Elevated rates of heterotrophic respiration in shrub-conditioned arctic tundra soils

Abstract

The response of arctic ecosystems to global change will be critical for future climate, due to their vast stores of carbon. Climate warming appears to be linked to the expansion of shrubs across tundra, but it is unclear how shrubs affect the activity of soil microbes. We investigated three potential mechanisms by which shrubs may stimulate soil microbial activity, by 1) increasing the rate of litter inputs, 2) promoting soil microbial adaptation to litter and 3) reducing nutrient limitation. We created microcosms of root-free soils collected from shrub (Alnus fruticosa, Betula nana, and Salix pulchra) and non-shrub plots in arctic Alaska and conducted two experiments. We quantified heterotrophic soil respiration rates in response to litter inputs (experiment 1), or nutrient inputs (nitrogen, phosphorus, or both nutrients together, experiment 2). We found that shrub-conditioned soils maintained higher rates of soil respiration in both experiments. Shrub litter increased respiration in both soil types, but the relative response was greater in non-shrub soils. We found no evidence that shrubs reduce nutrient limitation to heterotrophic respiration, although we observed a short-term increase in respiration after phosphorus addition in both soil types. Collectively, our results suggest that higher rates of respiration in shrub-conditioned soils may be the result of higher litter input rates, but other factors such as organic matter quality or microbial community structure may also contribute to our observed differences in respiration.