Diminished soil functions occur under simulated climate change in a sup-alpine pasture, but heterotrophic temperature sensitivity indicates microbial resilience

Abstract

The pressure of climate change is disproportionately high in mountainous regions, and small changes may push ecosystem processes beyond sensitivity thresholds, creating new dynamics of carbon and nutrient cycling. Given that the rate of organic matter decomposition is strongly dependent upon temperature and soil moisture, the sensitivity of soil respiration to both metrics is highly relevant when considering soil–atmosphere feedbacks under a changing climate. To assess the effects of changing climate in a mountain pasture system, we transplanted turfs along an elevation gradient, monitored in situ soil respiration, incubated collected top-soils to determine legacy effects on temperature sensitivity, and analysed soil organic matter (SOM) to detect changes in quality and quantity of SOM fractions. In situ transplantation down-slope reduced soil moisture and increased soil temperature, with concurrent reductions in soil respiration. Soil moisture acted as an overriding constraint to soil respiration, and significantly reduced the sensitivity to temperature. Under controlled laboratory conditions, removal of the moisture constraint to heterotrophic respiration led to a significant respiration-temperature response. However, despite lower respiration rates down-slope, the response function was comparable among sites, and therefore unaffected by antecedent conditions. We found shifts in the SOM quality, especially of the light fraction, indicating changes to the dynamics of decomposition of recently deposited material. Our findings highlighted the resilience of the microbial community to severe climatic perturbations, but also that soil moisture stress during the growing season can significantly reduce soil function in addition to direct effects on plant productivity. This demonstrated the sensitivity of subalpine pastures under climate change, and possible implications for sustainable use given reductions in organic matter turnover and consequent feedbacks to nutrient cycling.