Multiyear measurements on Δ17O of stream nitrate indicate high nitrate production in a temperate forest
2020
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 (Δ17O) 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.
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