Differences in soil organic carbon among soil layers caused by microbial necromass carbon accumulation under different tillage and carbon input regimes

Tillage and carbon input are considered to be important management measures for maintaining the sustainable production capacity of farmland soil. However, our understanding of the differences in the response of soil organic carbon (SOC) to tillage and carbon input between different soil layers remains limited. Therefore, we established no-tillage and plough tillage treatments alongside total straw return and exogenous carbon inputs of 600, 1,200, and 2400 kg C ha−1 yr−1 in four maize fields in Jilin Province. After one year, topsoil (0–20 cm) and subsoil (20–40 cm) were collected. Microbial high-throughput sequencing and amino sugar biomarkers combined with a partial least squares structural equation model (PLSPM) were used to analyze the role of microbial necromass carbon (MNC) in the SOC sequestration pathway of topsoil and subsoil, and the differences in SOC between different soil layers. We found that increasing the carbon input in addition to returning straw increased the SOC content of the topsoil as the carbon input increased, whereas the increase in SOC in the subsoil after plough tillage decreased with the increase in C input rates from 27.0% to 0.2%. In the topsoil, additional carbon input increased the soil total nitrogen (TN) content by 19.3%; β-glucosidase (BG) and cellobiohydrolase (CBH) activity by 29.8% and 24.2%, respectively; and microbial biomass carbon (MBC) by 35.0%, which, in turn, promoted fungal necromass carbon (FNC) and bacterial necromass carbon (BNC) by 18.5% and 6.7%, respectively. Through PLSPM analysis, it was found that FNC promoted the accumulation of mineral-associated organic carbon (MAOC) in the topsoil, whereas FNC and BNC together promoted the accumulation of particulate organic carbon (POC). In the subsoil, the changes in enzyme activity and the enzyme ratio showed that carbon input increased the preference of microorganisms for refractory carbon sources compared with returning straw alone, but the negative correlation between CBH/BG and FNC showed that carbon may be utilized more in respiration than in growth after plough tillage improved the aeration of the subsoil. Plough tillage promoted the decomposition of FNC by microorganisms such as Proteobacteria in the subsoil. The reduction in the precursor substance FNC limits the accumulation of MAOC. These findings suggest that carbon sequestration in the tillage layer is not entirely dependent on carbon input, but that higher farmland soil carbon sequestration benefits can be achieved through the comprehensive management of tillage, straw, and organic fertilizer.