Decoupled above‐ and belowground responses to multi‐decadal nitrogen and phosphorus amendments in two tundra ecosystems

Global change in the Arctic promotes deeper soil thaw and enhanced soil microbial activity, increasing nitrogen (N) and phosphorus (P) availability to plants and microbes in strongly nutrient‐limited ecosystems. This critical, positive climate feedback has been examined through fertilization experiments that describe short‐term (<10 yr) above‐ or belowground responses to combined NP additions, with evidence of enhanced shrub growth, nutrient availability, and soil organic matter decomposition. There has been less opportunity for long‐term comparisons of both above‐ and belowground responses with factorial N and P additions in different systems, despite broad awareness that ecosystem response can shift with time, and the potential for decoupled above‐ vs. belowground or N vs. P responses, currently and with further predicted global change. We examined the response of the plants, soil microbes, and soil nutrients, to factorial N and P additions in the moist acidic tundra (MAT; 26 yr of nutrient additions) and moist non‐acidic tundra (MNT; 16 yr). Aboveground, the MAT plant community continues to change as predicted by earlier studies: Functional groups responded independently to N and P, but NDVI‐biomass, especially of Betula nana, only increased with N addition. Unlike shorter‐term MNT studies, the MNT vegetation, which does not include B. nana, shows few new fertilization responses. Belowground responses were not predicted by aboveground responses in either MAT or MNT. In contrast to the N response aboveground, MAT microbial biomass responded positively and microbial phosphatase activity negatively to P additions, implying possible release from microbial P limitation. Critically, earlier published results of declines in soil total carbon (C) with combined NP addition in the MAT are not present in the long term. We make two conclusions: (1) Arctic ecosystems are not universally N‐limited but also exhibit complex responses to P alone or in combination with N; and (2) the presence or absence of key vegetation species can cascade from aboveground to belowground and restrict the extrapolation of responses of nutrient addition in a single arctic ecosystem to other arctic ecosystems, the short‐term to the long term, or aboveground to belowground. Arctic regions are warming more rapidly than other parts of the globe (Huang et al. 2017) resulting in rapid ecological changes in arctic ecosystems (Post et al. 2009). Organisms respond both directly to warmer temperatures, but also to changes in ecosystem properties resulting from the warming. For example, deeper