The responses of moss-associated nitrogen fixation and belowground microbial community to chronic Mo and P supplements in subarctic dry heaths

Aims: Although nitrogen (N) fixation by moss-associated bacteria is the main source of new N in N-limited ecosystems like arctic tundra, we do not know which nutrient, molybdenum (Mo) or phosphorus (P), is rate-limiting for sustaining this process in the long-term. Further, how moss-associated N fixation impacts the belowground microbial regulation of decomposition remains unresolved. Methods: Moss-associated N fixation and soil microbial process rates, abundance and community structure were assessed in long-term P and Mo field additions in the Subarctic during three years. Results: We found tendencies for stimulation of moss-associated N fixation by Mo in the short term, by P in the long-term, and tendencies for a stimulation of soil microbial activity by P. However, large variation in microbial activity within and below the moss exceeded any systematic variation induced by the field treatments. Our findings suggest that soil microbial activity is not limited by N at our site, and that Mo and P only occasionally limit N fixation during a growing season. Conclusions: Since increasing CO concentrations can induce nutrient limitation, the here reported transient limitation can easily shift into a chronic one with significant implications for ecosystem productivity and biogeochemistry.