Differences in the Temperature Sensitivity of Soil Organic Carbon Decomposition in a Semi-Arid Ecosystem across an Elevational Gradient

Abstract:

Semi-arid ecosystems are a significant component of the global carbon (C) cycle as they store approximately 20% of global soil C. Yet, projected increases in mean annual temperatures might alter the amount of soil organic C (SOC) currently stored in these ecosystems. Uncertainties about the temperature sensitivity of SOC decomposition have hindered accurate predictions of C cycle feedbacks to climate change. This study aims to elucidate how the temperature sensitivity of SOC decomposition varies along an elevational (1000m) and climatic (i.e. mean annual temperature and precipitation) gradient. The study sites are located at Reynolds Creek Critical Zone Observatory in Owyhee Mountains of Idaho, USA. We conducted stratified random sampling of soil up to 0-5cm across sagebrush canopy and inter-canopy areas at four elevations. We hypothesized decomposition of SOC pools at lower elevations to have greater temperature sensitivity (more CO₂ respired per unit C) compared to upper due to the quality of C that is inherently more temperature sensitive. To assess the temperature sensitivity of SOC decomposition, we used aerobic laboratory incubations (n=40) across a temperature gradient ((15, 20, 25, 30) ^oC) at constant soil moisture (60% water holding capacity) for 120 days and measured CO₂ respired. Cumulative CO₂ respired increased with increasing incubation temperature. Cumulative CO₂ respired also increased with elevation as upper elevations support greater amounts of C. However, when normalized by SOC, we found that the temperature response of CO₂ respiration was greater in soils derived from lower than higher elevations (p<0.05). These results indicate that the response of SOC decomposition to elevated temperatures differs strongly across the landscape in semi-arid ecosystems.