Different fates of deposited NH4+ and NO3- in a temperate forest in northeast China: a N tracer study.

Increasing atmospheric reactive nitrogen (N) deposition due to human activities could change N cycling in terrestrial ecosystems. However, the differences between the fates of deposited NH4+ and NO3- are still not fully understood. Here, we investigated the fates of deposited NH4+ and NO3-, respectively, via the application of (NH4NO3)-N-15 and (NH4NO3)-N-15 in a temperate forest ecosystem. Results showed that at 410 days after tracer application, most (NH4+)-N-15 was immobilized in litter layer (50 +/- 2%), while a considerable amount of (NO3-)-N-15 penetrated into 0-5 cm mineral soil (42 +/- 2%), indicating that litter layer and 0-5 cm mineral soil were the major N sinks of NH4+ and NO3-, respectively. Broad-leaved trees assimilated more N-15 under (NH4NO3)-N-15 treatment compared to under (NH4NO3)-N-15 treatment, indicating their preference for NO3--N. At 410 days after tracer application, 16 +/- 4% added N-15 was found in aboveground biomass under (NO3-)-N-15 treatment, which was twice more than that under (NH4+)-N-15 treatment (6 +/- 1%). At the same time, approximately 80% added N-15 was recovered in soil and plants under both treatments, which suggested that this forest had high potential for retention of deposited N. These results provided evidence that there were great differences between the fates of deposited NH4+ and NO3-, which could help us better understand the mechanisms and capability of forest ecosystems as a sink of reactive nitrogen.