Retention fraction of 15N-labelled deposited ammonium and nitrate in forests

The impacts of enhanced nitrogen (N) deposition on global forest carbon (C) sink and other ecosystem services may depend on whether N is deposited in reduced (mainly as ammonium) or oxidized forms (mainly as nitrate) and the subsequent fate of each. However, the fates of the two key reactive N forms and its contribution to forest C sink is unclear. We conducted ecosystem-scale paired 15N-labelling experiments in nine forests across China to quantify N retention fractions for both deposited ammonium and nitrate, including tropical and sub-tropical forests for the first time. By combining these results with four previous experiments from temperate Europe and North America, here we show that total ecosystem N retention is similar for ammonium and nitrate, but plants consistently take up more of the labelled nitrate  than ammonium  while soils retain more ammonium than nitrate. Nitrogen retention in plants and soils across sites is predicted by a combination of tree (NPP and woody biomass) ..., Nitrogen retention data is generated based on 15N tracer experiments while N deposition is from model analysis.
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