Biological, Labile Soil Carbon Pools and Fertility Attributes As the Pathway for Carbon Storage Under a Long-Term Tillage Chronosequence in a Subtropical Ecosystem.

Management of crop residues and attendant no-till (NT) systems are key determinants affecting microbial processes that lead to changes in soil organic C (SOC) pools. The aims of this study were to assess a 22-year tillage chronosequence effects on SOC, N and S pools, and on microbial activity to understand the changes induced under conventional plow-based tillage and no-tillage systems in a subtropical ecosystem in Southern Brazil. Biological pools in this study was comprised of soil microbial biomass C and N (MB-C and MB-R), basal respiration (BR), and microbial metabolic quotient (respiration-to-biomass ratio, qCO₂), and their interactions with the more labile C pool represented by total polysaccharides (TP), and fertility attributes (pH, S, P and basis saturation) were assessed. The chronosequence consisted of five treatments: (i) native field (NF); (ii) NT for 10 years (NT-10); (iii) NT for 20 years (NT-20); (iv) NT for 22 years (NT- 22); and (v) conventional tillage for 22 years (CT-22). Our results showed a strong interaction between soil management strategies, soil fertility parameters and soil microbial activity. Conventional tillage depletes the soil C, N, S, and TP concentrations when increases were observed in NT soils. A decrease of 34% in SOC, 42% in N, and 48% in S, respectively, was observed in CT-22 when compared with NT-22 in 0-5 cm depth. NF has the higher MB-C and MB-N but exhibited the lower basal respiration and metabolic quotient. Furthermore, conventional tillage showed a significantly lower soil microbial biomass and a high metabolic quotient, reflecting that the soil microbes compete intensely for the small amount of C available. The results for no-tillage systems emphasized high microbial biomass C and N, high microbial respiration and relatively high metabolic quotient. This suggests the presence of microbial communities with high C use efficiency contributing to SOC sequestration.