The effects of long-term warming on tundra soil enzyme dynamics

Background/Question/Methods

Globally, Arctic soils are among the largest stores of terrestrial organic carbon (C). Climate models unambiguously predict that Arctic temperatures will increase over the next century, with winter warming expected to dominate. Therefore, there is substantial interest in developing mechanistic descriptions of Arctic systems’ C dynamics as they respond to warming. We are using a greenhouse experiment initiated in 1988 at the Toolik, AK Long Term Ecological Research site (68^o38’N, 149^o34’W) to explore the consequences of over two decades warming on permafrost soil microbial dynamics and nutrient cycling. Recent research has demonstrated the importance of seasonal dynamics in regulating permafrost soils’ microbial activity; however, there is a paucity of information on the impact of long-term Arctic warming on the seasonality of soil biogeochemical dynamics, including extracellular enzyme activity – the rate limiting step in decomposition. To address this knowledge gap, we have characterized the impacts of greenhouse warming on the seasonal dynamics of six hydrolytic and two oxidative extracellular enzymes in conjunction with concurrently collected biogeochemical measurements.

Results/Conclusions

Our study suggests that more than two decades of greenhouse warming seasonally stimulates hydrolytic enzyme activity, with the greatest increase in enzymatic activity occurring during the spring thaw in the mineral horizon. Notably, long-term greenhouse warming has driven the greenhouse soils to be warmer on average than ambient soils to a depth of 40 cm during the winter. This effect is likely caused by increased shrub density and height in the greenhouse plots creating snow drifts that insulate the ground. Correspondingly, our sampling effort has revealed elevated concentrations of greenhouse soils’ extractable N prior to thaw not seen in soils sampled during the summer growing season, suggesting that the warmer winter temperatures are exerting a strong influence on the greenhouse soils’ microbial activity. This research suggests that the seasonal nature of warming appears to be a highly significant factor in regulating microbial enzymatic activity and thus the potential magnitude of tundra soils’ decomposition with climatic warming.