Effects of Sheep Grazing on Availability and Leaching of Soil Nitrogen in Low-Alpine Grasslands

Abstract

Alpine ecosystems are generally nitrogen (N) limited with low rates of N mineralization. Herbivory may affect N cycling and N losses and thus long-term productivity of ecosystems.

Using a controlled grazing experiment in a low-alpine region at Hol, southern Norway, with three density levels of sheep, we determined effects of grazing on in situ availability of inorganic N, potential N mineralization, and mobility of dissolved inorganic N (DIN) and dissolved organic N (DON) in soil water of O-horizons in grazing-preferred grassland habitats. In addition, we studied the within-season and spatial variation of these processes.

The low alpine grasslands at Hol were characterized by small rates of N mineralization and relatively large plant demands for N. Significantly greater rates of potential N mineralization were found at sites with high sheep density compared to those with low density or no grazing. Effects of grazing on bioavailable N (as determined by buried PRS™ exchange resins) were greater at low as compared to high altitudes. At low altitudes, low sheep density reduced amounts of bioavailable N. Nitrogen concentration of plants as a proxy of N availability in soils revealed, however, no significant effects of grazing. There was a strong seasonal effect on inorganic N and DIN∶DON ratios of the soil water, with decreasing values in the course of the growing season, probably due to increasing nutrient demand of plants and/or microbes.

We conclude that grazing may significantly stimulate N-cycling, but not sufficiently to release the system from its strong N deficiency, as we found no evidence for short-term increased risk in N loss via soil water due to herbivore activity. Nitrogen removal through grazing is small compared to the total soil N pool and at high sheep density is about half of the N deposition. This suggests that grazing in grassland habitats in this low alpine ecosystem is sustainable from a nutrient point of view.

Acknowledgments

Marit Ness and Franz Grund are acknowledged for assistance in data collection and sample preparation. We also acknowledge Line Tau Strand, Irene Dahl Eriksen, and Grete Bloch for helpful comments and technical assistance, and two anonymous referees for comments on a previous draft. Finally, we acknowledge Christian Ritz for helpful comments on statistics. The study was funded by the Research Council of Norway (Miljø 2015 program, project 183268/S30), the Directorate for nature management and the Norwegian University of Life Sciences (PhD scholarship to Vegard Martinsen).