels of phosphate ions in the soil strongly influence the soil efficacy and arsenic absorption by plants. This study investigated the effects of phosphate-solubilizing fungi (PSF) on environmental factors and structural changes in microbial community in soils contaminated with arsenic. Four experimental groups were created: control (CK), Penicillium GYAHH-CCT186 (W186), Aspergillus AHBB-CT196 (W196), and Penicillium GYAHH-CCT186 + Aspergillus AHBB-CT196 (W186 + W196), with Pakchoi (Brassica chinensis L.) as the test plant. Analysis of altered nutrient levels, enzyme activities and microbial community structure in the soil as well as the growth and physiological characteristics of Pakchoi, revealed a significant increase in the available phosphorus (AP), organic matter (OM), cation exchange capacity (CEC) and available arsenic (AAs) content of the soil following W186 + W196, W196 and W186 treatments. All experimental treatments enhanced the activity of soil β-glucosidase (β-GC) and soil catalase (S-CAT). W186 + W196 and W196 treatments significantly enhanced soil acid phosphatase (S-ACP) activity. Besides, W186 + W196 treatment significantly induced dehydrogenase (S-DHA) activity. Further, of the treatment with PSF increased the fresh weight, root length, plant height and chlorophyll levels while decreasing the arsenic accumulation in Pakchoi. Exposure to PSF also increased the activity of Ascomycota, Basidiomycota, Chytridiomycota, unclassified_Fungi, Mortierellomycota, Cryptomycota and Rozellomycota in the soil. The relative abundance of Ascomycota, Basidiomycota, and Mortierellomycota was positively correlated with the available nutrients (except iron) in the soil as well as enzyme activities. Consequently, the PSF improved the quality of soil and the safety of Pakchoi, suggesting that PSF can be utilized for the remediation of arsenic-contaminated soil.

Modelling biogeochemical reactions triggered by graphene's addition in a fertilized calcareous sandy soil

Graphene production has dramatically increased in the last years and new ways to recycle this engineered material need to be investigated. To this purpose, a reactive model network was developed using PHREEQC-3 code to quantify the relevant biogeochemical reactions induced by graphene scraps' incorporation in a calcareous sandy soil. The numerical model was calibrated versus a complete dataset of column experiments in water saturated conditions using two different fertilizers, a synthetic NPK fertilizer and fertigation water produced in a wastewater treatment plant. Column experiments consisted of 50 cm columns filled with a mixture of graphene scraps (0.015 % dry weight) and soil in the first 10 cm, while the remaining 40 cm had only soil. The model performance was tested using classical statistical indices (R2, Modelling Efficiency, and Index of Agreement), resulting to be satisfactory. Besides, a simple sensitivity analysis via the perturbation of relevant parameters showed a low degree of uncertainty. The main outcome of this study was the quantification of the increased denitrification rate triggered by graphene incorporation into the soil. Moreover, graphene incorporation substantially increased soil CEC and DOC sorption capacity, demonstrating a good adsorption capacity for ammonium and organic compounds, thus decreasing nutrients leaching that represents a major concern related to agricultural practice. Indeed, Graphene incorporation increased by 40 % the CEC in the first 10 cm of the CSG_NPK column (2.50e-02 mol/L) respect to the CS_NPK column (1.75e-02 mol/L) and increased it by 150 % in the first 10 cm of the CSG_FW column (2.50e-02 mol/L) in comparison with the CS_FW column 1.00e-02 (mol/L). pH fluctuations were most likely due to the precipitation of Ca5(PO4)3OH, indeed the consumption of H+ ions could have triggered the pH lowering during the experiment. These results could be relevant for future graphene applications as a soil improver or as suitable material to enhance soil bioremediation in order to include graphene in a circular economy loop.

Acidity of Soil and Water Decreases in Acid-Sensitive Forests of Tropical China

Acid deposition in China has been declining since the 2000s. While this may help mitigate acidification in forest soils and water, little is known about the recovery of soils and water from previous severe acidification in tropical China. Here, we assessed the chemistry of mineral soils, water, and acid gases (SO₂ and NO_x) from three successional forest types in tropical China from 2000 to 2022. Our results showed that soil pH increased synchronously from 3.9 (2000-2015) to 4.2 (2016-2022) across all three forest types, with exchangeable acid initially decreasing and thereafter stabilizing. Surface and ground water pH also gradually increased throughout the monitoring period. Soil pH recovery was stronger in the primary than in the planted forest. However, soil pH recovery lagged behind the increase in rainfall pH by approximately a decade. The recovery of soil pH was likely related to the positive effects of the dissolution of Al/Fe-hydroxysulfate mineral and subsequent sulfur desorption on soil acid-neutralizing capacity, increased soil organic matter, and climate warming, but was likely moderated by increased exchangeable aluminum and potentially proton-producing hydroxysulfate mineral dissolution that caused the lagged soil pH recovery. Surface and ground water pH recovery was attributed to increased water acid-neutralizing capacity. Our study reports the potential for the recovery of acidified soil and water following decreased acid deposition and provides new insights into the functional recovery of acid-sensitive forests.

The transformation of soil Hg oxidation states controls elemental Hg release in the greenhouse with applying organic fertilizer

The foliage vegetables cultivated in greenhouse of Hg-contaminated regions suffer from severe Hg contamination issues because of soil elemental Hg (Hg(0)) release. Application of organic fertilizer (OF) is the indispensable part of farming, but its influences on soil Hg(0) release are unclear. A new method of thermal desorption coupled with cold vapor atomic fluorescence spectrometry was developed to measure transformations of Hg oxidation states to elucidate the impact mechanism of OF on Hg(0) release process. Our results showed that the soil Hg(0) concentrations can directly determine its release fluxes. The application of OF causes that oxidizing reactions of Hg(0)/Hg(I) and Hg(I)/Hg(II) are excited; then soil Hg(0) concentrations decreases. Besides, the elevated soil organic matter by amending OF can complex with Hg(II), resulting in that the reductions of Hg(II) to Hg(I) and Hg(0) are inhibited. Additionally, the OF can directly adsorb soil Hg(0), decreasing the removability of Hg(0). Subsequently, the application of OF can significantly inhibit soil Hg(0) release, resulting in a pronounced decrease in interior atmospheric Hg(0) concentrations. Our results provide a novel perspective for enriching the fate of soil Hg that transformation of soil Hg oxidation states plays a crucial role in affecting soil Hg(0) release process.

Ryegrass extraction of heavy metals from municipal sewage sludge compost-amended soils assisted with citric acid

Land use is an effective way to reduce carbon emission in the recycling process of municipal sludge compost; meanwhile, heavy metals (HMs) in the sludge can be phytoextracted by ornamental plants. As an eco-friendly soil amendment, citric acid (CA) has been reported to be of great potential aid to phytoremediation, and its effect on ryegrass (Lolium perenne L.) extraction of HMs (Zn, Ni, Pb, Cu, and Cd) from municipal sewage sludge compost-amended (MSSC) soils has been investigated through pot experiments in the study. The growth of ryegrass was significantly promoted under 2 and 4 mmol kg^-1 CA treatments. The concentrations of HMs in MSSC soil after 45-day planting were significantly reduced ([Formula: see text]), and they were further reduced except for Cu while CA treated. The acid-extractable fraction of HMs in the soil was increased significantly as CA treated, and further improvement could be found when CA dose increased, which was due to the decreased soil pH and the complexation of CA with metal ions. The phytoremediation factor (PRF) was proposed to assess the phytoremediation efficiency, which was obtained as a ratio of the product of the biomass and metal concentration of plant shoot between the CA-treated group and the control group. When the CA dose was 6 mmol kg^-1, the average PRF of five heavy metals reached 2.29, and Cd was the highest (3.72), demonstrating that CA had great promotion on phytoremediation of heavy metals. This study made a contribution to the research of phytoremediation in sludge land use by demonstrating ryegrass as an ideal bioaccumulator for heavy metals, especially for Cd.

Application of Near-Infrared Spectroscopy to Detect Modification of the Cation Exchange Properties of Soils from European Beech and Silver Fir Forest Stands in Poland

This study investigated changes in the composition of the cation exchange capacity of soil samples caused by the acid leaching of soil cations under laboratory conditions. Furthermore, near-infrared (NIR) spectroscopy was used to evaluate the properties of forest soils. The potential influence of the species composition of stands (beech and fir) was also investigated. Eighty soil samples from the topsoil of plots located in central Poland were analyzed. Soil samples were leached 0 (non-leached), 5, 10, and 15 times and then analyzed to determine the contents of cations (Al³⁺, Ca²⁺, K⁺, and Mg²⁺), the total carbon content, and the pH. From NIR spectra obtained by scanning 54 samples and measurement results for soil sample properties, a calibration model was developed. The model was validated using 26 independent samples. The results showed that acid leaching decreased the pH of soil solutions and the carbon content. The amounts of Al³⁺, Ca²⁺, K⁺, and Mg²⁺ decreased with an increasing number of leaching treatments, but most leaching had occurred after five treatments. Data analysis showed that leaching with hydrochloric acid depleted alkaline cations and Al³⁺ in the soil, which reduced the stability of organic matter, causing its release. Modification of ion exchange properties is observable based on the analysis of the NIR spectra. Good calibration results were achieved for all tested parameters (R²C ≥ 0.89). The best validation results were obtained for Al³⁺ and C contents under fir stands, and for the pH and Al³⁺ content of soils under beech stands (R²V > 0.8). However, the differences between the measured and estimated mean values of the investigated soil were relatively small (no significant difference, p > 0.05). The species composition of stands (beech and fir) had no impact on the developed mathematical models. Soil assessment using NIR spectroscopy allowed calibration models to be obtained, which were successfully used to calculate soil properties at a much lower cost and in a much shorter time compared with other laboratory methods. The results of the paper affirmed that using a relatively small number of samples (3-4) to calculate an average of soil content properties provided satisfactory results.

The leaching of antimony and arsenic by simulated acid rain in three soil types from the world's largest antimony mine area

A simulated acid rain (SAR) experiment on leaching of antimony (Sb) and arsenic (As) in three soil types including paddy soils (PS), vegetable soils (VS) and slag based soils (SS) from Xikuangshan (XKS) Sb mine area was conducted. The SAR at pH 2.5, 3.5, 4.5 and 5.6 were sprayed to soil columns with intermittent pattern in a period of 50 days. Through the spraying duration, leaching Sb in PS, VS and SS showed decreasing trends regardless of pH values in SAR and were in the ranges of 0.026-0.064 mg L^-1, 0.19-2.18 mg L^-1 and 11.8-32.4 mg L^-1, respectively. By contrast, leaching As in these three soil types continuously increased at the initial five spraying times and then deeply decreased afterward, with ranges being 0-0.007 mg L^-1, 0.001-0.071 mg L^-1 and 0.17-1.07 mg L^-1, respectively. The leaching Sb in all the three soil types were extremely higher than the reference value in grade IV (0.01 mg L^-1) for groundwater quality of China (GB/T 14,848-2017). For leaching As, peck values in VS and all the values in SS were also greater than the corresponding reference value (0.05 mg L^-1). This indicated that leaching Sb and As could pollute the groundwater in XKS Sb mine area, especially those in slag based soils. The total leaching losses of Sb and As were affected by pH ambiguously, such as SAR at pH 2.5, 5.6 and 2.5 induced the greatest losses of Sb in PS, VS and SS, and pH 3.5, 5.6 and 2.5 resulted in the greatest leaching losses of As in these soils. After SAR treatment, the specific sorbed and Fe/Mn oxide-associated Sb and As significantly decreased. It demonstrated that these two fractions of both Sb and As were involved in leaching losses. The present study also found that the SAR treatment resulted in soil acidification in all the three soil types. In addition, available N, P and K in all the SAR treatments decreased regardless of pH values, except for available N and P in PS.

Nitrogen input enhances microbial carbon use efficiency by altering plant-microbe-mineral interactions

Microbial growth and respiration are at the core of the soil carbon (C) cycle, as these microbial physiological performances ultimately determine the fate of soil C. Microbial C use efficiency (CUE), a critical metric to characterize the partitioning of C between microbial growth and respiration, thus controls the sign and magnitude of soil C-climate feedback. Despite its importance, the response of CUE to nitrogen (N) input and the relevant regulatory mechanisms remain poorly understood, leading to large uncertainties in predicting soil C dynamics under continuous N input. By combining a multi-level field N addition experiment with a substrate-independent ¹⁸ O-H₂ O labelling approach as well as high-throughput sequencing and mineral analysis, here we elucidated how N-induced changes in plant-microbial-mineral interactions drove the responses of microbial CUE to N input. We found that microbial CUE increased significantly as a consequence of enhanced microbial growth after 6-year N addition. In contrast to the prevailing view, the elevated microbial growth and CUE were not mainly driven by the reduced stoichiometric imbalance, but strongly associated with the increased soil C accessibility from weakened mineral protection. Such attenuated organo-mineral association was further linked to the N-induced changes in the plant community and the increased oxalic acid in the soil. These findings provide empirical evidence for the tight linkage between mineral-associated C dynamics and microbial physiology, highlighting the need to disentangle the complex plant-microbe-mineral interactions to improve soil C prediction under anthropogenic N input.

Salinity Effects on Microbial Derived-C of Coastal Wetland Soils in the Yellow River Delta

Microorganisms play a crucial role in regulating the turnover and transformation of soil organic carbon (SOC), whereas microbial contribution to SOC formation and storage is still unclear in coastal wetlands. In this study, we collected topsoil (0–20 cm) with 7 salinity concentrations and explored the shifts in microbial residues [represented by amino sugar (AS)] and their contribution to the SOC pool of coastal wetlands in the Yellow River delta. The gradually increasing soil salinity reduced soil water content (SWC), SOC, and soil nitrogen (N), especially in high salinity soils of coastal wetlands. Total ASs and their ratio to SOC, respectively, decreased by 90.56 and 66.35% from low salinity to high salinity soils, indicating that coastal wetlands with high salinity restrained microbial residue accumulation and microbial residue-C retention in the SOC pool. Together with redundancy analysis and path analysis, we found that SWC, pH, SOC, soil N, and glucosamine/muramic arid were positively associated with the ratio of ASs to SOC. The higher available soil resource (i.e., water, C substrate, and nutrient) increased microbial residue accumulation, promoting microbial derived-C contribution to SOC in low salinity coastal wetlands. The greatly decreased microbial residue contribution to SOC might be ascribed to microbial stress strategy and low available C substrate in coastal wetlands with high salinity concentration. Additionally, the gradually increasing salinity reduced fungal residue contribution to SOC but did not change bacterial residue contribution to SOC. These findings indicated that changed fungal residues would substantially influence SOC storage. Our study elucidates microbial contribution to SOC pool through residue reservoir in coastal wetlands and pushes microbial metabolites to a new application in global wetland SOC cycling.

Data Integration Analysis Indicates That Soil Texture and pH Greatly Influence the Acid Buffering Capacity of Global Surface Soils

Soil acidification is a global environmental issue that decreases soil functions, and it has been significantly accelerated by anthropogenic activities in recent decades. Soils can resist acidification upon receiving acid inputs due to the resistance or/and resilience capacity of soils, which is termed the acid buffering capacity of soils, and it is often indicated by the soil pH buffering capacity (pHBC). An increasing number of studies have been conducted to quantify soil pHBC at various sites, but to date, integration of global data is lacking; therefore, the variations in large-scale soil pHBC and the factors that influence these variations are still unclear. In this study, we collected previously published data on soil pHBC to analyze its variations on a large scale, as well as investigate the underlying factors influencing these variations. The results showed that soil pHBC varied substantially from site to site, with a mean of 51.07 ± 50.11 mmol kg−1 pH−1. Soil texture and pH, separately or collectively, explained a considerable proportion of the total variation of global soil pHBC. It is well-established that a series of processes contribute to the soil acid buffering capacity in different pH ranges, and the global data analyses showed that pH 5.5 could be a key threshold value; different buffering systems may be active at pH > 5.5 and pH < 5.5. Moreover, tropical soils were more acid-sensitive than temperate and subtropical soils, and forest soils had significantly lower soil pHBCs than grassland and cropland soils. This could be attributed in part to the different soil properties, such as soil texture or pH, among the different climatic zones and ecosystems.

Soil Organic Carbon Pools and Associated Soil Chemical Properties under Two Pine Species (Pinus sylvestris L. and Pinus nigra Arn.) Introduced on Reclaimed Sandy Soils

The roles of different tree species and their impacts are key in assessing the dynamics of soil restoration in afforested post-mining sites. The objective of this study was to compare the effect of Scots pine (Pinus sylvestris L.), which is native to Central Europe and commonly used in afforestation, to that of the non-native black pine (Pinus nigra Arn.) on the development of carbon pools and the chemical properties of reclaimed soils after sand exploitation. The study was carried out in 20- and 35-year-old stands, and the results were compared to undisturbed forest sites. Samples of the litter horizon and mineral soils (0–5 and 5–20 cm) were analyzed for pH, soil organic carbon (SOC), and total nitrogen (Nt). In addition, electrical conductivity (EC), sorption complex properties, water-soluble carbon, and hot-water-extractable carbon were determined from the mineral soil samples. Scots and black pine had a similar effect on the properties of the reclaimed soils. However, the soils under Scots pine were characterized by lower pH values in the litter and 0–5 cm horizons, higher EC in the 0–5 cm horizon, and higher C stocks in the litter horizon. Changes in the C stocks and chemical properties with afforestation years were limited to the uppermost soil horizons (litter and 0–5 cm). For both pine species, soils under the older stands were characterized by lower pH, higher EC, higher exchangeable acidity, higher cation-exchange capacity, lower base saturation, higher SOC and Nt contents, and more stable soil organic matter than soil under younger stands. After 35 years, about 20% and 27% of the C stocks in the reclaimed mine soils had been restored under black pine and Scots pine, respectively (compared to undisturbed soils). This difference between the pine species resulted from the higher C stocks in the litter horizons under Scots pine. Pedogenesis in post-mining sites after sand exploitation under pine species tended to result in more acidic and oligotrophic soils in relation to the undisturbed soils in adjacent forest ecosystems with pine.

Depth-dependent drivers of soil microbial necromass carbon across Tibetan alpine grasslands

Microbial necromass carbon (C) has been considered an important contributor to persistent soil C pool. However, there still lacks large-scale systematic observations on microbial necromass C in different soil layers, particularly for alpine ecosystems. Besides, it is still unclear whether the relative importance of biotic and abiotic variables such as plant C input and mineral properties in regulating microbial necromass C would change with soil depth. Based on the combination of large-scale sampling along a ~2200 km transect across Tibetan alpine grasslands and biomarker analysis, together with a global data synthesis across grassland ecosystems, we observed a relatively low proportion of microbial-derived C in Tibetan alpine grasslands compared to global grasslands (topsoil: 45.4% vs. 58.1%; subsoil: 41.7% vs. 53.7%). We also found that major determinants of microbial necromass C depended on soil depth. In topsoil, both plant C input and mineral protection exerted dominant effects on microbial necromass C. However, in subsoil, the physico-chemical protection provided by soil clay particles, iron-aluminum oxides, and exchangeable calcium dominantly facilitated the preservation of microbial necromass C. The differential drivers over microbial necromass C between soil depths should be considered in Earth system models for accurately forecasting soil C dynamics and its potential feedback to global warming.

Active and synchronous control of nitrogen and organic matter release from sediments induced with calcium peroxide

In order to realize the active and synchronous control of nitrogen (N) and organic matter (OM) release from sediments, this study compared the spatiotemporal changes in the physical, chemical, and biological indicators in the water system under different CaO₂ dosing modes. Results from 90-day incubation experiment showed that CaO₂ formed a dense barrier layer near its dosing position, improved the anoxic condition of water system, increased the physical adsorption of pollutants by sediments, and reduced the nutrients in overlying water, interstitial water, and sediments. Comprehensive comparison, the improvement effect of shallow injection group (I1) was the most obvious. Meanwhile, the activities of ammonia oxidizing bacteria and nitrite oxidizing bacteria near dosing position and those of denitrifiers and anammox bacteria adjacent to dosing site were significantly increased in all test groups (p < 0.01), thereby realizing the biological removal of N and OM in sediments. In addition, DO and ORP were steadily higher than 5 mg L^-1 and 100 mV in I1, where the NH₄⁺-N concentration in overlying water was stable below 1 mg L^-1, and the easily released N content in the upper (0-3 cm) and middle (4-6 cm) sediments decreased by 41.64% and 43.56%, respectively. Compared with the large pollutant flux in control (14.31 TN mg m^-2 d^-1 and 194.05 mg TCOD m^-2 d^-1), I1 completely inhibited the pollutant release and reduced the original nutrients in overlying water. In general, CaO₂ efficiently and synchronously controlled the endogenous release of N and OM under the combined actions of physical interception, physical adsorption, chemical oxidation, and biological transformation. Therefore, this study may provide valuable reference and guidance for the active and synchronous removal of N and OM in sediments and inhibition of endogenous pollutant release under anoxic condition.

The proximity of a highway increases CO2 respiration in forest soil and decreases the stability of soil organic matter

Roadways traverse many forest areas and they often have harmful effects on forest soils, including the modified stability of soil organic matter (SOM). Soil CO₂ respiration is an important indicator of SOM biological stability. The aim of this study was to test the hypotheses that a roadway will (1) modify the composition of the cation exchange capacity of adjacent forest soils, and (2) significantly decrease the stability of SOM. Two study sites were established in Scots pine and Silver fir stands, located close to the S7 highway in central Poland, which was opened to traffic in 1984. From each site, samples were taken at 2, 12 and 22 m from the forest edge. Soil CO₂ respiration was determined using closed chamber incubation with an alkali trap. We also conducted a comprehensive analysis of soil chemical properties. The stoichiometric ratios of chosen chemical parameters to total carbon (C_t) were calculated. In both sites, we observed increased soil pH and CO₂ respiration in the vicinity of the highway, as well as increased ratios of exchangeable calcium (Ca), magnesium (Mg) and sodium (Na) to C_t. In the fir site, the humic and fulvic acids, the dissolved organic carbon (DOC) content and aluminum (Al) to C_t ratio were depleted in close proximity to the highway. We suggest that the combined effect of Ca and Na ions, originating from winter de-icing, caused the depletion of Al and hydrogen (H) in the soil close to the forest edge and, therefore, resulted in lower SOM stability expressed as the decreased DOC and pyrophosphate-extractable carbon content, as well as the release of CO₂. We conclude that the changes of SOM stability with distance were the effect of modification of ion-exchange relationships (particularly base cations versus Al³⁺ with H⁺) rather than forest stand species or intrinsic SOM properties (like functional groups, the recalcitrance of bindings etc.). Our work supports earlier studies, confirming the significant impact of Al and H on SOM stability.

Plantations of Cinnamomum camphora (Linn) Presl with Distinct Soil Bacterial Communities Mitigate Soil Acidity within Polluted Locations in Southwest China

Although the underlying mechanisms remain unknown, soils from different forest types exhibit distinct acidification-buffering capacities. We investigated soil properties and bacterial communities across five plantation types and different soil horizons in a severely acid-polluted site in Southwest China and evaluated the potential mechanisms driving differences in community structure. Soils collected from pure stands of Pinus massoniana Lamb.or Schima superba Gardn. et Champ. or mixed stands of these species showed the lowest soil pH and greater abundances of acid-producing and acidophilic taxa (Halanaerobiales and Rhodospirillales). Soils from pure stands of Cinnamomum camphora (Linn) Presl, or a mixture of C. camphora and P. massoniana, yielded the greatest concentrations of exchangeable calcium (Ca) and magnesium (Mg) and greater soil pH. Increased relative abundances of beneficial taxa may have contributed to soil aggregate formation (e.g., Bradyrhizobium canariense and Terracidiphilus sp.) and heightened environmental stress resistance (e.g., Gaiellales). Fewer acid-producing and acidophilic taxa found in soils associated with C. camphora suggest that planting C. camphora may help soils recover from acidification, while planting S. superba may not be as beneficial. Our findings illustrate how differences in soil microbial communities may impact soil-acidification-buffering capacity across different forest types, which have important implications for understanding environmental functions within the context of microbial diversity.

Site-specific responses of lowland rice to acidulated and calcined phosphate rock fertilizers in the Center-West region of Burkina Faso

Soil phosphorus (P) deficiency is a major constraint of crop production in Sub-Saharan Africa. In particular, in the Sudano-Sahelian zone of Burkina Faso, P is rarely replenished as fertilizer supplies are limited in rural areas and because of the socio-economic situation of farmers. There is however, an abundance of local phosphate rock resource in the country. The development of local inorganic P fertilizers to improve crop production and replace the nutrients removed after harvesting, as well as to promote to sustainable agriculture, is desired. This study evaluated the efficiency of low-grade Burkina Faso phosphate rock (BPR)-based P fertilizers, produced by acidulation and calcination-the major fertilizer processing methods, on lowland rice production and the soil factors influencing their effectiveness. The results showed that the acidulated P fertilizers were as effective as conventional commercial P fertilizers on various soil types, textures, and fertility. Calcined P fertilizers were consistently effective on fine-textured soils with high basic fertility. It was found that fine soil texture and basic fertility of the initial soils were important factors in agronomic efficiency of BPR-based fertilizers and the resilience of rice production to climatic variability. It is recommended that soil type, with respect to soil texture, soil properties, inherent fertility, and water availability, should be considered when using BPR-based fertilizers for rice cultivation.

Soil carbon persistence governed by plant input and mineral protection at regional and global scales

Elucidating the processes underlying the persistence of soil organic matter (SOM) is a prerequisite for projecting soil carbon feedback to climate change. However, the potential role of plant carbon input in regulating the multi-layer SOM preservation over broad geographic scales remains unclear. Based on large-scale soil radiocarbon (∆¹⁴ C) measurements on the Tibetan Plateau, we found that plant carbon input was the major contributor to topsoil carbon destabilisation despite the significant associations of topsoil ∆¹⁴ C with climatic and mineral variables as well as SOM chemical composition. By contrast, mineral protection by iron-aluminium oxides and cations became more important in preserving SOM in deep soils. These regional observations were confirmed by a global synthesis derived from the International Soil Radiocarbon Database (ISRaD). Our findings illustrate different effects of plant carbon input on SOM persistence across soil layers, providing new insights for models to better predict multi-layer soil carbon dynamics under changing environments.

Soil degradation index developed by multitemporal remote sensing images, climate variables, terrain and soil atributes

Studies on soil degradation are essential for environmental preservation. Since almost 30% of the global soils are degraded, it is important to study and map them for improving their management and use. We aimed to obtain a Soil Degradation Index (SDI) based on multi-temporal satellite images associated with climate variables, land use, terrain and soil attributes. The study was conducted in a 2598 km² area in São Paulo State, Brazil, where 1562 soil samples (0-20 cm) were collected and analyzed by conventional methods. Spatial predictions of soil attributes such as clay, cation exchange capacity (CEC) and soil organic matter (OM) were performed using machine learning algorithms. A collection of 35-year Landsat images was used to obtain a multi-temporal bare soil image, whose spectral bands were used as soil attributes predictors. The maps of clay, CEC, climate variables, terrain attributes and land use were overlaid and the K-means clustering algorithm was applied to obtain five groups, which represented levels of soil degradation (classes from 1 to 5 representing very low to very high soil degradation). The SDI was validated using the predicted map of OM. The highest degradation level obtained in 15% of the area had the lowest OM content. Levels 1 and 4 of SDI were the most representative covering 24% and 23% of the area, respectively. Therefore, satellite images combined with environmental information significantly contributed to the SDI development, which supports decision-making on land use planning and management.

Effect of ferrolysis and organic matter accumulation on chromate adsorption characteristics of an Oxisol-derived paddy soil

How paddy cultivation influences the adsorption isotherms, envelopes, and the kinetics of hexavalent chromate (Cr(VI)) on Fe (hydro)oxide-rich paddy soil, as well as the mechanisms involved, remain largely unaddressed. To this end, the Cr(VI) adsorption characteristics on a paddy soil, in comparison with its parent upland Oxisol, were studied. The results showed that Cr(VI) adsorption capacities (Q_m^ad) were higher in the surface Oxisol than in the same layer of paddy soil. The Q_m^ad increased by 18.0% and 41.3% after removal of soil organic matter (SOM) from the surface Oxisol and paddy soil layers, respectively, indicating that Cr(VI) adsorption was considerably inhibited by SOM. The adsorption and desorption isotherms demonstrated that non-electrostatic adsorption was mainly responsible for Cr(VI) adsorption, accounting for 59.37%-83.42% of Cr(VI) adsorption capacities. The negative shift of the zeta potential-pH curves with Cr(VI) loading further corroborated the finding that non-electrostatic adsorption is largely responsible for Cr(VI) retention. Cr(VI) adsorption at equilibrium, obtained by the stirred flow chamber technique, and the free Fe (hydro)oxides (Fe_d) contents were in the same order, suggesting that Fe_d are the main adsorbents for Cr(VI). Therefore, paddy cultivation has had a profound impact on the electrochemical properties of the Oxisol and on subsequent Cr(VI) adsorption characteristics.

Soil Organic Carbon Attenuates the Influence of Plants on Root-Associated Bacterial Community

Plant-derived carbon (PDC) released by roots has a strong effect on root-associated bacterial community, which is critical for plant fitness in natural environments. However, the freshly exuded PDC can be diluted by the ancient soil-derived carbon (SDC) at a short distance from root apices. Thus, the rhizosphere C pools are normally dominated by SDC rather than PDC. Yet, how PDC and SDC interact to regulate root-associated bacterial community is largely unknown. In this study, a grass species and a legume species were planted in two contrasting matrixes, quartz sand and soil, to assess the role of PDC and SDC in regulating root-associated bacterial community, and to explore whether SDC affects the influence of PDC on bacterial community in soil. Our results indicated that the legume plant showed significantly positive priming effect on soil organic matter decomposition but the grass plant did not. PDC significantly shaped bacterial community in sand culture as indicated by PCR-DGGE and high-throughput sequencing of bacterial 16S rRNA gene. Intriguingly, we found that dissimilarity of bacterial communities associated with two plant species and the percentage of specific OTUs in quartz sand were significantly higher than those in soil. Moreover, several biomarkers enriched by plants in quartz sand turned to be general taxa in soil, which indicated that SDC attenuated the regulation of bacterial community by PDC. Taken together, these results suggest that SDC interacted with PDC and the root-associated microbial community, thus acted as soil buffering component of biological process contributing to soil resilience. The importance of PDC in structuring rhizosphere bacterial community needs to be reconsidered in the context of wider contribution of other C pool, such as SDC.

Metal Mobility in Afforested Sites of an Abandoned Zn-Pb Ore Mining Area

Heaps of waste material constitute a serious environmental problem in regions where the historical exploitation and processing of metal ores has taken place. The presented paper describes the trace metal distribution in selected heaps in the lead-zinc mining area of an abandoned mine in Poland, as well as the soil horizons beneath. The study aims at the estimation of the metal remobilization rate in vertical profiles in the spontaneously afforested area in the context of the potential danger it poses to the local groundwater. Individual samples were taken from profiles dug in heaps found in deciduous and coniferous forests. The bulk density, pH, organic matter and carbonate content, as well as the concentration and chemical forms of metals were analysed. Buffer properties and the mineralogical composition were also determined for the selected samples. The investigation indicates excessive cadmium, zinc and lead concentrations in the analysed heap material and the significant secondary enrichment of former soil horizons. A large percentage of these metals occur in potentially mobile forms. It suggests that, despite the high pH of the heap material and the good buffer properties of soil, cadmium and to a lesser extent, zinc, has migrated downwards to depths of at least several dozen centimetres over a period of about 200 years. This is related to soil acidity, particularly in profiles abundant in organic matter resulting from the encroachment of forest communities, particularly of coniferous forest. Spontaneous afforestation forming the litter cover contribute to the stabilization of the heap material and limiting groundwater pollution. Even though specific remediation measures are not needed in this area, it requires long-term monitoring.

Watershed-scale distributions of heavy metals in the hyporheic zones of a heavily polluted Maozhou River watershed, southern China

Hyporheic zone (HZ) sediments in river systems are often contaminated with heavy metals as a legacy of natural processes and anthropogenic activities. The geochemical behaviors of heavy metals in the HZ sediments at the laboratory scale have been extensively studied. However, the watershed-scale distributions of heavy metals in the HZ sediments and the processes controlling their distributions have not been well studied. Here, we report a watershed-scale study of heavy metals (i.e., Cr, Ni, Cu, Zn, Cd, and Pb) distributions in the HZ of the Maozhou River watershed, a heavily polluted area within the Pearl River Delta, southern China. Statistical analysis revealed that the spatial distribution of studied heavy metal concentrations was highly correlated with that of the sediment-associated sulfide at the watershed-scale. Metal extraction analysis and double-spherical aberration-corrected scanning transmission electron microscope imaging (Cs-STEM) further confirmed the strong association of heavy metals with sulfur. These observations demonstrated that the formation of metals-sulfide precipitates was the key process controlling the watershed-scale distributions of heavy metals (especially for Cr, Ni and Zn) in the HZ sediments. Additionally, high permeability of the HZ sediments may prevent Ni, Zn, Cd and Pb accumulation in sediments. Specially, Cu distribution was mainly affected by organic-Cu complexation. In the estuary area, salinity input likely affected the distributions of Ni, Zn and Cd through cation exchange processes. The findings improved our understanding of the distributions of heavy metals and the processes controlling their distributions at the watershed-scale, and have implications for remediating and managing contaminated HZ sediments.

The influence of land-use on tropical soil chemical characteristics with emphasis on aluminium

Composition of soil vegetation cover and land management directly influences the cycling of chemical elements and is a key factor for soil biogeochemistry and also Al behaviour. Moreover, Al is an important factor limiting the growth of cultural plants. Our results are based on long-term observations of soils translocated from selected small areas of eight 1 ha plots of different land-use gradient, with identical geological, climatic and geographical conditions, located in the North of Congo Basin (near Mbalmayo, Cameroon). The plots are established in primary and secondary forests, cocoa agroforestry systems and a maize field (two plots per habitat). All soil plots were exchanged between each other in two layers; A. 0-5 cm, and B. 5-20 cm of depths. The soil was sampled at the times 0, +3, +6 months, and soil chemical parameters were determined. The most important differences between the particular habitats comprise of vegetation cover as a consequence of the land management. Particular plots differed mainly in their pH, organic C, exchangeable Al and contents of base cations. The most marked trends comprise of significant decrease of pH, increase of Al and decrease of the Ca/Al ratio in A layer after translocation to the agricultural plots. All translocations resulted into rapid loss of organic C and release of Al, which was more obvious when the forest-to-agriculture translocation took place.

Regulation of priming effect by soil organic matter stability over a broad geographic scale

The modification of soil organic matter (SOM) decomposition by plant carbon (C) input (priming effect) represents a critical biogeochemical process that controls soil C dynamics. However, the patterns and drivers of the priming effect remain hidden, especially over broad geographic scales under various climate and soil conditions. By combining systematic field and laboratory analyses based on multiple analytical and statistical approaches, we explore the determinants of priming intensity along a 2200 km grassland transect on the Tibetan Plateau. Our results show that SOM stability characterized by chemical recalcitrance and physico-chemical protection explains more variance in the priming effect than plant, soil and microbial properties. High priming intensity (up to 137% of basal respiration) is associated with complex SOM chemical structures and low mineral-organic associations. The dependence of priming effect on SOM stabilization mechanisms should be considered in Earth System Models to accurately predict soil C dynamics under changing environments.

Global soil carbon dynamics are regulated by the modification of soil organic matter (SOM) decomposition by plant carbon input (priming effect). Here, the authors collect soil data along a 2200 km grassland transect on the Tibetan Plateau and find that SOM stability is the major control on priming effect.

The chemical composition of forest soils and their degree of acidity in Central Europe

We conducted an extensive screening of forest soils in the whole area of the Czech Republic to determine their degree of acidification and potential degradation. Soils were sampled at 480 forest sites (in a 7 × 7 km grid covering the entire Czech Republic) from the upper 30-cm layer and included both organic and mineral horizons. Based on values of water extractable pH (pH_H2O), cation exchange capacity (CEC) and base saturation (BS), soils were divided into three categories by their degree of acidity, i.e., non-or-low-acidic (NLA; pH_H2O ≥ 4.2, CEC ≥ 150 meq kg^-1, BS ≥ 15%), moderately acidic (MA; at least one parameter is below the limits for the NLA category), and strongly acidic (SA; all three parameters are below the limits for the NLA category). Only 11% of sampled soils were classified in the NLA category, while 58% and 31% belonged to the MA and SA category, respectively. The SA soils had median values of pH_H2O, CEC, and BS of 3.9, 102 meq kg^-1, and 10.2%, respectively, and their molar ratios between exchangeable concentrations of base cations to aluminum (BC_ex/Al_ex) were <0.6, indicating a high likelihood of adverse Al effects on plant growth. Moreover, the SA soils exhibited lowest ratios between extractable nutrients (base cations and phosphorus) and dissolved N (DN), indicating other than N limitation of plant growth at these sites, and elevated risks of reactive N leaching. In contrast, the NLA soils had median values of pH_H2O, CEC, BS and BC_ex/Al_ex of 5.4, 199 meq kg^-1, 95%, and 0.7 respectively. For these soils, neither adverse effects of Al nor elevated N losses are likely.

Fabrication and characterization of pyridinium functionalized anion exchange membranes for acid recovery

In the present work, the fabrication of pyridinium functionalized anion exchange membranes (AEMs) for acid recovery using diffusion dialysis (DD) processes was developed using brominated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO) as a polymer backbone and 4-methylpyridine (MP) as an ion exchange element. The electrochemical and physiochemical properties of the developed AEMs were tested under various concentration of MP into the polymer matrix. Water uptake (W_R) of 17.18% to 30.55%, ion exchange capacity (IEC) of 1.94-2.24 mmol/g and linear swelling ratio (LSR) of 6.87-14.89% were obtained. In addition, the new membranes exhibited dense morphology, higher thermal and chemical stability in addition to dimensional and mechanical sturdiness. Acid dialysis coefficient (U_H) in the range of 0.011-0.066 m/h was obtained. In addition the developed AEMs had a separation factor (S) in range of 24.87-77.61 resulting in enhanced DD performance compared to commercial membrane DF-120B under comparable experimental conditions. The new prepared membranes showed potential for successful application in acid recovery via diffusion dialysis.

Significant build-up of soil organic carbon under climate-smart conservation farming in Sub-Saharan Acrisols

Conservation farming (CF) involving minimum tillage, mulching and crop rotation may offer climate change adaptation and mitigation benefits. However, reported effects of CF, as applied by smallholders, on storage of soil organic carbon (SOC) and soil fertility in Sub-Saharan Africa differ considerably between studies. This is partly due to differences in management practice, soil type and adoption level between individual farmers. Where CF involves planting basins, year-to-year changes in position of basins make SOC stock estimates more uncertain. Here we assess the difference in SOC build-up and soil quality between inside planting basins (receiving inputs of lime and fertilizer; basins opened each year) and outside planting basins (no soil disturbance or inputs other than residues) under hand-hoe tilled CF in an Acrisol at Mkushi, Zambia. Seven years of strict CF husbandry significantly improved soil quality inside planting basins as compared with outside basins. Significant effects were found for SOC concentration (0.74 ± 0.06% vs. 0.57 ± 0.08%), SOC stock (20.1 ± 2.0 vs. 16.4 ± 2.6 t ha^-1, 0-20 cm), soil pH (6.3 ± 0.2 vs. 4.95± 0.4) and cation exchange capacity (3.8 ± 0.7 vs. 1.6 ± 0.4 cmol_c kg^-1). As planting basins only occupy 9.3% of the field, the absolute rate of increase in SOC, compared with outside basins, was 0.05 t C ha^-1 yr^-1_. This corresponds to an overall relative increase of 2.95‰ SOC yr^-1 in the upper 20 cm of the soil. Also, hot water extractable carbon (HWEC), a proxy for labile organic matter, and potential nitrification rates were consistently greater inside than outside basins. The significant increase in quantity and quality of SOC may be due to increased inputs of roots, due to favorable conditions for plant growth through input of fertilizer and lime, along with increased rainwater infiltration in the basins.

The crucial factors of soil fertility and rapeseed yield - A five year field trial with biochar addition in upland red soil, China

Biochar has been used as an amendment to improve soil fertility and increase crop yield. However, the effects of biochar on soil properties and rapeseed yield in upland red soil have not been thoroughly investigated, and the factors crucial for rapeseed yield are not yet clear. A five-year field trial was conducted to investigate the effects the of biochar (biochar application rates of 0, 2.5, 5, 10, 20, 30, and 40 t ha^-1, respectively) on soil physicochemical and microbial properties as well as rapeseed yield in upland red soil in Jiangxi Province, China. Results showed that biochar can significantly increase soil pH, available phosphorus, organic carbon, Ks, and water retention, however, the influences of biochar on these indexes declined over time. Soil total nitrogen increased significantly when the dose of biochar exceeded 5 t ha^-1, and the content of total nitrogen in the 40 t ha^-1 biochar treatment increased each year. While the application of biochar gradually increased the contents of NH₄⁺-N, NO₃^--N and enhanced the soil microorganism and enzymatic activities during the first three years, they had returned nearly to their starting values by the end of this study. Rapeseed yield and yield components were significantly improved relative to the control for all biochar amendments in the first year, but the rapeseed yield in all biochar treatments decreased steadily after 2012. According to the principal components analysis and path analysis, the most responsive parameters in the upland red soil were soil acidity and hydraulic properties, meanwhile, soil acidity and hydraulic properties had greater impacts on rapeseed yield than did other indexes. Taken together, these results suggest that biochar can significantly improve soil fertility and rapeseed yield, but the improvements are not permanent. Soil acidity and hydraulic properties were the crucial factors that determined soil fertility and rapeseed yield in upland red soil.

Determination of Organic Fractions and Enzymatic Activity in Forest Spruce Soil of Tatra National Park

The formation and quality of soil organic matter (SOM) highly depends on the input of organic material and microbial enzymatic activities. Soil extractions with specific nonpolar and polar extractives can be used to identify qualitative changes in SOM. The aim of this paper was to understand the correlations among microbial enzymatic activity and specific organic fractions in acidic spruce forest soil. Klason lignin (KL), acid soluble lignin (ASL), holocellulose (HC), SOM content, and potential enzymatic activity (FDA and phosphatase) was measured and analyzed. We sampled Dystric Cambisol of forest spruce stands (Picea abies) in Tatra National Park (Slovakia). The SOM fractions were determined gravimetrically based on their extractivity in nonpolar (dichloromethane (DME)) and polar (acetone (AE), ethanol (EE), water (WE)) solvents Total extractives content was 0.079% and nonpolar extractives 0.036%. The mean amount of polar extractives tented to increase in the order EE<AE<WE. The total lignin content was determined to be 1.079% and HC 0.774%. FDA negatively correlated with KL (r=-0.873 p<0.05) and DME (r=-0.913 p<0.05). Phosphatase positively correlated with WE (r=0.972 p<0.01) and KL (r=0.957 p<0.01).

Long-term impact of cement plant emissions on the elemental composition of both soils and pine stands and on the formation of Scots pine seeds

We investigated the long-term impact of the largest Russian cement plant on mesopodzol sandy soils and Scots pine stands. We determined the distributions of the total and available pools of Ca, Mg, K, Na, Mn, Fe, Zn, Ni, Cu, Pb and Cd in the soil profile to a depth of 60 cm (illuvial horizon) as well as the accumulation patterns of these elements in the vegetative and generative organs of Scots pine trees. High Ca accumulation in the impact zone soils was a result of CaO emissions by a cement plant. Also, CaO became the main cause of soil profile alkalization due to neutralization of soil acids and formation of calcium hydroxide or carbonates. Alkalization immobilized substantial amounts of Fe, Mn, Zn and Ni in the soil, reducing their availability. The most prominent effect of long-term cement production was a prominent Mn deficiency in vegetative and generative Scots pine organs due to the exhaustion of the available Mn pool in the illuvial horizon. The miniaturization of cones, a decrease in seed yield and a reduction in seed germinability were observed in the emission impact zones. Pretreatment of Mn-deficient seeds with manganese eliminated Mn deficiency but did not increase seed germination.

Cation exchange capacity of biochar: An urgent method modification

Biochar, produced through pyrolysis of organic matter, is negatively charged, thus contributing to electrostatic adsorption of cations. However, due to its porous structure and contents of alkaline ashes, the determination of the cation exchange capacity (CEC) is challenging. Literature values for the CEC of biochar are surprisingly variable and are often poorly reproducible, suggesting methodological problems. Here, we modify and critically assess different steps in the existing ammonium acetate (NH₄OAc) method (pH 7), where ammonium (NH₄⁺) is displaced by potassium chloride (KCl), following removal of excess NH₄OAc with isopropanol, in batch mode. We used pigeon pea biochar to develop the method and conducted a test on three additional biochars with different acid neutralizing capacity. A pretreatment step of biochar was introduced, using diluted hydrochloric acid, to decrease biochar pH to near neutral, so that 1 M NH₄OAc effectively buffers the biochar suspension pH at 7. This allows the CEC of all biochars to be determined at pH 7, which is crucial for biochar comparison. The dissolution of ashes may cause relatively large weight losses (e.g. for cacao shell biochar), which need to be accounted for when computing the CEC of raw biochar. The sum of NH₄OAC-extractable base cations provided a smaller and better estimate of the CEC than KCl-extractable NH₄⁺. We hypothesize that the overestimation of the CEC based on KCl-extractable NH₄⁺ is due to the ineffectiveness of the relatively large isopropanol molecules to remove excess NH₄OAc in biochars rich in micro-pores, due to size exclusion. The amount of base cations removed in the pretreatment was about three (rice husk biochar) to ten times (pigeon pea biochar) greater than the amount of exchangeable cations. The CEC values of biochar increased from 10.8 cmol/Kg carbon to 119.6 cmol/Kg carbon. These values are smaller than reported CEC values of soil organic carbon.

Fading positive effect of biochar on crop yield and soil acidity during five growth seasons in an Indonesian Ultisol

Low fertility limits crop production on acidic soils dominating much of the humid tropics. Biochar may be used as a soil enhancer, but little consensus exists on its effect on crop yield. Here we use a controlled, replicated and long-term field study in Sumatra, Indonesia, to investigate the longevity and mechanism of the effects of two contrasting biochars (produced from rice husk and cacao shell, and applied at dosages of 5 and 15tha^-1) on maize production in a highly acidic Ultisol (pH_KCl3.6). Compared to rice husk biochar, cacao shell biochar exhibited a higher pH (9.8 vs. 8.4), CEC (197 vs. 20cmol_ckg^-1) and acid neutralizing capacity (217 vs. 45cmol_ckg^-1) and thus had a greater liming potential. Crop yield effects of cacao shell biochar (15tha^-1) were also much stronger than those of rice husk biochar, and could be related to more favorable Ca/Al ratios in response to cacao shell biochar (1.0 to 1.5) compared to rice husk biochar (0.3 to 0.6) and nonamended plots (0.15 to 0.6). The maize yield obtained with the cacao shell biochar peaked in season 2, continued to have a good effect in seasons 3-4, and faded in season 5. The yield effect of the rice husk biochar was less pronounced and already faded from season 2 onwards. Crop yields were correlated with the pH-related parameters Ca/Al ratio, base saturation and exchangeable K. The positive effects of cocoa shell biochar on crop yield in this Ultisol were at least in part related to alleviation of soil acidity. The fading effectiveness after multiple growth seasons, possibly due to leaching of the biochar-associated alkalinity, indicates that 15tha^-1 of cocoa shell biochar needs to be applied approximately every third season in order to maintain positive effects on yield.

Aliphatic hydrocarbons recovered in vegetables from soils based on their in-situ distribution in various soil humus fractions using a successive extraction method

Aliphatic hydrocarbons (AHs) are major petroleum contaminants in the environment. In this study, the AHs bound to various soil endogenetic humus fractions were separated through successive extraction. Most of the AHs (46.1%) in soils were adsorbed onto/into humic acids (HA) and a small quantity of AHs (9.6%) were organic solvent extractable. AHs in B. chinensis were also analyzed since their potential risks to the residents through ingestion. AHs from C₂₁ to C₃₄, so called high molecular weight AHs (HMWAHs), were dominant AHs in B. chinensis (85.5%) and soils (70.4%), followed by AHs from C₁₆ to C_21, whose mobility can be enhanced via binding to fulvic acids and then can be taken up by plant root lipids (soil-plant pathway). HMWAHs were mainly HA-bound and then were detained in the top soil layers. HMWAHs associated with fine topsoil particles could be transported to B. chinensis via the soil-air-plant pathway, including resuspension and aboveground plant cuticle capture. Results from Principal Component Analysis combined with Regression Analysis supported this assumption due to the positive correlations between HMWAHs concentration in B. chinensis and fine particle contents in soils. This work presents the distributions of petroleum contaminants that result from previously described behavior mechanisms.

Metal availability, soil nutrient, and enzyme activity in response to application of organic amendments in Cd-contaminated soil

The study investigated the effects of organic amendments: green tea amendment (GTA) and oil cake amendment (OCA) on Cd bioavailability, soil nutrients, and soil enzyme activity in Cd-contaminated soil. The amendments were added to the soil at the doses of 1, 3, and 5% and were incubated for 45 days. Then, pakchoi cabbage was planted to test the remediation effect of the above two organic amendments. The diethylenetriaminepentaacetic acid (DTPA)-extractable Cd in GTA and OCA treatments was reduced by 14.69-27.51 and 13.75-68.77%, respectively, compared to no amendment-applied treatment. The application of GTA and OCA notably decreased the proportion of exchangeable fraction of Cd, but increased the percentage of oxide and organic-bound fraction of Cd, thereby suppressing the uptake by pakchoi cabbage. Cd concentration of aboveground parts decreased by 8.21-18.05 and 7.77-35.89% in GTA and OCA treatments, respectively. Relative to the no amendment-applied treatment, both GTA and OCA had enhanced soil nutrients and enzyme activities largely. Redundancy analysis showed that organic matter, total P, available N, and DTPA-extractable Cd significantly affected the enzyme activities. Furthermore, the application of OCA at the dose of 5% was more effective in reducing bioavailable Cd, enhancing soil available nutrients and urease and catalase activities in contaminated soil. These results indicated that oil cake should be used to immobilize metal and improve fertility and quality of Cd-contaminated soil.

Responses of soil buffering capacity to acid treatment in three typical subtropical forests

Elevated anthropogenic acid deposition can significantly affect forest ecosystem functioning by changing soil pH, nutrient balance, and chemical leaching and so on. These effects generally differ among different forests, and the dominant mechanisms for those observed responses often vary, depending on climate, soil conditions and vegetation types. Using soil monoliths (0-40cm) from pine forest (pioneer), coniferous and broadleaved mixed forest (transitional) and broadleaved forest (mature) in southern China, we conducted a leaching experiment with acid treatments at different pH levels (control: pH≈4.5; pH=3.5; pH=2.5). We found that pH3.5 treatment significantly reduced dissolved organic carbon (DOC) concentrations in leachate from the pioneer forest soil. pH2.5 treatment significantly increased concentrations of NO3(-), SO4(2-), Ca(2+), Mg(2+), Al(3+), Fe(3+) and DOC in leachate from the pioneer forest soil, and also concentrations of NO3(-), SO4(2-), Mg(2+), Al(3+), Fe(3+) and DOC in leachate from the transitional forest soil. All acid treatments had no significant effects on concentrations of these chemicals in leachate from the mature forest soil. The responses can be explained by the changes in soil pH, acid neutralizing capacity (ANC) and concentrations of Al and Fe. Our results showed that acid buffering capacity of the pioneer or transitional forest soil was lower than that of the mature forest soil. Therefore preserving mature forests in southern China is important for reducing the adverse impacts of high acid deposition on stream water quality at present and into the future.

Effect of variable soil texture, metal saturation of soil organic matter (SOM) and tree species composition on spatial distribution of SOM in forest soils in Poland

In this study we investigated the effect of fine (ϕ<0.05mm) fraction, i.e., silt+clay (FF) content in soils, site moisture, metal (Al and Fe) of soil organic matter (SOM) and forest species composition on the spatial distribution of carbon (C) pools in forest soils at the landscape scale. We established 275 plots in regular 200×200m grid in a forested area of 14.4km(2). Fieldwork included soil sampling of the organic horizon, mineral topsoil and subsoil down to 40cm deep. We analysed the vertical and horizontal distribution of soil organic carbon (SOC) stocks, as well as the quantity of physically separated fractions including the free light (fLF), occluded light (oLF) and mineral associated fractions (MAF) in the mineral topsoil (A, AE) horizons. Distribution of C in soils was predominantly affected by the variation in the FF content. In soils richer in the FF more SOC was accumulated in mineral horizons and less in the organic horizons. Accumulation of SOC in mineral soil was also positively affected by the degree of saturation of SOM with Al and Fe. The increasing share of beech influenced the distribution of C stock in soil profiles by reducing the depth of O horizon and increasing C stored in mineral soil. The content of FF was positively correlated with the content of C in MAF and fLF fractions. The content of oLF and MAF fractions was also positively influenced by a higher degree of metal saturation, particularly Al. Our results confirmed that Al plays an important role in the stabilization of SOM inside aggregates (CoLF) and as in CMAF fractions. We also found a significant, positive effect of beech on the CfLF and fir on the CoLF content.