Sheep grazing effects on soil carbon and nitrogen and implications for runoff water quality at a low alpine site in southern Norway

Grazing in outlying fields has a long history and is important in local communities worldwide. Alpine ecosystems, representing a large proportion of the land area in Europe, are under pressure from changes in climate and land-use. During the last decades grazing pressure has both decreased and increased in alpine ecosystems, but little is known about the effects on soil carbon (C) storage and nitrogen (N) cycling. As part of a sheep grazing experiment in a low alpine system at Hol, southern Norway, with three density levels of sheep; no sheep (control), 25 sheep km-2 and 80 sheep km-2, we determined effects of grazing on C and N in organic horizons of grassland soils. In addition, effects of grazing on leaching of nutrients and coliform bacteria were assessed.\nAfter 7 years of grazing, high sheep density reduced soil carbon concentration and carbon stocks at equivalent soil mass as compared to control. By contrast, the opposite was observed at the low sheep density. The form of soil organic matter (SOM), expressed as ratios of particulate organic C to soil organic C, and the lability of SOM (potential carbon mineralization) were only slightly affected by grazing. The lability of SOM was not affected by grazing directly, but significantly related to the mineral content of the O-horizons. In general, there were large differences between plant communities (i.e. snowbed and grasslands with willow-shrub) for several soil attributes.\nThe grasslands were characterized by a tight N-cycling with low concentrations of inorganic N in soil water and surface runoff, and small rates of potential N mineralization. There was a strong seasonal effect on inorganic N of the soil water, with decreasing values in the course of the growing season, probably due to increasing nutrient demand. Effects of grazing on bioavailable N (as determined by buried PRSTM exchange resins) were greater at low as compared to high altitudes. At low altitudes, low sheep density reduced amounts of bioavailable N. In turn, significantly greater rates of potential N mineralization were found at sites with high sheep density compared to those with low density or no grazing. However, effects of grazing on distribution and cycling of N as determined using δ15N natural abundance and added 15NH4-N were small.\nSheep grazing significantly reduced infiltration rates and increased concentrations of coliform bacteria in stream water indicating risks for drinking water quality in areas affected by grazing. There was, however, no indication of grazing induced risk in losses of nutrients. A low grazing pressure in this system is positive for soil carbon storage. In terms of N, these findings suggest that grazing may stimulate N-cycling, but the availability of N remains low. Nitrogen removal through grazing is small compared to the total soil N pool. This suggests that grazing in this low alpine ecosystem is sustainable from a nutrient point of view.