Plant rhizosphere enhances biochar's immobilization of cadmium: Divergent effects in flooded and unsaturated cultivation soils.

Land use and soil management may affect both labile and humified soil organic matter (SOM) fractions, but the magnitude of these changes is poorly known in subtropical environments. This study investigated effects of four land use and soil management systems (forest, native pasture, and conventional tillage and no-tillage in a wheat/soybean succession) on (i) total soil organic carbon (SOC) stocks (0 to 250mm depth) and on (ii) carbon (C) stocks in labile (coarse, light) and humified (mineral-associated, humic substances) SOM fractions (0 to 25mm depth), in a Hapludox soil from southern Brazil. In comparison to the adjacent forest site, conventionally tilled soil presented 36% (46.2Mg ha-1) less SOC in the 0 to 250mm depth and a widespread decrease in C stocks in all SOM fractions in the 0 to 25mm depth. The coarse (>53 mum) and light (<1kg dm-3) SOM fractions were the most affected under no-tillage, showing 393% (1.22Mg C ha-1) and 289% (0.55Mg C ha-1) increases, respectively, in relation to conventional tillage. Similar results were observed for mineral-associated SOM and humic substance C pools (34% and 38% increases, respectively) under no-tillage. Compared with labile SOM fraction results, the percentual increments on C stocks in humified fractions were smaller; but in absolute terms this C pool yielded the highest increases (3.06 and 2.95Mg C ha-1, respectively). These results showed that both labile and humified organic matter are better protected under the no-tillage system, and consequently less vulnerable to mineralization. Humified SOM stabilization process involving interactions with variable charge minerals is probably important in maintaining and restoring soil and environmental quality in tropical and subtropical regions.

Reforestation can alter the chemical composition of soil organic matter (SOM) and humification; however, information on how specific plant types impact SOM lability and humification is not well documented. In this study, we used solid‐state 13C nuclear magnetic resonance spectroscopy, photometric analysis, and chemical fractionation to examine carbon (C) components and lability of SOM in a Japanese cedar (Cryptomeria japonica) forest and bamboo (Phyllostachys edulis) plantation that reforested a cutover primary broadleaf forest. The ∆logK value of soil humic acids, the inverse index of SOM humification, was lowest in the bamboo plantation, suggesting a higher SOM humification stage in the bamboo plantation. The soil labile C/Total C ratio was highest in the bamboo plantation, and this can be attributed to low aromaticity and alkyl‐C/O‐alkyl‐C ratio (A/O‐A) in the bamboo litter. Intensive cultivation of the bamboo plantation accelerated litter breakdown in the strongly acidic soil, resulting in the depletion of SOM. Cedar coniferous leaves, with their high recalcitrant substances and slow decomposition, only slightly lowered SOM humification due to the substantial broadleaf understory. Our results suggest that the type of plants involved in reforestation and understory reestablishment is critical to how SOM humification and lability change during the reforestation through the control of litter C components. Further research into the interaction between microclimate change and forest type in forest conversion will be useful for increasing understanding on the impact of forest conversion on SOM lability and humification in subtropical high mountain forest ecosystems.

Given that microbial mediated input of crop residues and their humification products may be more conducive to the transformation and sequestration of soil organic matter. The study aims to use 13C15N double labeling tracing technology in incubation experiments to quantify the differences in the fate and distribution contribution of the C and N of crop residues and their decomposition products under the mediation of different exogenous microorganisms with different life strategies in soil organic matter (SOM) and dissolved organic matter (DOM). The study utilized exogenous microorganisms, such as Trichoderma reesei, Trichoderma harzianum, and Phanerochaete chrysosporium (K-strategists), as well as Bacillus subtilis (r-strategist). Additionally, a combination microbial treatment comprised of Trichoderma harzianum, Phanerochaete chrysosporium, and Bacillus subtilis was also employed. The study also aims to reveal the variation patterns of soil active organic carbon and humic carbon components in response to different exogenous microorganisms. The main conclusions of the study are as follows:The addition of exogenous k-strategy microorganisms was more favorable than r-strategy microorganisms in mediating the increase in soil SOM contribution of crop residues derived C and N.Trichoderma treatments were more adept at mediating crop residues derived dissolvd organic carbon, and k-strategy microorganisms were more likely to stimulate soil production of dissolved nitrogen. Although the combination microbial treatment was the most effective at translational immobilization of crop residues in SOM, the Trichoderma reesei treatment had the best ability to increase soil organic carbon content by mediating crop residues translational immobilization with the lowest depletion of SOM. In addition, the addition of K-strategy microorganisms was more effective than r-strategy microorganisms in increasing the content of labile organic carbon and humic carbon fractions in the soil. Fungimediated humification products was significantly better than bacterial and no microbe mediated for translational immobilization in soil, and the fungal treatments contributed more to DOM and stimulated soil deriving dissolved nitrogen. The Trichoderma reesei treatment was the most effective in immobilizing the carbon and nitrogen dereived from humification products. Fungi mediated humification products was superior to bacteria in boosting labile organic carbon and humic acid fractions. The Trichoderma reesei treatment was the most effective in boosting contents of easily oxidizable organic carbon, microbial biomass carbon and humic acid carbon, whereas the combination microbial treatments significantly increased fvlic acid carbon and substantially reduced PQ values in the early part of the experiment, and the three fungal treatments were effective in increasing fvlic acid carbon in the later part of the experiment. In summary, these conclusions provide a theoretical basis for seeking suitable microbial regulation of farmland management measures to improve SOM transformation and humification effects and provide practical reference for scientifically guiding agricultural production and soil carbon sequestration and fertilization.

The aim of this study was to provide an overview of the approaches and methods used to assess the dynamics of soil organic matter (SOM). This included identifying relevant processes that describe and estimate SOM decomposition, lability, and humification for the purpose of sustainable management. Various existing techniques and models for the qualitative and quantitative assessment of SOM were evaluated to gain a better understanding of advances in organic matter transformation. This evaluation aimed to identify the strengths, limitations, and applications of these techniques and models, and to highlight new research directions in the field. Quantitative analysis of SOM can be performed using various parameters, including oxidation kinetics, lability, carbon management index, humification degree, humification index, and humification ratio. On the other hand, qualitative evaluation of SOM can involve techniques such as oxidizability, high-performance size-exclusion chromatography, electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, visual examination, smell, assessment of microorganism content, plant growth, cation exchange capacity, type of organic material, and decomposition. These techniques and parameters provide valuable insights into the characteristics and transformation of SOM, enabling a comprehensive understanding of its dynamics. Evaluating SOM dynamics is of utmost importance as it is a determining factor for soil health, fertility, organic matter stability, and sustainability. Therefore, developing SOM models and other assessment techniques based on soil properties, environmental factors, and management practices can serve as a tool for sustainable management. Long-term or extensive short-term experimental data should be used for modeling to obtain reliable results, especially for quantitative SOM transformation analysis, and changes in the quality and quantity of SOM should be considered when developing sustainable soil management strategies.

The changes of soil organic matter (SOM) humification induced by long-term combination of tillage and olive mill wastewater (OMW) application compared to natural and cultivated soil have been little investigated. This study aimed to compare effects of no cultivation with natural vegetation soil (NC), tillage (CT1) for 80 years and combination of tillage with OMW application (CT2) for 20 years on SOM humification degree. Fluorescence spectroscopy and UV-visible ratios (E4/E6 and CHA/CFA) were used to study soil humic acids (HAs). The SOM and humification distribution was determined for the whole field area using the Inverse Distance Weighting method. Results showed that SOM content, fluorescence emission area and E4/E6 and CHA/CFA ratios were higher in NC. Tillage reduced SOM amount, molecular size, aromatic condensation and humification degree as shown by the strong correlation between fluorescence area and CHA/CFA ratio in CT1 conversely to E4/E6. Contradictory results between fluorescence emission area and E4/E6 ratio found in NC and CT1 indicated that E4/E6 ratio was not a reliable indicator of SOM humification degree. The SOM amount, CHA/CFA ratio and emission fluorescence area increased conversely to E4/E6 ratio in CT2. This revealed a greatly humified organic matter and aromatic structure condensation with tillage and OMW application. Spatial distribution showed a progressive increase of SOM and CHA/CFA from north-west to south-east linked to the positive relationship between CHA/CFA ratio and SOM amount independent of soil management practices. Soil amended with OMW provided a favourable environment for the development of HAs which improved soil quality. The UV-visible ratio CHA/CFA with fluorescence emission area can be used as parameters to investigate SOM humification degree.

To compare the dynamic effects of straw and corresponding biochar on soil acidity, nutrients, and exchangeable capacity in red soil, a pot experiment was performed. The treatments included control (CK), rice straw (R1B0), rice straw biochar prepared at 350℃ (R1B1) and 550℃ (R1B2), rape stalk (R2B0), and rape stalk biochar prepared at 350℃ (R2B1) and 550℃ (R2B2). Straw at 1% and corresponding biochar were added to a strongly acidic red soil. The rice was planted as the experimental crop. Soils were collected at the seedling, tillering, filling and mature stages of rice growth, respectively. The changes in soil pH, exchangeable acidity, organic matter, nutrients (NH4+-N and NO3--N), and exchangeable cations in soils were measured. The results showed that soil pH, NH4+-N, and NO3--N concentrations decreased with the growth period of rice, while the organic matter content and cation exchange capacity (CEC) increased. Direct returning of straw and biochar could increase soil pH, organic matter content, and exchangeable cations content, and reduce the total amount of exchangeable acids. In the mature stage of rice, rice straw and rape stalk biochar at 350℃ increased the soil pH by 0.29 and 0.42, respectively, compared to the control treatment. Similarly, biochar decreased the exchangeable acidity and exchangeable Al3+ content significantly compared to the direct returning treatments of straw. The exchangeable acidity and exchangeable Al3+ contents of soils in R1B2 and R2B1 treatments decreased by 54.8% and 58.9%, respectively, compared to the control treatment. The soil organic matter (SOM) content and CEC in biochar treatments were significantly higher than those in direct returning treatments of straw. Overall, the effects of rape stalk biochar on soil properties were slightly stronger than those of rice straw. The correlation analysis showed that soil exchangeable acids had a significantly negative correlation with organic matter (R=-0.912, P<0.01), and CEC (R=-0.866, P<0.05). The CEC in soils was positively related to organic matter (R=0.833, P<0.05). Direct returning of straw and biochar applications can effectively improve soil acidity and increase nutrient contents. The effects of straw biochar on soils were stronger than the direct returning of straw in decreasing soil acidity, and increasing soil organic matter content and exchangeable capacity in acidic soils.

Cadmium (Cd) commonly coexists with selenium (Se) in the natural Se-rich soils of China, severely limiting their agricultural utilization. Dissolved organic matter (DOM) extracted from agricultural residues is considered a potential soil amendment capable of mobilizing Se and immobilizing Cd. This study investigates the mechanism by which DOM from different agricultural residues affects Se and Cd dynamics in natural Se-Cd-rich soils. Two-season pot experiments were performed to examine Se/Cd transformation and bioavailability under different residue-derived DOM treatments: chicken, ox, and earthworm manures, rice straw, and straw-manure mixtures. Our findings revealed that DOM application increased soil pH (+4.85 %), organic matter content (+33.47 %), and cation exchange capacity (+61.97 %). The 2 % chicken manure-derived DOM treatment achieved optimal results, increasing grain-Se content by 46.14 % while reducing the grain-Cd levels by 47.22 %. Structural equation modeling indicated that the increased soluble Se fraction (+54.81 %) and decreased oxidizable Cd fraction (-20.97 %) primarily influenced these results. We conclude that DOM mobilizes Se and immobilizes Cd in natural Se-Cd-rich paddy soils, with chicken manure-derived DOM demonstrating superior efficacy.

Grande parte da matéria orgânica de Organossolos das turfeiras é composta por substâncias húmicas, formadas pela transformação de resíduos orgânicos pelos microrganismos do solo e pela polimerização dos compostos orgânicos em macromoléculas resistentes à degradação biológica. Os processos de humificação da matéria orgânica do solo (MOS) ainda são pouco compreendidos e o conhecimento sobre os precursores das substâncias húmicas é limitado, sendo apresentadas rotas diferentes para a formação dessas substâncias. Contudo, em todas as rotas, destaca-se a participação da lignina. Isótopos estáveis (13C, 15N) podem ser utilizados para rastrear processos de humificação da MOS, por meio da identificação de seus precursores. Este trabalho teve como objetivo avaliar comparativamente a composição isotópica da vegetação das fitofisionomias que colonizam uma turfeira tropical de altitude composta de Campo Limpo Úmido (CLU) e de Floresta Estacional Semidecidual (FES), em relação à composição isotópica das substâncias húmicas da MOS. A turfeira estudada ocupa 81,75 ha. Para as análises isotópicas e lignocelulósicas da vegetação, foram identificadas as espécies dominantes em cada fitofisionomia. Amostras de solo foram coletadas em três locais representativos sob cada fitofisionomia, a cada 5 cm de profundidade, até 50 cm. As substâncias húmicas dessas amostras foram fracionadas, assim como calculados os valores de δ13C e δ15N nas frações húmicas, respectivamente a partir da determinação dos isótopos estáveis 12C e 13C e 14N e 15N. Os teores de lignina e seus valores de δ13C são mais elevados na vegetação e MOS sob FES em relação à vegetação e MOS sob CLU. Os teores de humina são mais elevados entre as substâncias húmicas na MOS, sob as duas fitofisionomias; os de ácidos húmicos são mais elevados na MOS sob CLU, em relação à FES; e os de ácidos fúlvicos são mais elevados na MOS sob a FES, em relação ao CLU. O δ13C da lignina apresenta similaridade elevada em relação ao δ13C da humina, dos ácidos húmicos e dos ácidos fúlvicos. As variações na composição lignocelulósica das espécies que colonizam o CLU e a FES promovem diferenças nas taxas e nos produtos da humificação da MOS.

This study investigates the composition and transformation of soil organic matter (SOM) across seven sites in Maritime Antarctica, focusing on the impact of bird activity and vegetation cover on SOM dynamics. There is limited knowledge of the stability of Antarctic SOM and the effects of seabird colonies on it. This study aims to address the knowledge gap regarding drivers of soil organic matter transformations in polar ecosystems. Hot water-extractable carbon (HWC) and carbon extracted with phosphoric acid (PHP-C) were chosen as parameters for the labile carbon pool. A stable carbon pool was here characterized as one with alkali-soluble organic compounds opposing microbial decomposition. This carbon pool has long (decades) turnover rates, and therefore is regarded stable. The mentioned carbon pools were used to calculate humification indices. The HWC in studied soils ranged from 1.5 to 4.3% of total carbon, while the PHP-C varied largely and was not correlated with HWC. Soils influenced by current or historical bird colonies (particularly penguins and skuas) exhibited elevated labile carbon fractions, indicating active microbial processing. In contrast, sites without bird influence showed lower biological activity. The stable carbon peaked at 18.9% of total carbon, indicating distinct soil transformation stages. The humification degree (HD) and labile-to-stable carbon (L/S) ratio were used to assess SOM stability, revealing that former bird rookeries had the most stabilized SOM, while recently deglaciated sites were in early stages of organic matter accumulation. Vegetation cover, though secondary to bird impact, was positively correlated with SOM humification, supporting the role of vascular plant-derived organic input in carbon stabilization. The study showed a clear link between bird activity and SOM dynamics, supporting the concept of biological legacies in soil formation in Antarctica. It highlighted the role of vegetation in SOM stabilization, which is crucial for understanding how terrestrial ecosystems may evolve as ice retreats and plant colonization expands.

Dryland soils consistently exhibit a low capacity for the long‐term accumulation and storage of organic matter, which has been primarily attributed to low plant biomass inputs under drought suppression. Whether, and how, soil organic matter (SOM) compositions contribute to the consistently low SOM storage have been puzzling. A fundamental understanding of this mechanism is particularly essential to achieve the aspiration of ‘4 per mille Soils for Food Security and Climate’. By screening the molecular composition of dissolved organic matter (DOM), the gatekeeper of SOM decomposition, we explored the transformation processes among the pools of SOM, DOM and microbial biomass carbon (MBC) in soils along a precipitation gradient on dryland grasslands of the Tibetan Plateau. The results revealed that the number and mean weight of DOM molecules significantly decreased, and the soil DOM composition gradually shifted to be more labile along the transition from meadow, steppe, to desert with decreasing precipitation, coinciding with the substantial reduction in SOM. Compared with meadow soils, DOM degradability increased by 8.7% in steppe soils and by 23.4% in desert soils. The ratio of soil MBC to total organic carbon was threefold higher in desert than in meadow, and positively correlated with DOM degradability, indicating that labile DOM accelerated microbial growth and SOM decomposition in desert soils. Structural equation model and correlation analyses demonstrated that the DOM degradability was primarily controlled by soil dissolved nitrogen and soil organic C and soil DOC/DN ratio. Synthesis and application. This study at a molecular level provides a novel insight into the important role of the degradability of dissolved organic matter in carbon accumulation in dryland soils with consistently low organic matter storage. The findings will inform better global managements of soil organic matter under consideration of both food security and climate change.

Abiotic solubilization of soil organic matter, a less-seen aspect of dissolved organic matter production

The role of biodegradation and photo-oxidation in the transformation of terrigenous organic matter

Low cost lime‐based waste materials have recently been used to immobilize metals in contaminated soils. This study was conducted to evaluate the effects of oyster shells and eggshells as lime‐based waste materials on immobilization of cadmium (Cd) and lead (Pb) in contaminated soil, as well as their effects on metal availability to maize plants (Zea mays L.). Oyster shells and eggshells were applied to soils at 1 and 5% w/w, after which they were subject to 420 days of incubation. The toxicity characteristic leaching procedure (TCLP) test was employed to determine the mobility of Cd and Pb in soils. The results showed that the addition of waste materials effectively reduced the metal mobility as indicated by the decrease in the concentration of TCLP‐extractable Cd and Pb, and this was mainly due to significant increases in soil pH (from 6.74 in untreated soil to 7.85–8.13 in treated soil). A sequential extraction indicated that the addition of such alkaline wastes induced a significant decline in the concentration of Cd in the exchangeable fraction (from 23.64% in untreated soil to 1.90–3.81% in treated soil), but it increased the concentration of Cd in the carbonate fraction (from 19.59% in untreated soil to 36.66–46.36% in treated soil). In the case of Pb, the exchangeable fraction was also reduced (from 0.67% in untreated soil to 0.00–0.01% in treated soil), and the fraction of Pb bound to carbonate was slightly increased (from 16.61% in untreated soil to 16.41–18.25% in treated soil). Phytoavailability tests indicated that the metal concentrations in the shoots of maize plant were reduced by 63.39–77.29% for Cd and by 47.34–75.95% for Pb in the amended soils, with no significant differences being observed for the amendment types and the application rates. Overall, these results indicate that oyster shells and eggshells can be used as low cost lime‐based amendments for immobilizing Cd and Pb in contaminated soils.

Reactive primary and secondary minerals play a critical role in the transformation and stabilization of organic matter (OM) in soil, a critical aspect that has been largely overlooked in existing literature. In this regard, we propose a new model known as the “reactive mineral sink” (RMS) to illustrate three primary mechanisms through which these minerals drive the bioprocessing, transformation, transport and stabilization of OM in soil. Firstly, from a biological perspective, reactive minerals influence enzymatic and microbial OM processing through binding enzymatic proteins or influencing the structure of microbial communities. Secondly, from a chemical standpoint, these minerals have the capacity to adsorb OM and/or coprecipitate with it, leading to a more diverse distribution of OM in the soil. This distribution, in turn, triggers OM transformation through chemical catalysis and redox reactions. Thirdly, on a physical level, reactive minerals have a substantial impact on soil architecture, aggregate dynamics, porosity development, and hydrological processes. These physical changes then affect the transport, reprocessing and stabilization of OM. The RMS model provides a conceptual framework that underscores the fundamental role of reactive minerals in driving the dynamics of OM and carbon (C) sequestration in natural soil. Furthermore, it promotes the restoration of soil biogeochemical processes and ecological resilience. We advocate for the implementation of strategies based on the RMS model to enhance the sequestration of organic C in soils for the purposes of rejuvenating soil health and mitigating CO2 emission.

Stabilization of dissolved organic matter by sorption to the mineral soil