Shifts in Timing and Magnitude of Precipitation Modulate Soil Carbon Pools in Semi-Arid Ecosystems

Semi-arid ecosystems cover a large part of continental land area and are predicted to be strongly influenced by climate change. Shifts in the seasonal timing and magnitude of precipitation are predicted to alter ecosystem function in semi-arid rangelands, which are especially sensitive to seasonal changes in precipitation. Of particular interest is how changing climate will impact carbon cycling and the flux of carbon between the land and atmosphere. In semi-arid regions, carbon fluxes are often dynamic, varying orders of magnitude both spatially and temporally; whether long-term changes in climate will diminish or exaggerate these processes is not well studied in cold desert systems. Utilizing a long-term eco-hydrologic site (est. 1993) located in southeastern Idaho, we attempt to study the modulation of soil carbon pools in a cold desert ecosystem containing native and exotic plant species subjected to precipitation treatments varying both magnitude and seasonality of rainfall. The site consist of native sagebrush (Artemisia tridentata spp. tridentata) and exotic crested wheatgrass (Agropyron cristatum) plots, each with three seasonal precipitation treatments (ambient control, 2x ambient added in the winter and 2 x ambient added in the summer). We expect increases in both above and below-ground productivity as precipitation increases, as well as increased heterotrophic activity in the soils; we hypothesized that changes in the timing and magnitude of precipitation provide a mechanism in semi-arid regions for reallocation of soil carbon pools from soil organic matter to precipitated inorganic carbon (IC). Specifically, additional precipitation during the summer season will lead to the greatest increased production of inorganic carbon. Results show summer plots with significantly more IC content (P=0.0091) than winter plots irrespective of vegetation type (14.57 + 0.42 kg IC/m2 and 12.79+ 0.39 kg IC/m2 respectively). Although not significant, summer plots also show more IC production than ambient controls (13.7458 + 0.39 kg IC/m2). Soil organic matter was found to be significantly higher in winter plots (P=0.047) compared to ambient controls (1.49 + 0.09% and 1.22 + 0.07% respectively), with summer plots containing intermediate SOM (1.4184 + 0.07%). As expected, an increase in water availability increased SOM in both summer and winter plots, with summer plots accumulating more IC. Although preliminary results suggest changes in timing and magnitude of precipitation modulating changes in the form of carbon (inorganic vs. organic) stored in desert soils, additional measurements examining the natural abundance of 13C are needed to clarify whether production of IC is driven by pedogenic properties or increased biological activity. If in fact biota does show a feedback on IC production in this system, it could provide broader insight into possible carbon cycle responses to climate change in arid regions.