Response of Semiarid Grassland Sites to Nitrogen Fertilization: II. Fertilizer Recovery

Abstract

AbstractThe fate of N fertilizer applied annually for 5 or 6 years at rates up to 200 kg N ha−1 year−1 was determined on eight perennial grassland sites in western North Dakota. The amount of fertilizer N found in plant tops, roots, and as residual soil inorganic N, and the sum of these three fractions were determined for each site. The amount of fertilizer N recovered in each of these fractions was linearly correlated with the amount of fertilizer N applied to each site in almost all instances. Fertilizer N recovered in top growth was highest for smooth bromegrass (Bromus inermis) (about 80% of that applied), and least for the native prairie grasses (about 15% of applied), with intermediate values for crested wheatgrass (Agropyron desertorum). Likewise fertilizer N recovered in roots was greatest for bromegrass (near 15%), whereas soil inorganic N content was usually greatest under native range. Total fertilizer N recovered in these three fractions was usually near 50% for bromegrass sites, but only about 30% for native range sites. There was no evidence of appreciable runoff or deep percolation of nitrates. Fertilizer N not recovered was either immobilized in other forms of soil organic matter or lost to the atmosphere in gaseous forms. Available evidence indicates that only small quantities of fertilizer N would be expected to be immobilized in organic forms other than in plant roots, thereby suggesting that gaseous loss of applied N may have been significant at these dryland perennial grass sites.