Though less evenly distributed over the globe than CO2 enrichment, the atmosphere is heavily loaded with soluble nitrogen compounds in certain regions. Sources are for the most part:
- Any high temperature combustion process, today largely fossil fuel burning (NOx).
- Intense agriculture, for the most part cattle farming and manure application (NH3).
The natural background soluble N-input into alpine ecosystems (from lightning, volcanism, wild fires) would be somewhere between 1-2 kg N ha-1 a-1. During the last few decades, the anthropogenic input has reached values between 3 and 12 kg N ha-1 a-1 in certain regions (e.g. the Alps, parts of the Rocky Mts., E-China).
For comparison: lowland rates of N-input may be as high as 40 kg N ha-1 a-1 in densely populated areas and fertiliser application in intense agriculture ranges from 100-400 kg N ha-1 a-1.)
Though small in absolute terms, the current rates of N-deposition represent a massive change in the diet of alpine plants and will favour fast growing species, grasses in particular, to the potential disadvantage of other species.
Accumulating in snow over 6-9 months, a heavy load of soluble N becomes available to alpine plants at snowmelt in seasonal climates.
It is expected that N-deposition in exposed regions has greater ecological effects than atmospheric CO2-enrichment and/or warming temperatures alone.
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1 - Simulation of N-deposition in a glacier forefield at 2500 m in the Alps.
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