Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 27, 2007
Publication Date: July 15, 2008
Citation: Venterea, R.T., Stanenas, A.J. 2008. Profile analysis and modeling of reduced tillage effects on soil nitrous oxide flux. Journal of Environmental Quality. 37:1360-1367. Interpretive Summary: Reduced tillage is being considered as a way to decrease the amount of greenhouse gases emitted by agricultural practices. In addition to possible effects on carbon dioxide (CO2), changes in tillage practices may also affect soil emissions of another greenhouse gas, nitrous oxide (N2O). Effects on N2O may be important, because one molecule of N2O emitted to the atmosphere has the same potential to alter global climate as 300 molecules of CO2. However, because changes in tillage practices can cause changes in several different soil properties, it is difficult to predict the net affect of these multiple changes on N2O emissions. The objective of this study was to compare the effects of differing long-term tillage practices on several soil properties, and to assess which effects were most critical in regulating field soil N2O emissions over two growing seasons in a corn/soybean rotation in Minnesota. Changes in a variety of factors due to tillage were detected, including soil temperature, water content, bulk density, pH, and soluble carbon. The most important tillage-induced factor appeared to be differences in the vertical distribution of microbes capable of producing N2O in the absence of oxygen. These denitrifying microbes were more concentrated in the upper 5 cm of the soil surface in soils that were not tilled, while they were more uniformly distributed in the intensively-tilled soils. The combination of this effect with differences in the initial vertical placement of N fertilizer explained the major patterns in field N2O emissions. These findings are important because they shed light on the major controls over N2O emissions in reduced tillage systems, and will be useful to scientists and resource managers involved in the development of management practices and policies aimed at minimizing greenhouse gas emissions from agriculture.
Technical Abstract: No-till (NT) and other reduced tillage (RT) practices can alter a range of soil properties that influence soil-to-atmosphere fluxes of the potent greenhouse gas (GHG) nitrous oxide (N2O). However, the net impact of RT on N2O fluxes is difficult to predict, and limited information is available regarding strategies for minimizing fluxes from RT systems. We measured vertical distributions of key physical and biochemical factors 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. Under drier, nitrification-dominated conditions, simulated N2O emissions in the presence of nitrite (NO2-) were two to 10 times higher in NT soil compared to soil under conventional tillage (CT). This effect increased as fertilizer nitrogen (N) was placed closer to the surface, due to differences in vertical distributions of aerobic enzyme activity and abiotic reaction potential. This effect was partially offset by higher soil temperatures under CT. Under wetter, denitrification-dominated conditions, 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 temperature and lower N2O reduction rates under CT. The NT:CT ratio of denitrified N2O fluxes increased as nitrate (NO3-) was placed closer to the surface due to vertical variations in enzyme activity. 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 indicate that N2O fluxes from RT systems can be minimized by subsurface fertilizer placement using a chemical form that does not promote substantial soil NO2- accumulation. Modeling the effects of RT on N2O emissions would benefit from improved methods for quantifying soil-specific N2O reduction kinetics, which are currently difficult to assess.