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) and soil (organic layer mass and soil C/N ratios)
variables. Greater proportions of deposited N are retained in N-limited
ecosystems with low soil N availability and high soil C/N ratios. We
estimate that N deposition-induced C sink in forests contributes more than
20% of the total terrestrial C sink. Although less N is deposited in
oxidized than reduced state, their total contributions to the global
forest C sink are approximately equal due to more efficient use by trees
of the oxidized than the reduced form. Our study demonstrates differential
fates of reduced and oxidized N deposition that improves current
understanding of the C-N interaction in forests and indicates a greater C
sink attributable to atmospheric N deposition than previous estimates.