Long-term low-level factorial nitrogen and phosphorus additions significantly impact a Low Arctic mesic tundra plant community, but species responses differ from high-level fertilization: Implications for predicting climate warming effects

Arctic climate warming is expected to enhance plant growth-limiting nutrients in tundra soils, thereby affecting community composition. Much of our understanding of nutrient influences on tundra plants is derived from very high level fertilization experiments. Here, we report effects of 11–19 years of unusually low-level factorial annual nitrogen (N) and phosphorus (P) additions (at rates 1/10th of most previous studies) to mesic birch hummock tundra vegetation. We measured aboveground biomass of all species and birch shoot lengths (total and current year’s extension) in 1 and 9 m 2 sampling areas respectively. Only the low N treatment had significant biomass effects, increasing the moss group and some infrequent vascular species, reducing others, and having no effects on the dominant shrubs (including birch). Hence, overall community composition was altered, but in markedly different ways to classic high-level fertilization responses. However, birch new shoot extension in the 9 times larger sampling areas was strongly stimulated by each of the separate low-level N and P additions, and even more so by their combination, indicating that its stem apical primary growth was NP co-limited. Overall, our results demonstrate that this tundra plant community is sensitive to very low-level (but climatically-realistic) soil nutrient enhancements, and suggest that changes will be slow (multiple decades), and likely to favour species whose growth is primarily N-limited.