Different fates of deposited NH4+ and NO3− in a temperate forest in northeast China: a 15N 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 15NH4NO3 and NH415NO3 in a temperate forest ecosystem. Results showed that at 410 days after tracer application, most 15NH4+ was immobilized in litter layer (50 ± 2%), while a considerable amount of 15NO3− 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 15N under NH415NO3 treatment compared to under 15NH4NO3 treatment, indicating their preference for NO3−–N. At 410 days after tracer application, 16 ± 4% added 15N was found in aboveground biomass under 15NO3− treatment, which was twice more than that under 15NH4+ treatment (6 ± 1%). At the same time, approximately 80% added 15N 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.