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Profile Analysis and Modeling of Reduced Tillage Effects on Soil Nitrous Oxide Flux

USDA-ARS, Soil and Water Research Management Unit, 1991 Upper Buford Cir., 439 Borlaug Hall, St. Paul, MN 55108. Mention of product names is for the convenience of the reader and implies no endorsement on the part of the author or the USDA

^* Corresponding author (rod.venterea{at}ars.usda.gov).

Received for publication June 1, 2007. The impact of no-till (NT) and other reduced tillage (RT) practices on soil to atmosphere fluxes of nitrous oxide (N₂O) are difficult to predict, and there is limited information regarding strategies for minimizing fluxes from RT systems. We measured vertical distributions of key microbial, chemical, and physical properties in soils from a long-term tillage experiment and used these data as inputs to a process-based model that accounts for N₂O production, consumption, and gaseous diffusion. The results demonstrate how differences among tillage systems in the stratification of microbial enzyme activity, chemical reactivity, and other properties can control N₂O fluxes. Under nitrification-dominated conditions, simulated N₂O emissions in the presence of nitrite (NO₂^–) were 2 to 10 times higher in NT soil compared to soil under conventional tillage (CT). Under denitrification-dominated conditions in the presence of nitrate (NO₃^–), higher bulk density and water content under NT promoted higher denitrification rates than CT. These effects were partially offset by higher soluble organic carbon and/or temperature and lower N₂O reduction rates under CT. The NT/CT ratio of N₂O fluxes increased as NO₂^– or NO₃^– was placed closer to the surface. The highest NT/CT ratios of N₂O flux (>30:1) were predicted for near-surface NO₃^– placement, while NT/CT ratios < 1 were predicted for NO₃^– placement below 15 cm. These results suggest that N₂O fluxes from RT systems can be minimized by subsurface fertilizer placement and by using a chemical form of fertilizer that does not promote substantial NO₂^– accumulation.

Abbreviations: CT, conventional tillage • DEA, denitrifier enzyme activity • GHG, greenhouse gas • NT, no till • RT, reduced tillage • SOC, soluble organic carbon • UAN, urea ammonium nitrate

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