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Combined straw and straw-derived biochar input is commonly applied by farmland management in low-fertility soils. Although straw return increases soil organic matter (SOM) contents, it also primes SOM mineralization. The mechanisms by which active microorganisms mineralize SOM and the underlying factors remain unclear for such soils. To address these issues, paddy soil was amended with 13C-labeled straw, with and without biochar (BC) or ferrihydrite (Fh), and incubated for 70 days under flooded conditions. Compound-specific 13C analysis of phospholipid fatty acids (13C-PLFAs) allowed us to identify active microbial communities utilizing the 13C-labeled straw and specific groups involved in SOM mineralization. Cumulative SOM mineralization increased by 61% and 27% in soils amended with Straw + BC and Straw + Fh + BC, respectively, compared to that with straw only. The total PLFA content was independent of the straw and biochar input. However, 13C-PLFAs contents increased by 35–82% after biochar addition, reflecting accelerated microbial turnover. Compared to that in soils without biochar addition, those with biochar had an altered microbial community composition-increased amounts of 13C-labeled gram-positive bacteria (13C-Gram +) and fungi, which were the main active microorganisms mineralizing SOM. Microbial reproduction and growth were susceptible to nutrient availability. 13C-Gram + and 13C-fungi increased with Olsen P but decreased with dissolved organic carbon and NO 3 - contents. In conclusion, biochar acts as an electron shuttle, stimulates iron reduction, and releases organic carbon from soil minerals, which in turn increases SOM mineralization. Gram + and fungi were involved in straw decomposition in response to biochar application and responsible for SOM mineralization. Highlights: Straw return accelerates microbial mineralization of SOM. Combined biochar and straw input raises SOM mineralization by 60%. Biochar addition increases 13C incorporation from straw into Gram + bacteria and fungi. 13C-Gram + bacteria and 13C-fungi are the dominant active microorganisms that mineralize SOM. [ABSTRACT FROM AUTHOR]

Stoichiometric control of input substrate (glucose) and native soil organic C (SOC) mineralization was assessed by performing a manipulation experiment based on N or P fertilization in paddy soil. Glucose mineralization increased with nutrient addition up to 11.6% with combined N and P application compared with that without nutrient addition. During 100 days of incubation, approximately 4.5% of SOC was mineralized and was stimulated by glucose addition. Glucose and SOC mineralization increased exponentially with dissolved organic C (DOC):NH4+-N, DOC:Olsen P, and microbial biomass (MB)C:MBN ratios. The relative abundances of Clostridia and β-Proteobacteria (r-strategists) were increased with combined C and NP application at the beginning of the experiment, while the relative abundances of Acidobacteria (K-strategists) were enhanced with the exhaustion of available resource at the end of incubation. The bacteria abundance and diversity were negatively related to the DOC:NH4+-N and DOC:Olsen P, which had direct positive effects (+ 0.63) on SOC mineralization. Combined glucose and NP application decreased the network density of the bacterial community. Moreover, P addition significantly decreased the negative associations among bacterial taxa, which suggested that microbial competition for nutrients was alleviated. The relative abundances of keystone species showed significant positive correlations with SOC mineralization in the soils without P application, revealing that microbes increased their activity for mining of limited nutrients from soil organic matter. Hence, bacteria shifted their community composition and their interactions to acquire necessary elements by increasing SOC mineralization to maintain the microbial biomass C:N:P stoichiometric balance in response to changes in resource stoichiometry. [ABSTRACT FROM AUTHOR]