The relative contribution of symbiotic N-2 fixation and other nitrogen sources to grassland ecosystems along an altitudinal gradient in the Alps

The significance of symbiotic N-2 fixation in legumes (Trifolium alpinum L. , T. nivale Sieber, T. pratense L., T. badium Schreber, T. thalii Vill., T. repens L., Lotus alpinus [DC.] Schleicher, L. corniculatus L., Vicia sativ a L.) and other N sources for the N budget of grassland ecosystems was stud ied along an altitudinal gradient in the Swiss Alps. The total annual symbi otic N-2 fixation was compared with other sources of N for plant growth of the total plant community (mineralisation and wet deposition). The contribu tion of symbiotically fixed N to total above-ground N yield of the swards d ecreased from at least 16% to 9% with increasing altitude where legumes wer e present. This decrease was due to a decrease in the yield proportion of l egumes from 15% at 900 and 1380 m a.s.l. to 5% at 2100 and 2300 m a.s.l. (n o legumes were found above 2750 m a.s.l.) and not to a decline in the activ ity of symbiotic N-2 fixation. With increasing altitude legumes are more pa tchily distributed. The high symbiotic N-2 fixation of individual plants up to their altitudinal limit is not primarily the result of low mineral N av ailability since an addition of NH4+ or NO3- fertiliser at 2300 m a.s.l. le d either to no decrease or only to a minor decrease in symbiotic N-2 fixati on. At 1380 m a.s.l., N mineralisation (13.45 g N m(-2) yr(-1)) appeared to be the main source of N for growth of the sward; N from symbiosis (at leas t 1.0 g to 2.6 g N m(-2) yr(-1)) and wet deposition (0.4 g to 0.6 g m(-2) y r(-)1) was not a significant N source for plant growth at this altitude. At 2100 m a.s.l., the combined amounts of N from symbiotic N-2 fixation (at l east 0.1 g N m(-2) yr(-1)) and wet deposition (0.3 g N m(-2) yr(-1)) appear ed to be similarly important for plant growth as soil N mineralisation (0.4 7 g N m(-2) yr(-1)). At high altitudes, wet N deposition and symbiotic N-2 fixation together represent a significant source of N for the grassland eco system while at low altitudes these N inputs appear to be much less importa nt.