Soil biotic associations play a key role in subsoil C mineralization: Evidence from long-term tillage trial in the black soil of Northeast China

Quantifying the potential of soil carbon (C) mineralization improves our understanding of changes in soil health in agricultural ecosystems. Previous studies rarely linked the trait of interactions inherent in soil communities to soil C mineralization rate (Cmin). This study provides field-based evidence of tillage-driven changes in biological associations and their impacts on soil Cmin. Data obtained from a long-term (15 years) conservation tillage field experiment in Northeast China, and showed that the soil Cmin (per unit soil mass) and specific Cmin (per unit soil C) were 79% and 33% higher in topsoil (0–5 cm) and 27% and 24% lower in subsoil (5–20 cm) under no tillage with residue (NT) compared to conventional tillage with residue removal (CT). Meanwhile, in topsoil, NT strengthened biotic associations in June and August; in subsoil, a more complex network was found in CT. In topsoil, soil Cmin was not related to soil biotic associations, but was positively (P < 0.05) correlated with soil organic C (SOC) and certain biotic properties (e.g., MBC (microbial biomass carbon), microbial phospholipid fatty acid (PLFA) biomass and the relative abundance of AMF (arbuscular mycorrhizal fungi) and PP (plant-parasites)). Among them, the increases in SOC under NT exerted a positive effect on soil Cmin directly or indirectly through mediating biotic biomass properties (MBC, the PLFA biomass of all microbial groups and BF biomass). In contrast, in subsoil, soil Cmin was positively (P < 0.05) correlated with soil biotic associations, SOC and the relative abundance of PP. Notably, the increase in soil Cmin value was mainly achieved by strengthening biological associations. These findings provide novel insights into the role of soil biota in C mineralization, highlighting the diverse mechanisms underlying soil Cmin in response to tillage systems at different soil depths.