THE EFFECT OF WHEAT-FALLOW TILLAGE SYSTEMS ON THE NITROGEN BUDGETS OF A NEWLY CULTIVATED GRASSLAND SOIL

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 using three tillage practices, no-till, stubble mulch, and plow (bare fallow). The experiment was located in Western Nebraska on a Duroc silt loam (fine silty, mixed, mesic, Pachic Haplustoll). After twelve years 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 plow tillage. Potential loss by erosion was small because of protection 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)(\u27-)-N greater than that found below the sod control plus crop removal of N accounted for essentially all soil N lost with stubble mulch and plow tillage systems. In the no-till environment, crop removal of N and NO(,3)(\u27-)-N leached below the root zone accounted for more N than had been lost from the soil since cultivation began. Plow tillage resulted in leaching of 100 kg (.) ha-1 more NO(,3)(\u27-)-N than occurred with no-till or stubble mulch. As a result of the positive budget of the no-till soil, another study was conducted to determine the mechanism responsible for the positive N budgets in the plant-soil system of no-till soils as compared with balanced budgets under conventional tillage conditions. From May 1982 to September 1983, nonsymbiotic N(,2) fixation was estimated using the acetylene reduction assay using intact cores from plow, stubble mulch, no-till, and native sod management practices. No-till soils had a two fold greater potential to fix N(,2) than plowed soils. Soil water content differences caused by the different tillage systems were highly correlated with differences in N(,2) fixation potentials among tillage systems. In no-till soil, one cycle of the wheat-fallow rotation according to the assay, fixed only 0.33 kg N (.) ha-1, so nonsymbiotic N(,2) fixation could not account for the positive amounts (66kg N (.) ha-1) found in the budget for no-till soil. Lower incubation temperatures, which reflected the effect of tillage systems on soil temperature, caused a lower estimate of N(,2) fixation than the 25(DEGREES)C incubation. These shortfalls could have been a result of improper sampling or lack of light during incubation