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| Nitrogen (N) deposition is impacting the services that ecosystems provide to humanity. However,
the mechanisms determining impacts on the N cycle are not fully understood. To explore the
mechanistic underpinnings of N impacts on N cycle processes, we reviewed and synthesised recent
progress in ecosystem N research through empirical studies, conceptual analysis and model simulations.
Experimental and observational studies have revealed that the stimulation of plant N
uptake and soil retention generally diminishes as N loading increases, while dissolved and gaseous
losses of N occur at low N availability but increase exponentially and become the dominant fate
of N at high loading rates. The original N saturation hypothesis emphasises sequential N saturation
from plant uptake to soil retention before N losses occur. However, biogeochemical models
that simulate simultaneous competition for soil N substrates by multiple processes match the
observed patterns of N losses better than models based on sequential competition. To enable better
prediction of terrestrial N cycle responses to N loading, we recommend that future research
identifies the response functions of different N processes to substrate availability using manipulative
experiments, and incorporates the measured N saturation response functions into conceptual,
theoretical and quantitative analyses. | |
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