Changes in soil carbon,aggregation and trace gas fluxes immediately following cultivationof an undisturbed soil profile

Grandy,A.S., and G.P. Robertson

Presented at the All Scientist Meeting (2003-09-12 )

Cultivation of previously undisturbed soils commonly results in the loss of 40-60% of initial C and N from surface horizons. In the tropics losses typically occur within months of cultivation, but a lack of data for temperate ecosystems prevents similar generalizations on a time scale less than about five years. We investigated changes in soil C pools, aggregation, nitrous oxide fluxes and denitrification in the first sixty days following cultivation of a previously untilled mid-successional community in southwest Michigan. Additionally, we designed a litter removal experiment to determine whether immediate, measurable changes were due to acceleration of soil organic matter dynamics or the incorporation of above-ground plant biomass and litter. Cultivation had an immediate impact on all measured properties: within two weeks we found differences in dissolved organic carbon, inorganic N, trace gas fluxes, and field-moist aggregates. A decline in water-stable aggregates coupled with a shift in the distribution of inter- and intra-aggregate light fraction organic matter likely contributed to changes in carbon dioxide fluxes. Increased nitrous oxide fluxes following cultivation were closely related to increases in N availability. Cultivation also reduced denitrification enzymes, probably due to rapid aggregate turnover which reduced the frequency and persistence of anaerobic microsites. Sites where above-ground plant biomass and litter were removed prior to cultivation had trace gas fluxes that were higher than in control sites but lower than in plots where above-ground resources were incorporated with cultivation. The rapid changes in nutrient mobilization observed here have implications for understanding the historical impact of agricultural expansion and for mitigating that impact via soil C sequestration and other means.

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