Although returning crop residue to fields is a recommended measure for improving soil carbon (C) stocks in agroecosystems, the response of newly formed soil C (NFC) to the integrated supply of residue and nutrients and the microbial mechanisms have not been fully understood. Therefore, an 84-day incubation experiment was conducted to ascertain the microbial mechanisms that underpin the NFC response to inputs of residue and nitrogen (N), phosphorus (P), and sulfur (S) in two black soils. The results showed that adding residue alone accelerated microbial nutrient mining, which was supported by decreases of 8–16% in the ratios of C:N and C:P enzyme activities (relative to soils with nutrient inputs). The NFC amounts increased from 1155.9 to 1722.4 mg kg⁻¹ soil in Gongzhuling and increased from 725.1 to 1067.5 mg kg⁻¹ soil in Hailun as the levels of nutrient supplementation increased. Boosted regression tree analysis suggested that β-glucosidase (BG), acid phosphatase (AP), microbial biomass C (MBC), and Acidobacteria accounted for 27.8, 18.5, 14.7, and 8.1%, respectively, of the NFC in Gongzhuling and accounted for 25.9, 29.5, 10.1, and 13.9%, respectively, of the NFC in Hailun. Path analysis determined that Acidobacteria positively influenced NFC both directly and indirectly by regulating BG, AP, and MBC, in which MBC acquisition was regulated more by AP. The intensity of NFC was lower in Hailun soil than in Gongzhuling soil and was directly affected by AP, thereby indicating the importance of soil status (e.g., SOC and pH) in determining NFC. Overall, our results reveal the response of NFC to supplementation by N, P, and S, which depends on Acidobacteria and Proteobacteria, and their investment in BG and AP in residue-amended soil.

Soil microbial biomass nitrogen (MBN) contains the largest proportion of biologically active nitrogen (N) in soil, and is considered as a crucial participant in soil N cycling.  Agronomic management practices such as crop rotation and mono-cropping systems, dramatically affect MBN in agroecosystems.  However, the influence of crop rotation and mono-cropping in agroecosystems on MBN remains unclear.  A meta-analysis based on 203 published studies was conducted to quantify the effect of crop rotation and mono-cropping systems on MBN under synthetic N fertilizer application.  The analysis showed that crop rotation significantly stimulated the response ratio (RR) of MBN to N fertilization and this parameter reached the highest levels in upland-fallow rotations.  Upland mono-cropping did not change the RR of MBN to N application, however, the RR of MBN to N application in paddy mono-cropping increased.  The difference between crop rotation and mono-cropping systems appeared to be due to the various cropping management scenarios, and the pattern, rate and duration of N addition.  Crop rotation systems led to a more positive effect on soil total N (TN) and a smaller reduction in soil pH than mono-cropping systems.  The RR of MBN to N application was positively correlated with the RR of mineral N only in crop rotation systems and with the RR of soil pH only in mono-cropping systems.  Combining the results of Random Forest (RF) model and structural equation model showed that the predominant driving factors of MBN changes in crop rotation systems were soil mineral N and TN, while in mono-cropping systems the main driving factor was soil pH.  Overall, our study indicates that crop rotation can be an effective way to enhance MBN by improving soil N resources, which promote the resistance of MBN to low pH induced by intensive synthetic N fertilizer application.