Abstract
If microbial degradation of carbon substrates in arctic soil is stimulated by climatic warming, this would be a significant positive feedback on global change. With data from a climate change experiment in Northern Sweden we show that warming and enhanced soil nutrient availability, which is a predicted long-term consequence of climatic warming and mimicked by fertilization, both increase soil microbial biomass. However, while fertilization increased the relative abundance of fungi, warming caused only a minimal shift in the microbial community composition based on the phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA) profiles. The function of the microbial community was also differently affected, as indicated by stable isotope probing of PLFA and NLFA. We demonstrate that two decades of fertilization have favored fungi relative to bacteria, and increased the turnover of complex organic compounds such as vanillin, while warming has had no such effects. Furthermore, the NLFA-to-PLFA ratio for 13C-incorporation from acetate increased in warmed plots but not in fertilized ones. Thus, fertilization cannot be used as a proxy for effects on warming in arctic tundra soils. Furthermore, the different functional responses suggest that the biomass increase found in both fertilized and warmed plots was mediated via different mechanisms.
Highlights
Microbial use of carbon substrates in soil is of critical importance when estimating impacts of climate change on the fate of soil carbon
Treatment Effects on Fatty Acid Concentrations Substrate additions did not cause any significant changes in the total phospholipid fatty acid (PLFA) or neutral lipid fatty acid (NLFA) concentration, and an average of each substrate is presented (Fig. 1)
Fertilization and warming significantly increased the total PLFA concentration in soil by about 31% and 20%, respectively (Fig. 1; P,0.001), which indicates that these treatments increased microbial biomass
Summary
Microbial use of carbon substrates in soil is of critical importance when estimating impacts of climate change on the fate of soil carbon. In the Arctic, where the amount of soil carbon exceeds the amount of carbon in aboveground plant biomass by an order of magnitude [1], microbial processes transforming the carbon compounds are crucial. In this region, climate change is predicted to increase the mean annual temperature by 2–9uC within this century [2]. As enhanced nutrient availability is an expected long-term consequence of warming, fertilization is often used as a long-term proxy for warming in experimental studies [5,6] Both warming and enhanced nutrient availability affect turnover of soil carbon, directly through effects on microbial activity [3] and indirectly by increasing plant growth [7] and altering vegetation composition [8,9]
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