Biogeochemical consequences of arctic shrub expansion
The response of arctic ecosystems to global change will have critical effects on future climate. Climate warming has already triggered the expansion of shrubs across grassy tundra, raising questions about how shrubs will affect ecosystem carbon balance. Shrub functional traits like litter quality and mycorrhizal symbionts may accelerate the activity of soil microorganisms that regulate carbon cycling and facilitate the release of large stores of soil carbon. We investigated how shrubs affect the activity of soil microorganisms. We collected soils in paired spatial blocks from shrub (Alnus fruticosa, Betula nana, Salix pulchra) and non-shrub areas in arctic Alaska. We anticipated that shrubs affect soils through their functional traits and hypothesized that relative to their non-shrub counterparts, heterotrophic respiration of shrub soils would: (1) be greater, (2) demonstrate greater response to additions of shrub litter, and (3) be less nutrient limited. We created mesocosms with root-free soils at constant moisture and temperature, and quantified basal heterotrophic soil respiration rates (CO2 per unit soil C), and respiration in response to leaf and root litter inputs. We also added nitrogen (NH4NO3), phosphorus (NaH2PO4), or both, to determine which nutrient(s) limits heterotrophic respiration.
(1) The presence of shrubs generally produced higher rates of basal soil respiration in both horizons with some species effects. In the organic horizon, the presence of Betula and Salix enhanced respiration, while the presence of Alnus reduced respiration relative to non-shrub soils. In the mineral horizon, shrubs nearly doubled rates of respiration relative to non-shrub soils. These results demonstrate that shrubs stimulate soil microbial activity. (2) Litter addition increased respiration in all soils, but disproportionately increased respiration for some organic shrub soils (Salix and Alnus) relative to non-shrub soils. In mineral soil, litter addition generated a smaller respiratory response in soils of Betula and Salix relative to non-shrub soils. These findings suggest that shrub-affected microbial communities preferentially decompose shrub litter in the organic horizon, but not in the mineral horizon. (3) N additions did not increase heterotrophic respiration, but P and N+P additions induced a short respiratory pulse in all soils. These results suggest mild P limitation, but that shrub presence is not responsible for alleviating, or inducing, nutrient limitation for soil heterotrophs. Collectively, our findings provide evidence that shrubs fundamentally modify soil microbial activity to increase carbon loss, but generate new questions about the mechanisms driving these patterns.