Long-Term Tillage Effects On the Distribution Patterns of Microbial Biomass and Activities within Soil Aggregates.

The effects of tillage on the interaction between soil structure and microbial biomass vary spatially and temporally for different soil types and cropping systems. We assessed soil total organic C (TOC), microbial biomass C (MBC) and soil respiration associated with water-stable aggregates (WSA) under a combination ridge with no-tillage (RNT) and conventional tillage (CT) for a subtropical rice soil. Surface soil (0-20 cm) was fractionated into aggregate-sizes (>4.76 mm, 4.76-2.0 mm, 2.0-1.0 mm, 1.0-0.25 mm, 0.25-0.053 mm, <0.053 mm). Soil TOC, MBC, respiration rate, and metabolic quotient were heterogeneously distributed among soil aggregates while their distribution patterns associated with aggregate size were similar, regardless of tillage regimes. The TOC within WSA followed the sequence: medium-aggregates (1.0-0.25 mm and 1.0-2.0 mm) > 4.76-2.0 mm > micro-aggregates (0.25-0.053 mm) > large aggregates (>4.76 mm) > silt + clay fractions (<0.053 mm). The highest levels of MBC were associated with 1.0-2.0 mm aggregates (1025 and 805 mg C kg^-1 for RNT and CT, respectively). Significant differences in respiration rates were observed among different sizes of WSA, ranging from 2.5 to 12.1 mg CO₂ kg^-1 h^-1.  Similar to MBC, the highest respiration rate was associated with 1.0-2.0 mm aggregates (10.0 and 12.1 mg CO₂ kg^-1 h^-1 for RNT and CT, respectively). The C _mic/C _org was greatest for the largest aggregates (> 4.76 mm) regardless of tillage regimes, and decreased with aggregate size. Soil metabolic quotient (qCO₂) ranged from 3.6-17.7 mg CO₂ g^-1 MBC h^-1. The distribution pattern of soil microbial biomass and activity was governed by aggregate size, whereas the tillage effect was not significant at the aggregate scale. Tillage regimes that contribute to greater aggregation, such as RNT, also improved soil microbial activity.  Soil TOC, MBC and respiration rate were at their highest levels for 1.0-2.0 mm aggregates, suggesting that these microorganisms are the most biologically active at this aggregate size for this ecosystem.