Responses of soil N transformations and N loss to three years of simulated N deposition in a temperate Korean pine plantation in northeast China

Nitrogen (N) losses are related to N deposition and activity of microbes that affect N dynamics. However, the effects of N deposition on gross N dynamics and mechanisms of N losses are not fully understood. Here, we quantified N losses from runoff and gaseous N (NO, N2O) emissions, assessed gross N transformation rates using 15N tracing, and recorded microbe abundance using real-time PCR in a temperate Korean pine plantation treated with zero N (control), low-N (20 kg N ha⁻¹ yr⁻¹), moderate-N (40 kg N ha⁻¹ yr⁻¹) and high-N (80 kg N ha⁻¹ yr⁻¹). The results showed that total N loss (N2O, NO and surface runoff) during the growing season was higher with N addition, where it was 0.86 kg N ha⁻¹ in control and 1.83 kg N ha⁻¹ in the high-N treatment. NH4⁺ and DON losses from surface runoffand NO loss from gas emissions were higher with N addition, whereas gross N mineralization and autotrophic nitrification rates were lower. Gross mineral N immobilization and dissimilatory nitrate reduction to ammonium rates were very low, and immobilization of NH4⁺ and NO3⁻ varied among the treatments. The contrast between stimulated mineral N losses and suppressed mineral N production processes (mineralization and nitrification) with N addition showed that greater mineral N losses were likely related to losses from recently added N, rather than from old N converted from the native N pool, as a result of mineralization and nitrification processes. Suppressed NH4⁺ and NO3⁻ immobilizations in some treatments also contributed to greater N losses.