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Biochar has gained significant attention in agricultural and environmental research over the last two decades. This comprehensive review evaluates the effects of biochar on soil organic carbon (SOC), emission of non-CO 2 greenhouse gases, and crop yield, including related mechanisms and major influencing factors. The impacts of biochar on SOC, methane and nitrous oxide emissions, and crop yield are controlled by biochar and soil properties and management practices. High-temperature biochar produced from lignin-rich feedstocks may decrease methane and nitrous oxide emissions in acidic soils and strengthen long-term carbon sequestration due to its stable aromatic structure. In contrast, low-temperature biochar from manure may increase crop yield in low-fertility soils. Applying biochar to farmlands in China can increase SOC content by 1.9 Pg C and reduce methane and nitrous oxide emissions by 25 and 20 Mt CO 2 -eq year −1 , respectively, while increasing crop yields by 19%. Despite the increasing evidence of the positive effects of biochar, future research needs to explore the potential factors that could weaken or hinder its capacity to address climate change and secure crop production. We conclude that biochar is not a universal solution for global cropland; however, targeted applications in fields, landscapes, or regional scales, especially in low fertility and sandy soils, could realize the benefits of biochar as a climate-smart measure. Highlights The findings of research on biochar's effects on soil C sequestration, GHG mitigation, and crop production were summarized. The factors influencing the impact of biochar on soil functioning were reviewed. The effects of biochar on soil C sequestration and GHG mitigation in farmlands of China were quantified. Graphical Abstract

Global warming is accelerating glacial retreat and vegetation succession in young developing soils. Soil microbial communities interact with plants, and they are key to vegetation succession, but the specific microbial groups that control plant‒soil feedback (PSF) are unclear. Vegetation and microbial successions were investigated in young soils after a glacial retreat in the Gongga Mountains. The direction and intensity of PSF were quantified by comparing the biomass of pioneer plants in early- (5-10 years), mid- (30-40 years) and late- (80-100 years) succession soils. The plant‒soil interactions shifted from negative feedback between plants and microorganisms in the early stages to positive feedback in the middle and late stages of soil development in the glacial retreat area. In early-stage soils, microorganisms nearly impaired the growth of pioneer plants present in all successional stages, showing negative biotic legacies. PSF and biotic legacies facilitated plant competitiveness. Poplar trees, which have strong competitive abilities, grow well in the soils of all succession stages because their rapid growth and rapid retrieval of soil nutrients prevents the invasion of other pioneer plants. The strongest drivers of soil legacy changed i) from saprophytic fungal specialists during the early stage, ii) to generalists and specialists of bacteria and arbuscular mycorrhizal fungi (AMF) in the middle stage, iii) to ectomycorrhizal fungal specialists in the late stage. The turnover of AMF and fungal pathogens across successional stages contributed to PSF. The effects of biotic legacies and microbial turnover accelerated primary succession and intensified PSF in the young soils of the glacial retreat area.

Paddy field abandonment has become a major concern in China, particularly among traditional rice-cultivation regions. Abandonment results in the alteration

Phosphorus (P) mobilization by microorganisms influences root morphological and exudation traits related to crop nutrient acquisition. However, the dynamics of root P acquisition strategies in response to P immobilization followed by release that accompany microbial growth and decay remain unclear. We evaluated root–microbe interactions governing P uptake by Brassica chinensis and Solanum lycopersicum grown for 56 days in low-P (10 mg P kg−1) and high-P (100 mg P kg−1) soil with or without straw addition. Straw addition increased the growth of bacteria and fungi (including microorganisms harbouring the phoD, phoC and pqqC genes) and enhanced the microbial P pool. The high microbial abundance 29 days after straw addition was associated with short roots and weak carboxylate exudation in B. chinensis and S. lycopersicum regardless of P fertilization. Phosphate-solubilizing microbes harbouring the phoD, phoC and pqqC genes together with microbial P release increased plant-available soil P, influencing variation in root P-acquisition strategies between the species. The high level of CaCl2-extractable P due to P release upon microbial decay and phosphate-solubilizing microorganisms was associated with short roots and weak carboxylate exudation in B. chinensis, but underpinned crop nutrient acquisition in low-P soil amended with straw after day 40. S. lycopersicum increased root elongation in response to mobilization of microbial P after day 44. The long roots coupled with the increased P availability due to microbial P release elicited large P acquisition by S. lycopersicum in straw-amended, low-P soil. By contrast, microbial abundance and microbially mediated P availability had little influence on root P-acquisition strategies in B. chinensis and S. lycopersicum in high-P, straw-amended soil. This study highlighted the importance of temporally varying root–microbe interactions in influencing crop P acquisition in low-P, straw-amended soil.

Chinese milk vetch (CMV) is widely used as a green manure due to its efficiency for nutrient preservation and soil fertility improvement, as well for providing more organic substances for soil organisms. Detritus micro-food web drives carbon and nutrient cycling and maintains ecosystem productivity. The effects of CMV amendment on detritus micro-food web were investigated in a six-year field experiment. Compared to the mineral fertilizers (NPK) alone, the fertilizers combined with CMV (NPK + CMV) increased soil organic carbon content, and thus enhanced the biomass of microorganisms, as well as protozoa abundance, while did not change micro-feeding nematode abundance (MN). This indicates a substrate-driven bottom-up effect. However, the green manure application increased omnivores-predators (OP) abundance and the OP/MN ratio, suggesting a top-down effect. Higher F/B ratio indicates a shift of the food web energy structure towards to fungi-dominated channel in the manured soil. Here, the green manure amendment should be encouraged to improve soil detritus micro-food web.

Sustainable crop production in a warming climate requires land management strategies that support plant-soil-microbe interactions to optimize nitrogen (N) availability. Here, we investigate the interacting effects of 10 years' experimental warming and management (conservation vs. conventional agriculture) on wheat N acquisition using in situ ¹⁵N-labeling, root metabolomics and microbial metagenomics. We find that warming amplifies the positive effects on wheat nitrate uptake by 25% in conservation agriculture compared to conventional agriculture, while alleviating microbial competition for N. Additionally, warming increases soil gross N mineralization and nitrification rates by 191% and 159%, but decreases microbial immobilization by 24% in conservation agriculture. Concurrently, microbial genes for mineralization and nitrification are enriched, while those for N immobilization and nitrate reduction are reduced under conservation agriculture with warming. These shifts are driven by alterations in root primary and secondary metabolites, which reshape N-cycling microbial functional niches and optimize multiple microbial N processes beyond mere organic N mining. This reconfiguration increases carbon-nitrogen exchange efficiency, enabling wheat to outcompete soil microorganisms for N. Collectively, our findings suggest that conservation agriculture enhances plant N acquisition by strengthening plant-soil-microbe interactions under climate change, providing a sustainable strategy for future food security.

Microbial biomass turnover and the associated recycling of carbon (Cmic), nitrogen (Nmic) and phosphorus (Pmic) depend on their stoichiometric relationships and plays a pivotal role for soil fertility. This study examines the responses of Cmic, Nmic, Pmic, the microbial respiration rate (CO2 efflux), and the total DNA content to C and nutrient addition in forest soils with very low (Low-P) and high P (High-P) contents. Both the Low-P and High-P soils were treated with a low and high level of C, N and P (5% and 200% of Cmic, Nmic and Pmic). Phosphorus (33P) was added before the addition of C (14C) and N (15N) to investigate the potential P limitation. We hypothesized two modes of microbial biomass C and nutrient turnover: 1) maintenance through intracellular metabolisms and/or 2) microbial growth and death through necromass reutilization. In Low-P soil, the 2-day-sooner increase of Cmic and Pmic compared to the increase of CO2 efflux and DNA content after high CN input showed the rapid initial uptake of C and limiting nutrients into microbial cells. It also demonstrated a lag period before microbial growth commenced. In High-P soil, however, the CO2 efflux and DNA content increased simultaneously with increases in microbial biomass, reflecting the microbial capacity for immediate growth. Afterwards, CO2 efflux and DNA content dropped to the level before CNP addition, with a decline of Cmic and Pmic in Low-P soil and a decline of Nmic in High-P soil, suggesting a C and P limitation in Low-P soil and N limitation in High-P soil. Under low CNP addition, the microorganisms in High-P soil are ready to grow, while those in Low-P soil are mainly in maintenance mode. The microorganisms under maintenance in low-P soil can switch to growth/death mode after removing the nutrient limitation. High CNP input caused a non-homeostatic response of Cmic: Nmic: Pmic stoichiometry from 691:105:1 to 33:1:1 in Low-P soil, mainly resulting from a higher storage of the limiting elements (C and P) in microbial biomass. The ratio remained stable under low CNP addition due to the endogenous metabolism of C and nutrient at maintenance. The C and nutrient were turnovered much faster by microorganisms in the growth/death mode, confirming a key principle of ecology: the stronger the limitation by an element, the more efficiently that element is retained within an organism, and the more intensively it is reused. The triple labeling approach linked with Cmic: Nmic: Pmic stoichiometry helped to identify the dominant maintenance and growth/death modes of microbial biomass CNP turnover in nutrient-limited and -unlimited soil.

No abstract is provided for this article.

Oil tea (Camellia spp.) is endemic to the hilly regions in the subtropics. Camellia yuhsienensis is resistant to diseases such as anthracnose and root rot, while Camellia oleifera is a high-yield species but susceptible to these diseases. We hypothesize that differences in the rhizosphere microbial communities and functions will elucidate the resistance mechanisms of these species. We used high-throughput sequencing over four seasons to characterize the rhizosphere microbiome of C. oleifera (Rhizo-Sus) and C. yuhsienensis (Rhizo-Res) and of the bulk soil control (BulkS). In Rhizo-Res, bacterial richness and diversity (Shannon index) in autumn and winter were both higher than that in Rhizo-Sus. In Rhizo-Res, fungal richness in autumn and winter and diversity in summer, autumn, and winter were higher than that in Rhizo-Sus. The seasonal variations in bacterial community structure were different, while that of fungal community structure were similar between Rhizo-Res and Rhizo-Sus. Gram-positive, facultatively anaerobic, and stress-tolerant bacteria were the dominant groups in Rhizo-Sus, while Gram-negative bacteria were the dominant group in Rhizo-Res. The significant differences in bacterial and fungal functions between Rhizo-Sus and Rhizo-Res were as follows: (1) in Rhizo-Sus, there were three bacterial and four fungal groups with plant growth promoting potentials, such as Brevibacterium epidermidis and Oidiodendron maius, and one bacterium and three fungi with pathogenic potentials, such as Gryllotalpicola sp. and Cyphellophora sessilis; (2) in Rhizo-Res, there were also three bacteria and four fungal groups with plant-growth-promoting potentials (e.g., Acinetobacter lwoffii and Cenococcum geophilum) but only one phytopathogen (Schizophyllum commune). In summary, the rhizosphere microbiome of disease-resistant C. yuhsienensis is characterized by a higher richness and diversity of microbial communities, more symbiotic fungal communities, and fewer pathogens compared to the rhizosphere of high-yield but disease-susceptible C. oleifera.

Reforestation with native species and resulting understory succession can exert important influences on soil organic matter (SOM) storage and chemistry, but a mechanistic understanding of these effects is lacking. We studied different aged Masson pine (Pinus massoniana L.) plantations with and without the understory fern, Dicranopteris dichotoma (Thunb.) Berhn., in subtropical China to assess how SOM over a 30 year sequence of pine growth and fern expansion. To do this, we measured total SOM, lignin-derived phenols, soil carbon (total C and 13C), soil nitrogen (total N and 15N), and soil microbial community composition via phospholipid fatty acid (PLFA) analyses. We found that the accumulation of newly-formed SOM outweighed decomposition of old SOM, with the majority of this increase being derived from fern detrital inputs. Where ferns were present, ferns contributed 54–61% of total soil C storage in surface (0–10 cm depth) soils, which was 62–91% higher than pre-reforestation soil C storage. We found that the abundance of lignin-derived compounds was lower in fern dominated soils, perhaps because soils under ferns supported more soil fungi, the primary decomposers of the lignin in soil. Fern soils also showed higher ratios of syringyls to vanillyls and decreased δ13C values, an indicator that ferns altered the composition of SOM at the molecular level while contributing significantly to SOM accumulation. Reforestation especially when accompanied by fern expansion also improved soil N and phosphorus (P) status, with observed declines in soil δ15N in fern dominated soils aligning with increased nutrient retention and observed increases in soil C storage. Our study highlights the potentially important role of understory ferns in mediating SOM chemistry and soil C storage during ecosystem recovery.

Agricultural soils receive large amounts of anthropogenic nitrogen (N), which directly and indirectly affect soil organic matter (SOM) stocks and CO2 fluxe

<p>Drying-rewetting cycles (DRC) affect litter and soil organic carbon (SOC) decomposition and mineralization, especially in Mediterranean ecosystems. Global climate change is expected to increase drought periods as well as heavy precipitation frequency, which in turn will increase soils DRC. However, the effects of DRC on the functioning of microbial communities and dynamics of dissolved organic carbon (DOC) remain elusive. Here, we investigate the effects of climate-change on organic carbon turnover rates based on a DRC approach.</p><p>Composite <em>dehesa</em> soil samples (0-10 cm) (Pozoblanco, Córdoba, Spain) were taken from three forced climatic treatment plots (W: warming (heat increase); D: drought (water restriction); C: Control). The plots were installed 4 yrs ago under two distinct habitats: evergreen oak canopy (designated as ‘tree’) and in the open pasture (‘open’). The soil samples were incubated for 26-days at a constant moisture (40% of water-holding capacity, WHC) and labelled <sup>14</sup>C-glucose (150 % of C from microbial biomass). Afterwards, to simulate drought in nature, ¾ of each sample were dried and further four rewetting treatments were established: 1) constant-moisture at 40% WHC, 2) slow DRC with 5-days water addition to 40% WHC, 3) fast DRC with all water added during the first day of the experiment, and 4) dry DRC with 7-days drying and no rewetting. Following DRC period, there was an extended incubation (26 d in total), where samples were taken at three times after rewetting (4, 7 and 26 days) for further analyses. Total and <sup>14</sup>C-glucose-derived dissolved organic carbon (DOC), microbial biomass (MBC), C, N and P related enzymatic activities, and other parameters of microbial growth were measured. During the incubation period total and <sup>14</sup>C-CO<sub>2 </sub>were also monitored.</p><p>The results obtained and the discussion of the DRC effects detected and main threads regarding climate change in Mediterranean <em>dehesa</em> agroforestal system such as increasing temperatures and drought events on microbial biomass, respiration and C turnover, will be detailed. Changes in DRC can alter organic C mineralization, in turn such effect can strongly depend on previous field-induced conditions in Mediterranean savannas. In addition, our results will help to understand the responses of soil MBC and DOC to DWC in Mediterranean ecosystems and could improve the prediction of CO<sub>2</sub> emission under a changing environment in the future.</p><p><strong>Acknowledgment:</strong> EU-EJC 2<sup>nd</sup> Call Projects MIXROOT-C and MAXROOT-C. L.M. San Emeterio thanks Ministerio de Ciencia Innovación y Universidades (MICIU) FPI research grant (BES-2017-07968) and the German Academic Exchange Service (DAAD) for funding. A.M. Carmona, M.D. Hidalgo, P. Campos and K. Schmidt are acknowledged for technical assistance.</p>