Temperature Impacts on Soil Organic Carbon Decomposition Across an Environmental Gradient in a Semi-Arid Ecosystem

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Delvinne, Hasini Hiranya
soil carbon pools; temperature sensitivity; laboratory incubations; elevational gradients in semi-arid ecosystems; physical fractionation; carbon model; Soil Science
thesis / dissertation description
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. Incomplete understanding of the temperature sensitivity of SOC decomposition has hindered accurate predictions of changes in C cycle feedbacks to climate change. We aimed to elucidate the temperature sensitivity of SOC decomposition along an elevational-climatic gradient in a semi-arid desert, and understand the influence of physicochemical protection of SOC and other biotic and abiotic factors. The study sites (four) were located in Reynolds Creek Experimental Watershed in Owyhee Mountains of South Western Idaho. All ecosystems were sagebrush dominated and spanned across a 1000m elevational-climatic gradient (i.e. 5 oC and (240-795) mm). We collected soils (0-5cm depth) at all four elevations, and assessed decomposition using aerobic laboratory incubations (n=5), in which soils were exposed to a temperature gradient ((15, 20, 25, 30) oC) at constant soil moisture (60% water holding capacity) for 360 days. To describe temperature sensitivity, Q10 or the proportional change in respiration with 10 0C increment in temperature, was used. Respiration data were used to derive a two-pool C model that estimated the size and dynamics of more and less persistent C pools, respectively. Different physico-chemically protected C pools were quantified using a physical fractionation (i.e. coarse and fine particulate organic matter, silt and clay), with C associated with coarse fractions attributed to be more persistent and fine fractions vice versa.Absolute cumulative CO2 respiration and apparent temperature sensitivity was highest at the highest elevation, compared to other sites. However, greater normalized CO2 respiration (i.e. C respired relative to total soil C present), and greater model estimated less persistent C pool demonstrated greater temperature response per unit C at the lowest elevation. Greatest percent POM-C was found at the highest elevation, while percent Clay-C was highest at the lowest elevation, implying relatively a greater relative abundance of more persistent C at the lower elevation. Increasing Q10 with incubation time suggested more persistent C being more temperature sensitive. However, there was no consistent relationship between the percent C associated with particulate organic matter, silt or clay pools and observed temperature sensitivity of SOC. This was believed to be largely due to limitations in ascribing physically protected C pools. Assessing SOC pools using diverse fractions (i.e., chemical, biological and physical) that are more microbial relevant may aid us to describe the relationship between discrete C pools, and their relationship with temperature sensitivity of decomposition. Future studies should also incorporate how soil moisture mediates the temperature sensitivity of decomposition, given its predominant role and heterogeneity across the semiarid landscape.Our study elucidated that temperature sensitivity of SOC decomposition strongly differs across the landscape in semi-arid ecosystems. This reaffirms significant erroneous estimates on projected soil carbon stocks, if empirically validated Q10 values are not used. Therefore, stabilization and destabilization mechanisms that govern the temperature sensitivity of SOC decomposition should be integrated into robust climate prediction models.