Nitrogen Mineralization, Immobilization, and Nitrification Following Urea Fertilization of a Forest Soil Under Field and Laboratory Conditions1
- D. W. Johnson,
- N. T. Edwards and
- 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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