By taking an agronomic point of view, N-cycling by biological processes in the Alpine (fixation, decomposition, mineralisation, uptake) is slow and limits biomass production. However, as can easily be demonstrated (Fig. 1), adding nutrients can rapidly destroy typical alpine vegetation and its robustness.

Plant litter decomposes slowly and often accumulates over several years. Thick organic layers are quite abundant, representing an important intermediate nutrient reservoir.

Winter is not a "dead season" for microbes! Under snow, soil rarely freezes and if it does, not to very low temperatures. Microbes are active under these conditions and release nutrients which accumulate over the dormant season.

Alpine soils have a high affinity to soluble nitrogen input. Tracer experiments with the ¹⁵N stable isotope have shown that half of the marker can be found almost 30 years after application. Alpine soils thus contribute to N retention and the clean water supply to the lowlands.

1 - Regular dung addition at cattle rest places or near stables can convert a low stature alpine heath (as shown in Fig. 2) into a luxurious "herbage crop" (Rumex alpinus, Polygonaceae; 2300 m, Swiss Alps).

2 - Large amounts of nitrate and ammonium are deposited together with precipitation during winter (snow) and are released over alpine vegetation in spring.

Animals play a key role in N-cycling. Grasshoppers alone have been found to harvest one fifth of annual graminoid production in an alpine heath. Rodents, such as voles, marmots or pikas, but also ungulates such as chamois, guanacos, etc. shape alpine ecosystems by both their intake and dung deposition.

3 - Miramella alpina (Acrididae), a major herbivore in the Alps

4 - A young ibex, Capra ibex (Bovidae), in the Alps

5 - Ibex feces

Man has replaced wild ungulates in many areas with domestic animals, which therefore play an important role in nutrient cycling. It had been estimated that 2 % of every patch of alpine landscape receives dung deposition every year (50 years return time), an important contribution to alpine heathland nutrition and biodiversity.

6 - Sheep at 2500 m, Swiss Alps

7 - Cattle have been part of the European Alpine for the past millennia (Swiss Alps).

8 - Cattle manure. A killing and nutritious dump recurring every 50 years.

29 August 2011
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