Fate of Fertilizer Nitrogen Applied to a Northern Great Plains Rangeland Ecosystem

A field study was conducted at Mandan, North Dakota to provide information on the fate of fertilizer nitrogen (N) in a Northern Great Plains rangeland ecosystem. Mixed prairie grasses were fertilized with up to 540 kg N/ha applied (a) all in one year, (b) divided equally among 3 years, or (c) divided equally among 6 years. Up to 200 kg fertilizer N/ha was immobilized the first year in grass roots, soil organic matter, and fixed ammonium combined, plus gaseous losses (no leaching occurred). Immobilization and losses increased to about 350 kg fertilizer N/ha after 3 to 4 years, and remained rather constant thereafter. About half the immobilized N was found in the grass roots at the termination of the experiment. Collectively these results indicate that addition of high N rates to grasslands results in saturating the capacity of the soil-plant system to immobilize N. The system can then be maintained in an N-saturated condition if annual fertilizer additions plus mineralization equals immobilization plus irreversible losses. Thus, N can be eliminated as a growth-limiting factor, providing maximum grass production from the available water supply.