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 the relationship between soil structure induced by tillage and soil microbial activity at the level of soil aggregates. To this aim, organic C (OC), microbial biomass C (MBC) and soil respiration were measured in water-stable aggregates (WSA) of different sizes from a subtropical rice soil under two tillage systems: conventional tillage (CT) and a combination of ridge with no-tillage (RNT). Soil (0-20 cm) was fractionated into six different aggregate sizes (>4.76, 4.76-2.0, 2.0-1.0, 1.0-0.25, 0.25-0.053, and <0.053 mm in diameter). Soil OC, MBC, respiration rate, and metabolic quotient were heterogeneously distributed among soil aggregates while the patterns of aggregate-size distribution were similar among properties, regardless of tillage system. The content of OC within WSA followed the sequence: medium-aggregates (1.0-0.25 mm and 1.0-2.0 mm)> macro-aggregates (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 the 1.0-2.0 mm aggregate size class. Significant differences in respiration rates were also observed among different sizes of WSA. and the highest respiration rate was associated with 1.0-2.0 mm aggregates. The C-mic/C-org was greatest for the large-macroaggregates regardless of tillage regimes. This ratio decreased with aggregate size to 1.0-0.25 mm. Soil metabolic quotient (qCO(2)) ranged from 3.6 to 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 OC, MBC and respiration rate were at their highest levels for 1.0-2.0 mm aggregates, suggesting a higher biological activity at this aggregate size for the present ecosystem. (C) 2011 Elsevier B.V. All rights reserved.