Soil aggregate microstructure and microbial community structure mediate soil organic carbon accumulation: Evidence from one-year field experiment

Soil organic carbon (SOC) accumulation is easily susceptible to tillage managements, which strongly affect soil structure and microbial community structure. Subsoiling is developed for mitigating soil compaction and thus improving soil structure. Coupled with straw residue incorporation, it is expected not only to enhance this improvement but also to promote organic carbon (C) storage in soil. However, how subsoiling tillage managements change soil aggregate microstructure, microbial community structure and SOC fractions, and how soil aggregate microstructure and microbial community structure affect SOC in the topsoil and subsoil layers are equally unknown. An in-situ incubation field experiment was conducted in a Mollisol in northeastern China and included conventional tillage (CT), subsoiling tillage (ST) and subsoiling tillage with straw residue incorporation (ST + S). The methods of synchrotron-based X-ray micro-computed tomography scanning, phospholipid fatty acid and SOC density fractionation were used to analyze aggregate pore characteristics, microbial community structure and SOC fractions, respectively. The results showed that both ST and especially ST + S not only improved soil aggregate microstructure but also enhanced microbial biomass and then favored SOC accrual in the topsoil (0–20 cm). In the subsoil (20–35 cm), ST + S exerted similar positive effects as those in the topsoil. However, ST only improved soil aggregate microstructure but decreased microbial community biomass and SOC in the subsoil. Moreover, soil aggregate structure characteristics explained (solely explained 6 % in the topsoil and 12 % in the subsoil, respectively) a smaller part of the variation in SOC within aggregates than soil microbial characteristics did (solely explained 15 % in the topsoil and 25 % in the subsoil, respectively). Specifically, bacteria explained 26 % and 66 % of the variation in SOC within aggregates in the topsoil and subsoil, respectively. The porosity of macropores (>100 μm) also explained 25 % of the variation in SOC within aggregates in the topsoil, whereas the porosity of micropores (<30 μm) explained more variation (12 %) than that of macropores (3 %) in the subsoil. Notably, the substantially different interaction mechanism-processed of pore characteristics, microbial structure and SOC between topsoil and subsoil was probably traceable for the variation in the influx of organic C, pore size distribution and microbial adaptability in soil layers. These results would provide new insights on tillage managements strategies for improving soil conditions in terms of optimizing soil aggregate microstructure–microbes–SOC interactions in the topsoil and subsoil layers.