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Short-Term Effects of Nitrogen Fertilization on Soil Organic Nitrogen Availability

Department of Soil Science, 1525 Observatory Dr., Univ. of Wisconsin, Madison, WI 53706-1299

*Corresponding author.

ABSTRACT

Long-term N fertilization affects soil organic N reserves, N mineralization potential, and crop response to applied N, but little information is available on the influence of short-term N fertilizer (STN) management on soil organic N availability and crop response. This study was conducted to determine if STN changes soil N supplying capability to corn (Zea mays L.) after 3 yr of differential N fertilization on a Fayette silt loam soil (fine-silty, mixed, mesic Typic Hapludalf) in Wisconsin. Various rates of N fertilizer (0–402 kg N ha^–1) were applied to corn in 1983, 1984, and 1985, and their residual effects on corn response were evaluated in 1986. Soil profile No₃-N levels in spring 1986 were very low in all plots (48 ± 4 kg ha^–1 [90 cm]^–1), yet grain yields and N uptake were significantly increased by STN applications. Corn N uptake was linearly related to the total amount of N returned to soil in crop residues during the previous 3 yr. Increased organic N availability under high STN management was equivalent to a 78 kg N ha^–1 rate, or 47% of the N fertilizer required for optimum crop yields. In aerobic incubations (40 wk) of spring 1986 soil (0–30 cm), STN additions increased N release only in the first few weeks. Kinetics of N mineralization were best described by a two-component model in which the active fraction (N_A) of soil organic N was highly correlated with corn N uptake (r = 0.88). Simulation of field conditions showed that 95% of N_A is available before crop maturity. A phosphate-borate buffer organic N availability index was significantly and consistently related to STN treatments. Relative increases in total soil organic N corresponded with the 3-yr N balance between fertilizer additions and grain removals, and were about 10 times larger than mineralizable N. These results indicate that immobilization of excess mineral N into stable soil organic N during decomposition of crop residues should be considered in determining the environmental risk of N fertilization. Although labile organic N is a small fraction of the total fertilizer N contribution to soil N, its quantification should allow a more accurate assessment of crop N needs.

Research supported by the College of Agricultural and Life Sciences, Univ. of Wisconsin, Madison, through Projects 2764 and 3449.

Received for publication August 10, 1994.

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