Responses in microbes and plants to changed temperature, nutrient, and light regimes in the arctic

Previous research has shown that experimental perturbations of arctic ecosy stems simulating direct and indirect effects of predicted environmental cha nges have led to strong responses in the plant communities, mostly associat ed with increased plant nutrient availability. Similarly, changes in decomp osition and nutrient mineralization are likely to occur if the soil warms a nd the soil moisture conditions are altered. Plant and microbial responses have usually been investigated separately, and few, if any, studies have ad dressed 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 factori al fertilizer addition, air warming, and shading. We expected increased N a nd P uptake by microorganisms after fertilizer addition but less after warm ing, soils. Plant productivity and N and P uptake were expected to increase after fertilizer addition but less after warming, because microbes were ex pected to absorb most of the extra released nutrients. Shading was expected to decrease plant production and also microbial biomass, due to the reduce d production of labile carbon (C) in plant root exudates associated with re duced photosynthesis. We found that the plants responded strongly to fertilizer addition by incre ased biomass accumulation and N and P uptake. They responded less to warmin g, but more than expected, showing a decline in N and P concentrations in m any cases. There were few significant responses to shading. The strongest r esponse was found in combined fertilizer addition and warming treatments. A ll functional vascular plant groups responded similarly. However, mosses de clined under those conditions when vascular plant growth was most pronounce d. Contrary to our expectation, microbial C, N, and P did not increase after w arming, but microbial N and P increased after shading. As expected, fertili zer addition led to increased microbial P content, whereas microbial N eith er increased or did not change. In general, microbial C did not change in a ny treatment. The microbes accumulated extra N and P only when soil inorgan ic N or P levels increased, suggesting that the soil microorganisms absorbe d extra nutrients only in cases of declining N and P sink strength in plant s.