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This article in SSSAJ

  1. Vol. 59 No. 5, p. 1350-1359
    Received: Aug 10, 1994

    * Corresponding author(s):
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Short-Term Effects of Nitrogen Fertilization on Soil Organic Nitrogen Availability

  1. M. B. Vanotti,
  2. S. A. Leclerc and
  3. L. G. Bundy 
  1. Department of Soil Science, 1525 Observatory Dr., Univ. of Wisconsin, Madison, WI 53706-1299



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 (NA) of soil organic N was highly correlated with corn N uptake (r = 0.88). Simulation of field conditions showed that 95% of NA 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.

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