Soil aggregate dynamics and the retention of organic matter in laboratory-incubated soil with differing simulated tillage frequencies

It is generally accepted that aggregate dynamics are a significant control on the dynamics of organic C and that aggregate dynamics differ under cultivated and uncultivated (or no-till) conditions. Cultivation may alter soil organic matter (SOM) dynamics by changing its position within the soil matrix, either releasing organic materials from within aggregates during disruption or occluding materials during aggregate formation. The goal of this study was to observe a 'agregate dynamics under differing regimes of simulated tillage and relate these to organic matter dynamics. The experiment specifically examined the mechanism of physical protection by inducing different rates of soil aggregate turnover without changes in environmental conditions. Soil samples were incubated with dysprosium-labeled tracer spheres and finely ground corn residues for 8 weeks under imposed rates of aggregate turnover: no simulated tillage, three simulated tillage events, or five simulated tillage events. Total soil respiration and (CO2)-C-13 evolution were used to determine the relative degree of physical protection afforded by the induced rates of aggregate turnover. Increased frequency of simulated tillage increased the incorporation of tracer spheres into stable macroaggregates, and reduced the total amount Of CO2 evolved during the experiment. We propose that organic matter retention in tilled samples was achieved through a reduction of the priming effect afforded by the increased aggregate turnover and the disruption of the microbial biomass decomposing the added POM and native organic matter. While there appears to be a disparity between short-term tillage-enhanced organic matter protection and the long-term decrease in organic matter content observed in cultivated soils, the results suggest that there may be threshold rates of aggregate turnover that will protect rather than release organic C. We propose that the physical protection available under differing rates of soil aggregate turnover will differ for incoming organic materials versus previously protected organic matter, and that soil C sequestration is maximal at an intermediate aggregate turnover rate. (C) 2002 Elsevier Science B.V. All tights reserved.