Impact of warming and drying on microbial activity in subarctic tundra soils: inferences from patterns in extracellular enzyme activity

Abstract:

Permafrost contains a large pool of carbon that has accumulated for thousands of years, and remains frozen in organic form. As climate warms, permafrost thaw will increase rates of microbial breakdown of old soil organic matter (SOM), accelerating release of carbon to the atmosphere. Higher rates of microbial decomposition may also release reactive nitrogen, which may increase plant production and carbon fixation. The net effect on atmospheric carbon, and the strength of climate feedback, depends on the balance between direct and indirect effects of increased microbial activity, which depends on changes in soil conditions and microbial responses to them. In particular, soil moisture and availability of C and N for microbes strongly influence soil respiration and primary production. Current understanding of changes in these factors as climate warms is limited. We present results from analysis of soil extracellular enzyme activities (EEA) from a long-term warming and drying experiment in subarctic Alaskan tundra (the CiPEHR experiment) as an indicator of changes in soil microbial activity and relative availability of C and N for microbes. We collected soil samples from control (C), warming (W), and warming + drying (WD) treatments and used fluorometric methods to estimate EEA in shallow (0-5 cm) and deep (5-15) soils. We measured soil moisture, SOM, and C:N, and plant tissue C:N as an indicator of N availability. Activity of N-acquiring enzymes was higher in WD soils at both depths. Carbon EEA in W soils was lower in surface, but higher in deeper soils. We also found significantly lower soil C:N in both W and WD in deeper soils, where C:N was generally lower than surface. In general, EEA results suggest drying leads to increased C availability relative to N. This may be due to lower soil moisture leading to greater aeration of soils in WD plots relative to W plots, which may be saturated due to significant land subsidence. Greater aeration may increase efficiency of carbon use by microbes, effectively increasing carbon availability for microbes in WD soils relative to W soils. This suggests that drying may reduce N recycling by microbes, potentially reducing plant production and increasing soil respiration relative to carbon fixation, increasing the likelihood that permafrost thaw will be a positive feedback on climate change.