Multiyear Measurements on Delta O-17 of Stream Nitrate Indicate High Nitrate Production in a Temperate Forest

Nitrification is a crucial step in ecosystem nitrogen (N) cycling, but scaling up from plot-based measurements of gross nitrification to catchments is difficult. Here, we employed a newly developed method in which the oxygen isotope anomaly (Delta O-17) of nitrate (NO3-) is used as a natural tracer to quantify in situ catchment-scale gross nitrification rate (GNR) for a temperate forest from 2014 to 2017 in northeastern China. The annual GNR ranged from 71 to 120 kg N ha(-1 )yr(-1) (average 94 +/- 10 kg N ha(-1) yr(-1) over the 4 years in this forest. This result and high stream NO3- loss (4.2-8.9 kg N ha(-1) yr(-1)) suggest that the forested catchment may have been N-saturated. At the catchment scale, the total N output of 10.7 kg N ha(-1)yr(-1), via leaching and gaseous losses, accounts for 56% of the N input from bulk precipitation (19.2 kg N ha(-1) yr(-1)). This result indicates that the forested catchment is still retaining a large fraction of N from atmospheric deposition. Our study suggests that estimating in situ catchment-scale GNR over several years when combined with other conventional flux estimates can facilitate the understanding of N biogeochemical cycling and changes in the ecosystem N status.