Impacts of reindeer grazing on phosphorus sorption and nutrient availability in a tundra site

Reindeer Rangifer tarandus , a large circumpolar herbivore, can influence whether nitrogen (N) or phosphorus (P) is the primary limiting nutrient in tundra plant communities. Specifically, findings from a site in northern Scandinavia suggest that under conditions where reindeer grazing stimulates inorganic N availability, grazing may drive ecosystems towards P limitation. However, the mechanisms driving such effects on tundra soil P biogeochemistry remain poorly understood. We conducted a study, where heavily grazed areas locate close to long‐term lightly grazed areas in both meadows and heaths in northern Norway, to assess how reindeer grazing moderates soil inorganic N and P concentrations, and particularly the relationship between P availability and soil P sorption capacity. We measured water extractable inorganic N (NH 4 ‐N and NO 3 ‐N) and water extractable P (PO 4 ‐P) concentrations, P sorption index, and the P‐sorbing elements aluminium (Al) and iron (Fe). Our results demonstrate that high reindeer grazing pressure was related to higher extractable inorganic N concentrations in the organic soil layer in meadows, and a higher extractable inorganic N:P ratio in both vegetation types. Also, the higher reindeer grazing pressure significantly increased the soil P sorption index, which was positively related to soil Al + Fe concentrations and decreased extractable PO 4 ‐P in the organic layer in the heath. In the meadow, extractable PO 4 ‐P concentrations remained unchanged while the extractable N:P ratio increased from 21 to 30, thus decreasing the amount of extractable PO 4 ‐P per unit of inorganic N. We conclude that herbivores shift both habitats in our study area in the direction of P limitation, but the mechanism seems to vary from increased P sorption in the heath to increased N availability in the meadow. This shift in mechanism could be important for understanding how herbivores can influence the relative importance of N and P limitation in ecosystems across the Arctic.