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 (NO_x).

• 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.

1 - Simulation of N-deposition in a glacier forefield at 2500 m in the Alps.

2 - Frontal ranges are affected more than central or lee ranges.

29 August 2011
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