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Long-Term Recovery of Fertilizer Nitrogen Applied to a Native Mixed Prairie¹

ABSTRACT

Previous research on perennial grassland soils has generally shown that a large part of any fertilizer nitrogen (N) added is immobilized in grass roots or in soil organic matter. Few studies have measured the amount of such immobilized N that eventually becomes mobilized and taken up by plant growth in subsequent years. In this experiment, up to 540-kg fertilizer N/ha was applied to native mixed prairie in North Dakota during a 1- to 6-year period, and residual effects of these treatments on grass production were followed until no further plant growth response was measured. When a total of 270- or 540-kg N/ha had been applied, plant growth responses were measured for up to 9 years after fertilization ceased. As fertilizer N rate decreased, length of residual effects likewise decreased. Part of the residual response may have resulted from increased density of cool-season grasses where higher N rates were applied. Growth responses after fertilization ceased were often greater when fertilizers were applied over a 6-year period, rather than applying an equal quantity of fertilizer N all in 1 year. For the 540-kg N/ha treatment, as much as 12% of the fertilizer N applied was recovered in top growth after fertilization ceased. Assuming that all increased inorganic N in the soil resulting from fertilization was used by plants during the residual period, and that the remaining fertilizer N taken up by top growth after fertilization ceased came from mobilization of fertilizer N present in grass roots in 1968, then an estimate can be made of availability of fertilizer N in roots. Maximum value for such an estimate was 44-kg N/ha, leaving 60- to 160-kg fertilizer N/ha permanently immobilized as this root material decomposed. Thus, when 270- or 540-kg fertilizer N/ha was applied, typically about 30% was permanently immobilized by absorption into root material during the period of fertilization, 35% was removed in top growth, and 35% was either immobilized permanently in soil organic matter or lost by gaseous means.

¹ Contribution from Agric. Res., Science and Ed. Admin., USDA.

² Research Leader, USDA-SEA-AR, Lincoln, Nebr., formerly located at Mandan, N. Dak.

Received for publication December 3, 1980. Accepted for publication March 16, 1981.