Trade-offs in carbon-degrading enzyme activities limit long-term soil carbon sequestration with biochar addition

Effects of organic material addition on carbon cycling and soil fertility in paddy soil

Soil organic carbon (SOC) plays a crucial role in plant nutrient supply and soil physical, chemical, and biological function regulation. Factors such as climate change, human activities, and farm management practices can adversely affect SOC. Here, paddy soil in the double-cropping rice area of Hunan Province of China was cultured with eight kinds of organic materials, and the effects of organic fertilization on SOC content, humus content, and soil carbon sequestration efficiency were analyzed. The addition of straw, green fertilizer, and organic fertilizer had a positive influence on active SOC content. Straw addition had the most prolonged impact on soil microbial biomass carbon content, and green manure had the most rapid influence on soil dissolved organic carbon content. All organic materials enhanced the soil humus content; furthermore, the addition of organic fertilizers significantly improved the carbon sequestration efficiency among all treatments. The results of controlled culture experiments in paddy soils established that applying organic materials can increase SOC content, active carbon components, soil humus content, and the carbon fixation rate, thereby improving soil fertility. This study provides a theoretical basis for the optimal fertilization of paddy soil with organic material addition in China.

Effects of biochar and nitrogen fertilizer on microbial communities, CO2 emissions, and organic carbon content in soil

This study examined the effects of biochar and nitrogen fertilizer application on CO2 emissions, microbial communities, and soil organic carbon (SOC) in irrigated wheat fields through a 3-year field experiment. Eight treatment groups were established for this study: (1) CK, without fertilizer or biochar, (2) N1 group, with nitrogen fertilizer application (300 kg/ha), (3) B group, with biochar application (20 t/ha), (4) BN1 group, with nitrogen fertilizer and biochar application, (5) N2, with a 15% reduction in nitrogen fertilizer (255 kg/ha), (6) BN2, with a 15% reduction in nitrogen fertilizer + biochar. (7) N3, with a 30% reduction in nitrogen fertilizer (210 kg/ha); and (8) BN3, with a 30% reduction in nitrogen fertilizer + biochar. The results revealed an increase in active organic carbon (AOC) and SOC contents of soil after the addition of biochar and N fertilizer, particularly with their combined application. In the BN2 treatment, SOC and AOC contents reached 27.48 g/kg and 1.47 g/kg, representing increases of 3.04% and 30.91%, respectively, compared to N1. In comparison to CK, cumulative CO2 emissions increased by 9-48% with the addition of both biochar and nitrogen fertilizer, possibly due to biochar's influence on the composition and functional diversity of soil microbial communities. The functional diversity of soil microbes in the BN1 group differed significantly from that in CK (p < 0.01). In the B group, soil microbial attributes were lower than those in BN1, BN2, and BN3 groups. Furthermore, the bulk density of biochar-amended soil was 0.19 g/cm3 lower than that of untreated soil in CK. Overall, the combination of biochar application and a nitrogen dose of 255 kg/ha emerged as the most effective strategy for irrigated wheat fields in northern Xinjiang, enhancing SOC content while reducing carbon emissions. However, further research is required to assess the long-term effects of this approach on soil health and sustainability.

Altered precipitation affects soil enzyme activity related to nitrogen and phosphorous but not carbon cycling: A meta-analysis

Altered precipitation significantly influences soil function in terrestrial ecosystems. As a bioindicator of soil function, soil extracellular enzyme activity (EEA) plays a crucial role in mediating ecosystem responses to altered precipitation. However, the global patterns and regulatory mechanisms of altered precipitation impacts on soil EEAs remain unclear. We conducted hierarchical mixed-effects meta-analyses to explore the responses and regulators of carbon, nitrogen, phosphorus hydrolytic EEAs, and carbon oxidative EEA to changes in precipitation, using the largest dataset to date, comprising 1185 observations of 14 soil EEAs from 73 publications. The results indicated that soil nitrogen hydrolytic EEA increased by 14.3% under increased precipitation, while phosphorus hydrolytic EEA decreased by 8.8% under decreased precipitation, showing higher sensitivity to altered precipitation compared to carbon-degrading EEAs. These responses varied across ecosystem types and depended on the magnitude of precipitation manipulation (MPM). Specially, decreased precipitation significantly reduced phosphorus hydrolytic EEA in forests, while increased precipitation enhanced nitrogen hydrolytic EEA in grasslands. Furthermore, these effects were linearly correlated with MPM, deviating from the expected nonlinear double asymmetric model. The response of soil hydrolytic EEAs was predominately regulated by soil water content, organic carbon, and microbial biomass. These findings underscore the higher sensitivity of nitrogen and phosphorus cycling EEAs to altered precipitation compared to carbon cycling EEAs and extend the application of the double asymmetric model for understanding soil EEAs' responses to precipitation changes. This synthesis provides essential insights for predicting biogeochemical cycling and improving ecosystem models to evaluate ecosystem functions under altered precipitation.

Fate of Fertilizer Nitrogen in the Field 2 Years After Biochar Application

This study aimed to clarify the scientific quantification of fertilizer nitrogen (N) uptake and utilization, its destination, and its residual distribution in the soil at a depth of 0-30 cm after biochar application using 15N tracer technology. The purpose was to provide a theoretical basis for developing a scientific application strategy for N fertilizer and biochar in irrigated farmland areas. Two levels of N fertilizer application were set up using the 15N labeling method in microareas of large fields: the regular amount of N fertilizer (N1: 300 kg·ha-1) and a reduction of N fertilizer by 15% (N2: 255 kg·ha-1). Further, three levels of biochar application were set up: no biochar (B0: 0 kg·ha-1), a low amount of biochar (B1: 10 × 103 kg·ha-1), and a medium amount of biochar (B2: 20 × 103 kg·ha-1). The tested biochar was derived from corn stover (maize straw). The natural abundance of 15N-labeled fertilizer N, the total N content of each aboveground organ, and the total N content of soil at a depth of 0-30 cm in a spring wheat field at maturity were determined, and the yield was measured in the corresponding plots. The proportion of 15N-labeled fertilizer N uptake by each organ of spring wheat and the soil N uptake was 20.60-35.32% and more than 64.68%, respectively. Moreover, the proportion of soil N uptake showed a decreasing trend with an increase in biochar application. The spring wheat N uptake and utilization rate, the residue rate in the soil at a depth of 0-30 cm, the total utilization rate, and the rate of loss of 15N-labeled fertilizer N ranged from 15.21% to 29.61%, 23.33% to 28.93%, 38.54% to 58.54%, and 41.46% to 61.46%, respectively. The spring wheat N fertilizer utilization rate, fertilizer N residue rate in soil, and total fertilizer N utilization rate all increased gradually with an increase in biochar application, except for the N loss rate, which decreased gradually. When N fertilizer reduction was combined with medium biochar (B2N2), the yield of spring wheat significantly improved, mainly due to an increase in the number of grains in spikes. Under this treatment, the number of grains in spikes of spring wheat was 41.9, and the yield reached 7075.54 kg·ha-1, which was an increase of 9.69-28.25% and 10.91-25.35%, respectively, compared with other treatments. Yield increased by up to 25.35%, and nitrogen loss decreased by 48.24% under the B2N2 treatment. Biochar application could promote the amount and proportion of fertilizer N uptake in various organs of spring wheat as well as in the soil at a depth of 0-30 cm. In this study, a 15% reduction in N fertilizer (255 kg·ha-1) combined with 20 × 103 kg·ha-1 biochar application initially helped achieve the goal of increasing spring wheat yield and N fertilizer uptake, as well as improving fertilizer N utilization, providing an optimal scientific application strategy for N fertilizer and biochar in the farmland of the irrigation area. These results substantiate the hypothesis that biochar application enhances spring wheat (Triticum aestivum L.) assimilation of fertilizer-derived nitrogen (15N) while concomitantly improving fertilizer nitrogen retention in the soil matrix, which could provide a sustainable framework for nitrogen management in irrigated farmlands.

Potential of biochar to restoration of microbial biomass and enzymatic activity in a highly degraded semiarid soil

Biochar is an effective material for enhancing soil ecosystem services. However, the specific impacts of biochar on microbial indicators, particularly in degraded soils, remain poorly understood. This study aimed to evaluate the effects of biochar produced from cashew residues and sewage sludge, in a highly degraded soil, on microbial indicators. We analyzed soil chemical composition and microbial biomass C and N, enzyme activity, and stoichiometry. Cashew biochar increased soil respiration, indicating a higher availability of C to microorganisms compared to sewage sludge biochar and a better adaptation of soil microbial communities to C-rich organic material obtained from a native plant. Although the biochar differentially impacted microbial biomass C, both significantly increased N in the microbial biomass. Arylsulphatase activity did not respond to biochar application, while β-glucosidase, urease, and phosphatases showed significant changes with biochar treatments. Importantly, stoichiometry and vector analysis revealed that both types of biochar increased P limitation for soil microbes. Conversely, both biochar alleviated C and N limitations for the soil microbes. Thus, biochar applications in highly degraded soils should be supplemented with external P sources to maintain soil functions, mainly for cashew residues. Our results provide evidence that biochar can restore soil biological properties and enhance the availability of C and N to microorganisms. These findings have significant implications for restoration practices in degraded lands of semiarid regions.

Food waste biochar for sustainable agricultural use: Effects on soil enzymes, microbial community, lettuce, and earthworms

This study investigates the effects of food waste biochar (FWB) on the biological properties of soil, including the microbial community structure, enzyme activities, lettuce growth, and earthworm ecotoxicity. This holistic assessment of various soil organisms was used to assess the potential of FWB as a soil amendment strategy. Pot experiments were carried out over a 28-d period using various FWB concentrations in soil (0-3% w/w). The presence of FWB enhanced the activity of alkaline phosphatase and beta-glucosidase in proportion to the FWB concentration. Similarly, the dehydrogenase activity after 28 d was positively correlated with the FWB concentration. Notably, the application of FWB improved the bacterial diversity in the soil, particularly among hydrocarbonoclastic bacteria, while also prompting a shift in the fungal community structure at the class level. Measures of lettuce growth, including total fresh weight, shoot length, and leaf number, also generally improved with the addition of FWB, particularly at higher concentrations. Importantly, FWB did not adversely affect the survival or weight of earthworms. Collectively, these findings suggest that FWB can enhance soil microbial enzyme activity and support plant growth-promoting rhizobacteria, potentially leading to increased crop yields. This highlights the potential of FWB as an eco-friendly soil amendment strategy.

Effect of long-term radish (Raphanus sativus var. sativus) monoculture practice on physiological variability of microorganisms in cultivated soil

Long-term monoculture may affect soil environment biodiversity. An example of such a plant is radish (Raphanus sativus var. sativus), an economically important crop in Poland, a quick-growing vegetable with intensified harvest throughout the season. The aim of this study was to determine changes in biodiversity of soil under radish cultivation and to compare the research methods applied. The monoculture practice affected soil pH, but the organic carbon content remained stable. 16S RNA-seq analysis revealed changes in soil microbial population, with the dominant phyla Proteobacteria (37.3%), Acidobacteria (19%), and Actinobacteria (16%), and the dominant taxa Gaiella (1.59%), Devosia (1.51%) and Nocardioides (1.43%). These changes have not fully expressed in the number of culturable microorganisms, where only fungal abundance changed significantly. However, the physiological state of microbial cells (λ) indicated that oligotrophs and copiotrophs were in a vegetative (λ > 3.0) state at the beginning of the season and fungi at the end of the year. Changes in the biodiversity of soil microorganisms were visualised using Community Level Physiological Profiling, where an oscillation in Average Well Colour Development (OD560 = 0.78-1.48) was observed in successive months of radish culture, with biodiversity indices (Shannon and Substance richness) remaining similar. The greatest variation in the influence of monoculture practice on soil factors was observed for the soil enzymes activities (for dehydrogenase and peroxidase activities - 0.5 μg TPF/h/g DW and 1.5 μmolPYGL/h/g DW respectively). Alkaline phosphatases predominated among this group of enzymes, and the activity of carbon metabolism enzymes decreased over the season, except for invertases, where an increase in activity of up to 50 μg Glc/h/g DW was observed. All the parameters studied indicated changes in the soil environment. Nevertheless the microbial community remains stable during the whole experiment returning to equilibrium in a quite short time after changing conditions.

A critical review of biochar as an environmental functional material in soil ecosystems for migration and transformation mechanisms and ecological risk assessment

At present, biochar has a large application potential in soil amelioration, pollution remediation, carbon sequestration and emission reduction, and research on the effect of biochar on soil ecology and environment has made positive progress. However, under natural and anthropogenic perturbations, biochar may undergo a series of environmental behaviors such as migratory transformation, mineralization and decomposition, and synergistic transport, thus posing certain potential risks. This paper outlines the multi-interfacial migration pathway of biochar in "air-soil-plant-animal-water", and analyzes the migration process and mechanism at different interfaces during the preparation, transportation and application of biochar. The two stages of the biochar mineralization process (mineralization of easily degradable aliphatic carbon components in the early stage and mineralization of relatively stable aromatic carbon components in the later stage) were described, the self-influencing factors and external environmental factors of biochar mineralization were analyzed, and the mineral stabilization mechanism and positive/negative excitation effects of biochar into the soil were elucidated. The proximity between field natural and artificially simulated aging of biochar were analyzed, and the change of its properties showed a trend of biological aging > chemical aging > physical aging > natural aging, and in order to improve the simulation and prediction, the artificially simulated aging party needs to be changed from a qualitative method to a quantitative method. The technical advantages, application scope and potential drawbacks of different biochar modification methods were compared, and biological modification can create new materials with enhanced environmental application. The stability performance of modified biochar was compared, indicating that raw materials, pyrolysis temperature and modification method were the key factors affecting the stability of biochar. The potential risks to the soil environment from different pollutants carried by biochar were summarized, the levels of pollutants released from biochar in the soil environment were highlighted, and a comprehensive selection of ecological risk assessment methods was suggested in terms of evaluation requirements, data acquisition and operation difficulty. Dynamic tracing of migration decomposition behavior, long-term assessment of pollution remediation effects, and directional design of modified composite biochar materials were proposed as scientific issues worthy of focused attention. The results can provide a certain reference basis for the theoretical research and technological development of biochar.

Biochar addition promotes soil organic carbon sequestration dominantly contributed by macro-aggregates in agricultural ecosystems of China

Soil aggregates play pivotal roles in soil organic carbon (SOC) preservation and climate change. Biochar has been widely applied in agricultural ecosystems to improve soil physicochemical properties. However, the underlying mechanisms of SOC sequestration by soil aggregation with biochar addition are not well understood at a large scale. Here, we conducted a meta-analysis of 2335 pairwise data from 45 studies to explore how soil aggregation sequestrated SOC after biochar addition in agricultural ecosystems of China. Biochar addition markedly enhanced the proportions of macro-aggregates and aggregate stability, and the production of organic binding agents positively facilitated the formation of macro-aggregates and aggregate stability. Soil aggregate-associated organic carbon (OC) indicated a significantly increasement by biochar addition, which was attributed to direct and indirect inputs of OC from biochar and organic residues, respectively. Biochar stimulated SOC sequestration dominantly contributed by macro-aggregates, and it could be interpreted by a greater improvement in proportions and OC protection of macro-aggregates. Furthermore, the SOC sequestration of soil aggregation with biochar addition was regulated by climate conditions (mean annual temperature and precipitation), biochar attributes (biochar C/N ratio and pH), experimental practices (biochar addition level and duration), and agronomic managements (land type, cropping intensity, fertilization condition, and crop type). Collectively, our synthetic analysis emphasized that biochar promoted the SOC sequestration by improving soil aggregation in agricultural ecosystems of China.

Microbial-driven mechanisms for the effects of heavy metals on soil organic carbon storage: A global analysis

Heavy metal (HM) enrichment is closely related to soil organic carbon (SOC) pools in terrestrial ecosystems, which are deeply intertwined with soil microbial processes. However, the influence of HMs on SOC remains contentious in terms of magnitude and direction. A global analysis of 155 publications was conducted to integrate the synergistic responses of SOC and microorganisms to HM enrichment. A significant increase of 13.6 % in SOC content was observed in soils exposed to HMs. The response of SOC to HMs primarily depends on soil properties and habitat conditions, particularly the initial SOC content, mean annual precipitation (MAP), initial soil pH, and mean annual temperature (MAT). The presence of HMs resulted in significant decreases in the activities of key soil enzymes, including 31.9 % for soil dehydrogenase, 24.8 % for β-glucosidase, 35.8 % for invertase, and 24.3 % for cellulose. HMs also exerted inhibitory effects on microbial biomass carbon (MBC) (26.6 %), microbial respiration (MR) (19.7 %), and the bacterial Shannon index (3.13 %) but elevated the microbial metabolic quotient (qCO2) (20.6 %). The HM enrichment-induced changes in SOC exhibited positive correlations with the response of MBC (r = 0.70, p < 0.01) and qCO2 (r = 0.50, p < 0.01), while it was negatively associated with β-glucosidase activity (r = 0.72, p < 0.01) and MR (r = 0.39, p < 0.01). These findings suggest that the increase in SOC storage is mainly attributable to the inhibition of soil enzymes and microorganisms under HM enrichment. Overall, this meta-analysis highlights the habitat-dependent responses of SOC to HM enrichment and provides a comprehensive evaluation of soil carbon dynamics in an HM-rich environment.

Role of Soil Microbiota Enzymes in Soil Health and Activity Changes Depending on Climate Change and the Type of Soil Ecosystem

The extracellular enzymes secreted by soil microorganisms play a pivotal role in the decomposition of organic matter and the global cycles of carbon (C), phosphorus (P), and nitrogen (N), also serving as indicators of soil health and fertility. Current research is extensively analyzing these microbial populations and enzyme activities in diverse soil ecosystems and climatic regions, such as forests, grasslands, tropics, arctic regions and deserts. Climate change, global warming, and intensive agriculture are altering soil enzyme activities. Yet, few reviews have thoroughly explored the key enzymes required for soil fertility and the effects of abiotic factors on their functionality. A comprehensive review is thus essential to better understand the role of soil microbial enzymes in C, P, and N cycles, and their response to climate changes, soil ecosystems, organic farming, and fertilization. Studies indicate that the soil temperature, moisture, water content, pH, substrate availability, and average annual temperature and precipitation significantly impact enzyme activities. Additionally, climate change has shown ambiguous effects on these activities, causing both reductions and enhancements in enzyme catalytic functions.

Bioremediation of PBDEs and heavy metals co-contaminated soil in e-waste dismantling sites by Pseudomonas plecoglossicida assisted with biochar

Biochar-assisted microbial remediation has been proposed as a promising strategy to eliminate environmental pollutants. However, studies on this strategy used in the remediation of persistent organic pollutants and heavy metals co-contaminated soil are lacking, and the effect of the combined incorporation of biochar and inoculant on the assembly, functions, and microbial interactions of soil microbiomes are unclear. Here, we studied 2,2',4,4'-tetrabrominated diphenyl ether (BDE-47) degradation and heavy metal immobilization by and biochar-based bacterial inoculant (BC/PP) in an e-waste contaminated soil, and corresponding microbial regulation mechanisms. Results showed that BC/PP addition was more effective in reducing Cu and Pb availability and degrading BDE-47 than inoculant alone. Notably, BC/PP facilitated bound-residue formation of BDE-47, reducing the ecological risk of residual BDE-47. Meanwhile, microbial carbon metabolism and enzyme activities (related to C-, N-, and P- cycles) were enhanced in soil amended with BC/PP. Importantly, biochar played a crucial role in inoculant colonization, community assembly processes, and microbiome multifunction. In the presence of biochar, positive interactions in co-occurrence networks of the bacterial community were more frequent, and higher network stability and more keystone taxa were observed (including potential degraders). These findings provide a promising strategy for decontaminating complex-polluted environments and recovering soil ecological functions.

Carbon Sequestration Strategies in Soil Using Biochar: Advances, Challenges, and Opportunities

Biochar, a carbon (C)-rich material obtained from the thermochemical conversion of biomass under oxygen-limited environments, has been proposed as one of the most promising materials for C sequestration and climate mitigation in soil. The C sequestration contribution of biochar hinges not only on its fused aromatic structure but also on its abiotic and biotic reactions with soil components across its entire life cycle in the environment. For instance, minerals and microorganisms can deeply participate in the mineralization or complexation of the labile (soluble and easily decomposable) and even recalcitrant fractions of biochar, thereby profoundly affecting C cycling and sequestration in soil. Here we identify five key issues closely related to the application of biochar for C sequestration in soil and review its outstanding advances. Specifically, the terms use of biochar, pyrochar, and hydrochar, the stability of biochar in soil, the effect of biochar on the flux and speciation changes of C in soil, the emission of nitrogen-containing greenhouse gases induced by biochar production and soil application, and the application barriers of biochar in soil are expounded. By elaborating on these critical issues, we discuss the challenges and knowledge gaps that hinder our understanding and application of biochar for C sequestration in soil and provide outlooks for future research directions. We suggest that combining the mechanistic understanding of biochar-to-soil interactions and long-term field studies, while considering the influence of multiple factors and processes, is essential to bridge these knowledge gaps. Further, the standards for biochar production and soil application should be widely implemented, and the threshold values of biochar application in soil should be urgently developed. Also needed are comprehensive and prospective life cycle assessments that are not restricted to soil C sequestration and account for the contributions of contamination remediation, soil quality improvement, and vegetation C sequestration to accurately reflect the total benefits of biochar on C sequestration in soil.