Effect of Tillage, Cropping, and Fertilizer Management on Soil Nitrogen Mineralization Potential

Nitrogen mineralization potentials (Nₒ) were determined on soil from a long-term crop rotation tillage experiment on a Palouse silt loam (fine-silty, mixed, mesic Pachic Ultic Haploxerolls). Crop rotations included continuous winter wheat (Triticum aestivum L.), alternate winter wheat and (pea Pisum sativum L.), alternate winter wheat and spring wheat, and pea-alfalfa (Medicago sativa L.)-green manure, followed by 5 y of alternate spring wheat and winter wheat. Tillage variables were moldboard plowing, chisel plowing, or no-till. Long-term N fertilizer rate plots were also studied on a Ritzville silt loam (coarse-silty, mixed, mesic Calciorthidic Haploxerolls). The tillage plots were cropped annually, whereas the fertilization rate plots were alternately fallowed and cropped to winter wheat with and without spring supplemental irrigation. Moldboard plowing resulted in uniform Nₒ values throughout the top 15 cm of soil, but N mineralization potential (Nₒ) was greater for chisel plowing and no-till than for moldboard plowing at the 0- to 5-cm depth and less at the 5- to 10- and 10- to 15-cm soil depths. The net result was that average Nₒ for 0 to 15 cm was unaffected by tillage or crop rotation in the fall sampling. In the spring sampling, average Nₒ for either chisel plowing or no-till was significantly higher than for moldboard plowing. Also, peas-alfalfa-green manure followed by alternate spring wheat-winter wheat had a significantly higher Nₒ averge than both continuous winter wheat and winter wheat-pea but was not different from winter wheat-spring wheat. The Nₒ of the tillage and the crop rotation management treatments were significantly greater with samples obtained in the fall than from those obtained in the spring. Nitrogen mineralization potentials increased linearly with increased N rate on both the dryland and supplemental irrigated treatments. However, supplemental irrigation uniformly increased Nₒ compared with the corresponding nonirrigated treatments.