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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]

Tracking photosynthesized carbon (C) allocation into different C pools is crucial for management of C sequestration, especially in agroecosystems. However, the effects of long–term tillage management on allocation and dynamics of recently fixed C in the crop–soil–atmosphere system have been rarely investigated under dryland conditions. Using in–situ 13CO 2 repeat labeling, this study quantified the photosynthesized C input to soil, and assessed the responses of allocation dynamics, microbial utilization, and aggregate protection of newly fixed C in three long–term (>10y) tillage practices (no–till, chisel–till, and plow–till) in a dryland wheat cropping system. Regardless of tillage practice, the 13C included into the shoots, roots, soil respiration, rhizosphere, and bulk soils accounted for 46–64%, 4.6–6.0%, 17–38%, 6.4–11.7%, and 2.8–7.2%, respectively, of added 13C over a 35–d chase period. Owing to relatively low plant biomass, compared to plow–till, long–term no–till and chisel–till on average lowered plant 13C fixation and its allocation in rhizosphere soil by 17% and 11%, and 21% and 15%, respectively. Nevertheless, the 13C relocated to bulk soil was significantly higher under both no–till (0.24 g m−2) and chisel–till (0.22 g m−2) than that under plow–till (0.18 g m−2) over the chase period. This could be partially attributed to the decreased allocation of belowground 13C to root–derived CO 2 releases and the increased C retention in both macroaggregates and microaggregates in conservation tillage plots. Moreover, the reduced microbial utilization of new C in rhizosphere soil, as indicated by the low 13C amount in microbial biomass, may further facilitate new C accumulation in bulk soil under reduced tillage. Our findings suggest that despite low photosynthetically–fixed C input to belowground pools during the early growth stages, soils under long–term conservation tillage, in particular no–till, can have a distinct efficiency advantage in soil carbon sequestration by enhancing photosynthesized C preservation in soil aggregates and decreasing new C loss from root–derived CO 2 release in the dryland wheat–soil system. • In–situ 13C labeling was used to assess newly assimilated C allocation in crop–soil system. • Long–term conservation tillage significantly affected C allocation and microbial use in the belowground system. • Both no–till and chisel–till increased C retention in macroaggregates and microaggregates. • Conservation tillage improved new C sequestration efficiency in dryland cropping system. [ABSTRACT FROM AUTHOR]

The epidemicity of bacteria facultatively associated with eukaryotes, involves not only housekeeping genes but also genes linked with the pathogenesis and symbiosis. Here, by characterizing both housekeeping ( rpoB and 16S rRNA gene) and nodulation ( nodD ) genes, we explore processes shaping epidemic patterns of facultative microsymbionts from rhizosphere soils of faba bean in three ecoregions. Although total bacterial communities in rhizosphere were not significantly differentiated across ecoregions, rpoB amplicon sequencing uncovered that Rhizobium laguerreae and Rhizobium anhuiense were predominant in different samples with contrasting pH or salt content. However, R. anhuiense can outcompete R. laguerreae in certain sterilized soils where R. laguerreae originally dominated, and viceversa . Contrasting bacterial taxa associated with either R. laguerreae or R. anhuiense in soils. The biogeographical pattern of nodD was more clear than that of rhizobial species in both rhizosphere soils and nodules. Competitive nodulation experiments demonstrated a hierarchical selection on nodD genotypes and their genomic backgrounds by faba bean cultivars. Taken together, abiotic and biotic factors in soils and the selection by legume hosts are either indirectly or directly involved in shaping rhizobial species-level taxonomic biogeography, which however cannot be used to infer spatial patterns of nodulation gene. [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]

Fertilized agricultural soils can be a major source of soil nitrous oxide (N 2 O) emissions to the atmosphere. Conservative soil management may have the ability to reduce N 2 O emissions through affecting a number of N-cycling-related soil biophysical properties. Using in situ N 2 O measurements combined with the techniques of quantitative polymerase chain reaction (qPCR), amplicon sequencing, and metagenomic sequencing, we aimed to understand the effects of long-term (>10 y) conservation tillage (i.e., zero- and chisel-till vs. conventional plow-till) on soil N 2 O production and associated microbial guilds following inorganic N fertilizer application in maize. Between 2017 and 2019, continuous in situ measurements of N 2 O fluxes indicated that both zero- and chisel-till significantly lowered cumulative emissions within the growing season, compared to plow-till, mainly through shortening the duration and reducing the magnitude of post-fertilization emission events. Conservative soil management, in particular zero-till, consistently increased the Shannon diversity index of bacterial community over the growing season, compared with plow-till. High-frequency qPCR analyses further revealed a clear tillage-induced niche differentiation between nosZI- and nosZII- N 2 O reducers, as evidenced by the dominant gene abundance of nosZII compared to nosZI in the conservation tillage soil, which eventually probably contributed to the transformation of N 2 O to N 2. Moreover, compared to plow-till, zero-till significantly decreased gene abundances involved in N 2 O production including the nirS , nirK , and narG genes, but increased abundances of N 2 O reduction genes such as nosZ during peak N 2 O emissions. Critically, the abundances of detected species involved in denitrification, such as Deltaproteobacteria_bacterium spp. and Alphaproteobacteria_bacterium spp. were clearly inhibited by zero-till. Overall, the reduced soil N 2 O emissions under reduced tillage positively and strongly depended on the nosZI -to- nosZII ratio, while increased emissions due to conventional tillage were positively associated with intensified denitrification. Such improvements in understanding of the responses of N-cycling gene abundances to tillage intensity can certainly help in the development of updated soil management practices and adaptative N application strategies to reduce reactive N emissions in agricultural ecosystems. • Conservation tillage shortened the duration and lowered the magnitude of post-fertilization N 2 O emissions. • nosZI , and nosZII genes suggested a tillage-induced niche differentiation among N 2 O reducers. • Conservation tillage decreased abundance of genes involved in N 2 O production. • The reduced N 2 O emissions were mainly attributed to the low nosZI -to- nosZII ratio. • Intensified denitrification resulted in higher emissions of N 2 O under conventional tillage soil. [ABSTRACT FROM AUTHOR]