Effects of Stimulated N Deposition on Soil Net N Mineralization under a Temperate Korean Pine and Broadleaf Mixed Forest

Soil samples at 7 cm and 15 cm depths under a Korean pine and broadleaf mixed forest at Changbai Moun- tain, Northeast China, were collected with PVC tubes with resin cores at bases to study seasonal and annual changes in soil net N ammonification, net N nitrification, and net N mineralization after the addition of N sources such as (NH₄)₂SO₄, NH₄Cl, and KNO₃. In the non-fertilized plots, net N ammonification of the soils at both depths accounted for 53%-72% of soil net N mineralization, whereas the ratio of soil net N ammonification to net N mineralization was re- duced by 37%-66% in the KNO₃ added soils at a high dose, and the ratio was increased by 86%-92% in the NH₄⁺-N treated soils. The addition of N sources, particularly NH₄⁺-N, appeared to increase soil net N mineralization, and the stimulating effect reduced with time of fertilization. Both net N ammonification and net N mineralization rates of the soils at the 7 cm depth were greater than those of the soils at the 15 cm depth, particularly after the addition of NH₄Cl. In combination with results of previous studies, stepwise regression analysis showed that 38% of the variability in annual net N mineralization of forest soils at various global sites could be attributed to the annual wet atmospheric N deposition. In addition, 50% of the variability in annual soil net N mineralization could be attributed to the annual wet atmospheric N deposition, the pH value of forest organic horizon, and the ratio of total C to total N. The results are beneficial for pre- dicting soil net N mineralization at regional forest stands and its responses to environmental changes.