RESPONSES IN MICROBES AND PLANTS TO CHANGED TEMPERATURE, NUTRIENT, AND LIGHT REGIMES IN THE ARCTIC

Previous research has shown that experimental perturbations of arctic ecosystems simulating direct and indirect effects of predicted environmental changes have led to strong responses in the plant communities, mostly associated with increased plant nutrient availability. Similarly, changes in decomposition and nutrient mineralization are likely to occur if the soil warms and the soil moisture conditions are altered. Plant and microbial responses have usually been investigated separately, and few, if any, studies have addressed simultaneous responses to environmental changes in plants and soil microorganisms, except in models. We measured simultaneous responses in biomass, nitrogen (N), and phosphorus (P) incorporation in plants and microorganisms after five years of factorial fertilizer addition, air warming, and shading. We expected increased N and P uptake by microorganisms after fertilizer addition and also after warming, due to increases in mineralization rates in warmer soils. Plant productivity and N and P uptake were expected to increase after fertilizer addition but less after warming, because microbes were expected to absorb most of the extra released nutrients. Shading was expected to decrease plant production and also microbial biomass, due to the reduced production of labile carbon (C) in plant root exudates associated with reduced photosynthesis. We found that the plants responded strongly to fertilizer addition by increased biomass accumulation and N and P uptake. They responded less to warming, but more than expected, showing a decline in N and P concentrations in many cases. There were few significant responses to shading. The strongest response was found in combined fertilizer addition and warming treatments. All functional vascular plant groups responded similarly. However, mosses declined under those conditions when vascular plant growth was most pronounced. Contrary to our expectation, microbial C, N, and P did not increase after warming, but microbial N and P increased after shading. As expected, fertilizer addition led to increased microbial P content, whereas microbial N either increased or did not change. In general, microbial C did not change in any treatment. The microbes accumulated extra N and P only when soil inorganic N or P levels increased, suggesting that the soil microorganisms absorbed extra nutrients only in cases of declining N and P sink strength in plants.