Wheat Fallow Tillage Systems' Effect on a Newly Cultivated Grassland Soils' Nitrogen Budget

Abstract

The objective of this study was to examine soil nitrogen (N) losses from grassland soil as affected by time since the beginning of cultivation. A native grassland site was cultivated for winter wheat (Triticum aestivum L.) production in a crop-fallow rotation under three tillage systems; no-till, stubble mulch, and plow (bare fallow). The experiment was located in Western Nebraska on a Duroc loam (fine silty, mixed, mesic, Pachic Haplustolls). After 12 yr of cultivation, losses of soil N from the 0 to 30 cm depth were 3% for no-till, 8% for stubble mulch, and 19% for the plow tillages. Potential loss by erosion was small because of protection from wind by the surrounding grasslands. These results suggest decreased stirring of the soil resulted in major N savings beyond erosion losses. The sum of NO−3-N greater than that found below the sod control plus crop removal of N accounted for essentially all of the soil N lost from stubble mulch and plow tillages. In the no-till environment, crop removal of N and leached NO−3-N accounted for more N than had been lost from the soil since cultivation began. Plow tillage system resulted in leaching of 100 kg ha−1 more NO−3-N than occurred with no-till or stubble mulch. Soil N in the 0-to 30-cm soil depth was fractionated into exchangeable NH+4-N, nonexchangeable NH+4-N, and nonhydrolyzable N. The nonexchangeable and exchangeable NH+4-N fractions were not affected by cultivation. The nonhydrolyzable N fraction was reduced by all forms of tillage and accounted for a substantial part of the soil N losses.