Immobilization and mineralization of tracer nitrogen in forest raw humus

Summary An experiment was conducted to permit an assessment of the effect of the N-carrier on the interchange of fertilizer-N in humus maintained under waterlogged conditions. The experiment was carried out at 4°; 12°; and 20°C; and the incubation varied from 0 to 90 days. N¹⁵-enriched urea, ammonium chloride and potassium nitrate were used. The incorporation of fertilizer-N into a non KCl-extractable fraction occurred relatively fast in the urea treatment regardless of the temperature. The highest recovery of tracer-N in the mentioned fraction was attained at 20°C after 90 days' incubation. The urea was hydrolyzed at a high rate, and within 30 days of fertilizer application the inorganic N¹⁵-pool was almost stabilized at a level in the vicinity of 60% of the added N at 12° and 20°C. Nitrogen added as ammonium was incorporated into the non-extractable fraction at a lower rate than was the N added in the form of urea. The data reveal a continual N-immobilization, but at 20°C a substantial net re-mineralization took place during the last part of the experiment. The nitrate treatment was followed by a sizeable temporary net immobilization of tracer-N. The extremely great changes in the inorganic N¹⁵-pool and the net re-mineralization of tracer-N were positively correlated with temperature and incubation time. Within 30 days, about 95% of the added nitrate-N had disappeared from the inorganic pool at 12° and 20°C. Approximately 7.6% of the added NO₃-N was re-mineralized at 20°C after 90 days' incubation. The data reveal great losses of gaseous N from this waterlogged acid forest soil after treatment with nitrate. Contribution from the Forest Soil Fertilization Research Group, Vollebekk, Norway. This work was supported by the Agricultural Research Council of Norway.