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This article in SSSAJ

  1. Vol. 44 No. 3, p. 610-616
    Received: June 7, 1979
    Accepted: Dec 14, 1979

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Nitrogen Mineralization, Immobilization, and Nitrification Following Urea Fertilization of a Forest Soil Under Field and Laboratory Conditions1

  1. D. W. Johnson,
  2. N. T. Edwards and
  3. D. E. Todd2



Following a 200-kg urea-N/ha fertilization in a loblolly pine stand (Pinus taeda), soil mineral N levels (almost entirely NH4+) declined from 200 ppm 20 days after fertilization to < 10 ppm within 161 days. Similar patterns had been previously observed following urea fertilization in a Douglas-fir stand. After the decline in soil mineral N, 20% (40 ppm) of fertilizer N was mineralized within 4 weeks of aerobic incubation in the laboratory at 25°C. Nitrogen mineralization in control soils did not occur after 7 weeks incubation.

In contrast to field results, urea additions to control soils in the laboratory resulted in rapid net N mineralization. The difference between laboratory and field fertilization results is thought to be due principally to differences in available C supplies, i.e., large (> 2 mm) litter and roots were excluded from laboratory tests.

The maximum rate of nitrification during incubation was related to the supply of NH4+ substrate, whereas the delay period was probably related to the initial populations of nitrifiers.

Additions of 100 kg P/ha as triple superphosphate in the field and equivalent additions of NaH2PO4 in the laboratory had little or no effect on results.

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