Soil N and P cycles are not always coupled and may respond differently to global change. In Alaskan tundra, biogeochemical research for the past three decades has focused on the availability of nitrogen (N), whereas the availability of phosphorus (P) has received much less attention. This focus results from early studies at these sites which reported stronger limitation of vegetation by N than P. However, N and P factorial fertilization experiments in Alaskan, Canadian and Swedish tundra ecosystems report potential N and P co-limitation of numerous arctic ecosystems suggesting that a reassessment of the importance of P in Alaskan arctic tundra is important for understanding potential ecosystem responses to shifting nutrient supplies. The objective of this study was to identify ecosystem-specific variation in responsiveness to N vs. P of vegetation community structure and microbial processing of soil organic matter. We examined this objective using long-term factorial N and P fertilization experiments in both moist acidic tundra (MAT) and moist non-acidic tundra (MNT) at Toolik LTER in northern Alaska. We examined a suite of variables that characterize the vegetation community (species cover), microbial community (microbial biomass and exoenzyme activity) and soil nutrient pools (extractable and total carbon (C), N and P).
Overall vegetation responses to both N and P additions were stronger in MAT than MNT. In MAT numerous plant functional groups responded independently to N and P, with few responding interactively to both variables. In contrast, in the MNT few functional groups responded to the addition of either nutrient, likely because the site-specific flora did not contain the species with large responses in MAT. Similarly, the microbial community responded more strongly in MAT. In MAT, there were strong responses to P-addition (increases in microbial biomass C and P but decreases in N) but fewer responses to N-addition. Microbial biomass showed few responses to nutrient addition in MNT. The response of soil nutrient pools to N and P additions also differed between the two ecosystems. Similar to the vegetation and microbial community responses, in MAT soil nutrient pools generally responded independently to N and P. In contrast, soil nutrient pools in MNT showed strong and interactive responses to N and P addition. Our findings of multiple indications of ecosystem response to P in addition to N and their interactions suggests that further exploration of the role of P in determining plant and microbial community structure and function is necessary.