Long-Term Tillage and Compost Shape Soil Microbes Under Soil Organic Carbon Equilibrium

Soil microorganisms are crucial in regulating soil organic matter dynamics and nutrient cycling, mediating the effects of agricultural management on soil health. Although the microbial responses to changes in soil organic carbon (SOC) are well-documented, a knowledge gap remains regarding microbial dynamics when soils reach SOC equilibrium. This study investigated how tillage and fertilizer types (compost and mineral fertilizer) influence microbial properties in a continuous corn system with surface soils at SOC equilibrium. We evaluated a 28-year experiment comparing conventional tillage (CT) and no-till (NT), combined with either manure or compost (OF), mineral fertilizer (MF), or no nitrogen addition (CO), measuring soil microbial biomass, extracellular enzyme activity, and soil physicochemical properties to a depth of 90 cm. In the 0–5 cm layer under NT-OF, SOC concentration had stabilized since 2003 despite annual compost additions, indicating a near-equilibrium state. Upon reaching this threshold, microbial biomass and β-glucosidase (bG) activity plateaued, suggesting additional organic carbon inputs no longer enhanced these properties but instead contributed to SOC movement into deeper soil horizons, where increased microbial activity was observed. Long-term CT-OF resulted in 30% less SOC and total nitrogen compared to NT-OF, suggesting tillage disrupted SOC accumulation and enhanced decomposition. Both NT-MF and NT-CO had minimal effects on microbial properties and SOC, potentially due to insufficient organic residue returned. Although NT-OF increased SOC, total nitrogen, available phosphorus, and microbial biomass to 30 cm depth, it also reduced oxidative enzyme activity and arbuscular mycorrhizal fungi abundance, indicating shifts in microbial functional strategies in response to the continuous addition of compost. Our study demonstrated that once surface soils reach SOC equilibrium, additional compost additions no longer increased microbial processes in the surface layer but instead promoted SOC translocation to deeper horizons. This dynamic underscores the need for depth-conscious management strategies that balance soil microbial activity, SOC storage, and the capacity for SOC stabilization across soil profiles.