Reindeer grazing and soil microbial processes in two suboceanic and two subcontinental tundra heaths

In oceanic, nutrient‐rich Fennoscandian arctic‐alpine tundra heaths, grazing by reindeer has been found to increase herbs and graminoids in relation to dwarf shrubs. In continental lichen heaths in the inland with nutrient‐poor conditions, however, slowly decomposable dwarf shrubs are favoured by grazing. According to a hypothesis, by favouring easily decomposing plants in nutrient‐rich conditions and slowly decomposing plants in nutrient‐poor conditions, herbivory enhances soil nutrient cycling in nutrient‐rich and retards it in nutrient‐poor areas. We tested this hypothesis by comparing the impact of reindeer grazing on soil C and N mineralization between two oceanic and two continental arctic‐alpine tundra heaths. 
Although soil respiration and microbial metabolic activity were enhanced by grazing in the suboceanic but not in the subcontinental tundra heaths, gross N mineralization rates were higher in the grazed areas in soils from all study sites, indicating that reindeer grazing leads to increased rates of nutrient cycling in both nutrient‐poor and nutrient‐rich tundra heaths. Thus, in the subcontinental tundra heaths, the increase in soil N concentrations due to mammalian waste products enhances N mineralization rates, even though the organic C quality is not improved by reindeer grazing. There was some site‐specific variation in the strength of the reindeer effects on various microbial processes and soil properties, which can be related to spatial variation in grazing intensity and timing, as these factors in turn affect the nutrient sink strength of the vegetation.