Nitrogen sources for alpine vegetation

• Direct, natural input of soluble N-compounds with precipitation (nitrite, nitrate, ammonium) as a result of volcanism, lightning, fires, nowadays also by anthropogenic emissions (NO_x, NH₃ from combustion processes and agriculture). The natural background N-deposition is believed to be smaller than 2 kg N per hectare and year. Current deposition rates in the Swiss Central Alps and part of the North American mountains exceed 4 kg with rates of 10-12 kg ha^-1a^-1 not uncommon.

1 - At high elevation, large amounts of N are stored in snow and ice and are released during short periods in spring and early summer.

Alpine pioneer vegetation in glacier forefields is swamped in dissolved nitrogen and dissolved mineral nutrients for a short period after snowmelt.

1 - At high elevation, large amounts of N are stored in snow and ice and are released during short periods in spring and early summer.

• Biological nitrogen fixation by free cyanobacteria (for instance in so-called cryptogam crusts or cyanobacteria in symbiosis with lichens) or by N-fixing bacteria living in symbiosis with roots of plants belonging to certain plant families or genera such as Fabaceae (e.g. Trifolium = clover), some Rosaceae (e.g. Dryas) or Alnus (alder). Biological N-fixation creates a rather small input, commonly insufficient to meet instantaneous ecosystem demand, but over the years these processes contribute substantially to the build-up of the organic N-pool in soils, the ultimate source of biologically recycled N.

2 - Lichen crusts on rock with cyanobacteria lichens

2 - Lichen crusts on rock with cyanobacteria lichens

3 - N-fixing Stereocaulon sp. (foreground, blue-green) and non-N-fixing Cladina sp. (background, white)

3 - N-fixing Stereocaulon sp. (foreground, blue-green) and non-N-fixing Cladina sp. (background, white)

4 - Massive cryptogram crusts including free cyanobacteria on loose substrate

4 - Massive cryptogram crusts including free cyanobacteria on loose substrate

5 - Symbiotic N-fixation in Fabaceae (Trifolium alpinum, purple flowers)

5 - Symbiotic N-fixation in Fabaceae (Trifolium alpinum, purple flowers)

6 - Symbiotic N-fixation in Dryas octopetala

8 - Symbiotic N-fixation in Alnus viridis

Alnus viridis (Betulaceae), one of the two highest climbing deciduous broad-leafed tree species in the Alps (the other one is Sorbus aucuparis, Rosaceae), preferentially grows in wet, disturbed ground. N-fixing symbionts permit rapid growth, important for slope stabilisation.

8 - Symbiotic N-fixation in Alnus viridis

• Microbial N-recycling, i.e. the decomposition of dead biomass and breakdown of humus N-pools. This is by far the most important process in late successional communities and well developed soils, but may contribute little to early pioneer vegetation on raw substrate.

9 - Microbial N-recycling

Because alpine soils are cold, and because the season can be very short, microbial processes run slowly, and alpine soils are considered nutrient poor. By agricultural standards they are. However, as will be shown, this does not reflect the reality for alpine plants, which are adapted to these life conditions.

Note: Most of the extra-anthropogenic N deposits pass through the system or get trapped in recalcitrant forms in soil humus. Yet, by passing through the system these easily available forms of N may change species composition and biodiversity.