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Biocrusts are critical biological components of drylands and play an important role in soil carbon (C) cycling. However, the effect of biocrusts on soil CO 2 exchange across global gradients of temperature and moisture is poorly understood. Moreover, their response to climate change remains highly uncertain. Bayesian hierarchical meta-analyses were performed on 47 published studies to quantify the impact of biocrusts on net soil exchange (NSE) of carbon- the difference between respiration and photosynthesis. Meta-analyses were also used on 23 studies to examine the effects of experimental warming on NSE in biocrusts. Meta-regressions further explored the thermal and wetness sensitivities of biocrust NSE and potential adaptation of biocrust responses to climate change. The development of biocrusts in dryland soils significantly increased NSE by 66.5 [22.2, 112.2] g C m−2yr−1, despite seasonal fluctuations, indicating a net loss of carbon to the atmosphere. Experimental warming, on average, increased biocrust NSE by 22.9 [-0.1, 40.8] g C m−2yr−1 per °C. However, across the spatial climate gradient, aridity limited the effects of warming, while high temperature decreased the thermal sensitivity of biocrust NSE, thus supporting the thermal adaptation of biocrusts. These results emphasize the critical role of biocrusts in modulating soil carbon exchange in response to climate warming across drylands, with particularly high thermal sensitivity in cool and moist regions. This highlights the need to incorporate biocrusts into global carbon budgets and models for a comprehensive understanding of their impact on the carbon cycle. • Biocrusts increase soil CO2 efflux by 66.5 g C m-2yr-1 in drylands worldwide. • Climate warming increases CO2 efflux in biocrust soil by 22.9 g C m-2yr-1 per °C. • Thermal adaptation reduces thermal sensitivity of biocrust carbon efflux. [ABSTRACT FROM AUTHOR]

Straw return has been widely used to increase SOC storage globally, but such C accrual is frequently constrained by other nutrients which are needed to maintain soil ecological stoichiometry balance. However, the impact of fertilization on SOC stocks under straw return remains unexplored at broad spatial scales. We constructed a national-scale dataset to explore the fertilization effect on SOC stocks (0–20 cm) under straw return and identified the dominant drivers and pathways in upland and paddy systems across China. On average, straw return enhanced SOC stocks and sequestration rate by 1.74 Mg C ha−1 and 0.27 Mg C ha−1 y−1 relative to straw removal (CK). Yet, straw co-applied with nutrient enrichment further increased SOC stock and sequestration rate by 0.81 Mg C ha−1 and 0.12 Mg C ha−1 y−1, compared to straw return only. These results indicated a general nutrient limitation on SOC sequestration under straw return. The fertilization effect on SOC stocks varied greatly across nutrient enrichment types and cropping systems, with greater soil C gain occurring under balanced fertilization (i.e., NPK) and more in uplands than in paddy fields. In the uplands, SOC sequestration with fertilization was positively correlated with increased belowground biomass. In paddy fields, however, improved biomass with fertilization did not translate into increased SOC accumulation, owing to the co-limitations of soil acidification and initial SOC. We further clarified the differential pathways by which chemical fertilization altered SOC stocks, i.e., productivity pathway in uplands but decomposition pathway in paddy fields. These mechanistic insights on the fertilization control over SOC can help us optimize the context-specific integrated straw and fertilization management strategies for greater SOC sequestration, and also inform the development of soil C projections by incorporating fertilization management into biogeochemical models in agroecosystems. [Display omitted] • Nutrient limitation of SOC stocks under straw return was assessed across China. • Balanced fertilization promoted SOC sequestration under straw return. • SOC gain due to fertilization was much higher in uplands than in paddy fields. • Improved productivity with fertilization did not translate into increased SOC in paddies. • Fertilization modified SOC stocks via differential pathways across cropping systems. [ABSTRACT FROM AUTHOR]

Biochar amendment can alter native soil organic carbon (SOC) mineralization via the priming effect (PE); however, its direction, intensity, and controls over a broad geographic scale are not clear, undermining the predictions of SOC dynamics as impacted by biochar inputs. Here, we synthesized 5,720 measurements of CO 2 effluxes from 329 soil samples with and without 13C/14C labeled-biochar additions to quantify the spatial patterns and temporal dynamics of the PE and assess the underlying environmental drivers. Across all data, biochar amendment has led to a slight but significant positive PE (i.e., 56 mg C kg−1 soil), with stronger PEs in natural ecosystems than in agricultural soils. Negative PE occurred in the short term (i.e., <9 days after biochar addition) and subsequently shifted to a strong positive PE (i.e., 364–966 days) and remained in an insignificant PE thereafter (i.e., >1450 days). Notably, soils from rainfed croplands had the lowest negative PE (−28.76 mg C kg−1 soil). Grass-derived biochar produced at a low pyrolysis temperature (i.e., 300–400 °C) induced the strongest positive PE (244 mg C kg−1 soil). The results of our structural equation model indicated that biochar pyrolysis temperature and soil C:N ratio had the largest negative and direct association with biochar-induced PE, whereas incubation temperature and microbial biomass C exerted the greatest positive and direct effects on the PE. Variance partitioning analyses further revealed that both biochar and soil properties together accounted for 73% of the explained variance in biochar-induced PE. Overall, these results add to our understanding of biochar-induced SOC priming as impacted by different land-use types, soils, and biochar properties. The amendment of wood-derived biochars produced at high pyrolysis temperatures (>500 °C) to rainfed croplands could serve as a promising strategy to achieve maximum soil C sequestration. [Display omitted] • Biochar amendment led to a significant positive priming effect (PE). • Soils from natural ecosystems exhibited stronger PEs than agricultural soils. • Grass-derived biochar produced higher PEs than wood- and crop-derived biochar. • The PEs decreased as biochar pyrolysis temperature and soil C:N ratio increased. • The PEs increased as incubation temperature and microbial biomass C increased. [ABSTRACT FROM AUTHOR]

Altered drying-rewetting patterns due to climate change may affect soil nitrogen (N) and phosphorus (P) cycling in terrestrial ecosystems. The responses of soil N and P cycling to drying and rewetting cycles can vary with drying-rewetting patterns, experimental methods, ecosystems, and soil types, thus making a synthesis of these studies necessary for understanding mechanisms and predicting future responses to climate change. Here, we compiled data of 1882 observations from 79 studies for a meta-analysis of the responses of soil N and P pools and fluxes to drying and rewetting and how these responses are modified by experimental conditions. Results showed that 1) experimental drying increased NH 4 +, extractable organic nitrogen (EON), and available P in the soil significantly by 22, 27, and 72%, respectively. In contrast, soil NO 3 −, enzymatic activities, microbial biomass, net nitrification, and N 2 O emissions significantly decreased by 37, 13–21, 21–28, 39, and 93%, respectively. 2) Upon rewetting of dried soil, soil EON, extractable organic phosphorus (EOP), net N mineralization, nitrification, phosphatase activity, dissolved organic N leaching, dissolved inorganic P leaching, and N 2 O emissions significantly increased by 59, 27, 19, 15, 12, 60, 116, and 218%, respectively, while soil NO 3 − and NO 3 − leaching significantly decreased by 9 and 74%, respectively. Soil microbial N and P as well as enzymatic activities recovered from drought during the rewetting phase. The mean effect sizes of drying and rewetting generally increased with drying intensity, which was probably also the main reason for greater effect sizes observed in laboratory than in field experiments. Our meta-analysis showed stronger positive responses of available P to drying and rewetting than mineral N, which agreed with greater effect sizes on P than on N leaching. This suggests that drying and rewetting induce an imbalance between N and P, which was more pronounced in soils from forests than from agricultural systems. Overall, these results imply that the expected increase in the frequency and intensity of droughts potentially decouples the cycling of P and N, with consequences for nutrient leaching and the supply of plants and microorganisms with these nutrients. • We compiled data of 1882 observations and conducted a meta-analysis. • Drying and rewetting significantly affected soil N and P pools and fluxes. • Soil microorganisms could recover from drought on rewetting. • There were higher effect sizes of N and P in laboratory than in field. • The responses of soil N and P also depended on soil and land use types. [ABSTRACT FROM AUTHOR]