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Do Productivity and Environmental Trade-offs Justify Periodically Cultivating No-till Cropping Systems?

^a W.K. Kellogg Biol. Stn. and Dep. of Crop and Soil Sci., Michigan State Univ., Hickory Corners, MI 49060
^b Dep. of Crop and Soil Sci., Michigan State Univ., East Lansing, MI 48824-1325; A.S. Grandy, current address: Dep. of Geol. Sci., Univ. of Colorado, Boulder, CO 80309-0399

View larger version (19K): [in a new window]   Fig. 1. Average crop yields and soil NO3–N levels over 14 yr of conventional and no-till production at the KBS LTER site between 1989 and 2002. The crop rotation consisted of corn and soybean grown in 1:1 rotation until 1995 when the rotation was switched to corn–soybean–wheat. Corn was grown in 6 yr, soybean in 5, and wheat in 3. Asterisks indicate significant differences between tillage treatments for a particular crop (n = 6 replicate blocks). Data are adapted from annual crop yields presented in Grandy et al. (2006).

View larger version (23K): [in a new window]   Fig. 2. A conceptual model of immediate changes in soil aggregation and organic matter availability that lead to changes in soil C dynamics, environmental nutrient losses, and substantially reduced soil structure following tillage in long-term no-till soils. Immediately following tillage is a reduction in soil aggregate mean weight diameter (MWD). The release of labile light fraction soil organic matter (SOM) from within aggregates plus the incorporation of aboveground biomass leads to increased SOM availability. Microbial activity increases, leading to accelerated CO2 emissions and N mineralization rates. Although some of the NO3– produced from tillage is taken up by plants, the release of this N is often poorly synchronized with plant N needs, which usually do not peak for eight or more weeks after tillage, making it highly susceptible to loss via leaching and denitrification, including N2O loss. Associated with these changes in aggregation and organic matter availability are increases in soil temperature and O2 concentration that further stimulate decomposition. The process can be reversed by eliminating tillage, but the recovery of aggregates and aggregate-associated C pools takes far longer (years) than their destruction (days to weeks).

View larger version (12K): [in a new window]   Fig. 3. Simplified N pools and pathways in long-term no-till ecosystems highlighting the changes in soil environmental conditions and management that increase plant available N over time. Organic N pools are increased by decreased decomposition rates; increased organic matter leads to greater N mineralization at a time when plants can use it, stimulated by plant growth and warmer weather rather than tillage. Inorganic N pools are maintained by N mineralization and also by reduced fertilizer N losses to the environment as changes in aggregation and porosity attenuate N losses via runoff, leaching, and denitrification. Plant available N is enhanced by increased inorganic N concentrations and by expanded producer expertise in applying appropriate N forms at times and in soil regions that maximize plant uptake, rather than environmental losses.