Soluble nitrogen deposition
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-1a-1. During the last few decades, the anthropogenic input has reached values between 3 and 12 kg N ha-1a-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-1a-1 in densely populated areas and fertiliser application in intense agriculture ranges from 100-400 kg N ha-1a-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.
