Subarctic ecosystems are often considered nitrogen (N)-limited, but emerging evidence suggests an important role for phosphorus (P). We tested how P-fertilization (0, 5, 15 g m−2) in Subarctic tundra affected microbial growth rates, the limiting factor for growth and microbial carbon (C) use efficiency (CUE). In unfertilized soils, microbial growth was co-limited by C and P. P-fertilization tended to increase microbial growth rates, whilst weakening P-limitation and shifting the limiting nutrient towards N. Microbial CUE increased systematically with P-fertilization, reflecting greater C allocation towards anabolism. Together, these results show that Subarctic tundra microorganisms are nutrient-limited by P, suggesting enhanced microbial C sequestration under higher P availability, but also that intensified plant-microbe competition for N may affect overall ecosystem productivity.
