Soil Quality Differences Among Long-Term No-till Irrigated Crop Rotations to Replace Burning and Plowing Continuous Wheat.

Heavy residue in irrigated wheat (Triticum aestivum L.) production in the Inland Pacific Northwest of the United States (PNW) is a problem often resolved by burning.  This management option can reduce air quality and alternative practices are needed.  We investigated the soil quality benefits of a diversified no-till crop rotation with various residue management practices compared to the burn and plow system for producing continuous winter wheat.  We established a 6-year no-till field experiment to evaluate soil quality and soil microbial changes occurring during two complete cycles of a 3-year irrigated crop rotation.  Winter wheat, spring barley (Hordeum vulgare L.), or winter canola (Brassica napus L.) were sown (1) into standing residue; (2) with residue removed and, (3) after residue was burned.  The traditional practice of continuous annual winter wheat sown after burning residue and inverting the topsoil with a moldboard plow was included as a check treatment.  Soil organic carbon (SOC) increased each year at the surface depths for all treatments.  The SOC increase was least for plow and direct seed burn treatments and was greatest for stubble remaining or removed treatments.  We found enzyme activities to correlate with the SOC levels.  The soil microbial communities changed with time.  The burn plow and burn direct seed treatments developed similar communities, while the direct seed residue standing and removed treatments were similar to each other and to the undisturbed noncultivated sites compared to the other plots.  The communities found in direct seed approached that of the undisturbed soils as time progressed and did not differ between residue removal or left on the surface.  We found that intact soil was a greater determinate in improving soil quality than residue handling.  This information will help growers and field persons assess management practices to increase yield, improve soil carbon and protect the environment.