Ecosystem respiration responses to experimental manipulations of winter and summer precipitation in a Mixedgrass Prairie, WY, USA

Changes in timing or amount of precipitation may be of great consequence for carbon cycling in the Mixedgrass Prairie of N. America, because CO₂ fixation and efflux are tightly coupled to soil water properties. The objective of our project was to quantify how ecosystem respiration ($R_{\text{e}}$) responds to experimental changes in winter and summer precipitation in a Mixedgrass Prairie using in situ field manipulations of snow depth and summer rain. Our study was conducted at the USDA-ARS High Plains Grasslands Research Station, west of Cheyenne, Wyoming. We installed three replicated 50 m snow fences to increase winter snow on the leeward side of the snow fence and experimentally manipulated summer precipitation by either increasing (+50%) or decreasing (-50%) precipitation amounts. We also measured ambient conditions.$R_{\text{e}}$rates in May were around 2 g C$\text{m}^{-2}\ \text{d}^{-1}$for all treatments and increased to their greatest values in June, up to 10 g C$\text{m}^{-2}\ \text{d}^{-1}$, with the ambient treatment having the largest flux rates. There were no treatment effects during the early summer, but by midsummer,$R_{\text{e}}$rates were least in the reduced rainfall plots and greatest in the snow plots. Soil moisture and gross photosynthesis had strong influence on the daily$R_{\text{e}}$rates, but soil temperature had little correlation with daily$R_{\text{e}}$rates. In summary, the$R_{\text{e}}$rates in this Mixedgrass Prairie are strongly influenced by changes in precipitation, especially winter snow accumulation. Thus, carbon cycle estimates under future climate change scenarios need to include not only the affects of changes in summer rain, but also, the consequences of deep snow in winter and its' affect on carbon cycling processes in winter and subsequent summers.