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

Abstract

Abstract Following a 200‐kg urea‐N/ha fertilization in a loblolly pine stand ( Pinus taeda ), soil mineral N levels (almost entirely NH 4 + ) 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 NH 4 + 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 NaH 2 PO 4 in the laboratory had little or no effect on results.