Effects of Decreased Snowpack On Nutrient Cycling In a Sagebrush-Steppe Ecosystem.

Snow cover is important to soil biogeochemical cycles because it provides protective insulation for the soil during the freezing winter months. Projections in climate change suggest that a decrease in snowpack will occur throughout much of North America and this will alter freezing regimes of soil. Soil will be exposed to the ambient winter air temperatures creating an increased frequency of freeze-thaw cycles without having the protection of insulating snow cover. Freeze-thaw cycles have been shown to cause soil instability, microbial mortality, and nutrient release. Arid and semi-arid ecosystems are likely to be most vulnerable to a decreased snow cover because these ecosystems experience less snow accumulation than alpine ecosystems but arid ecosystems have not been studied. We have conducted a field experiment in a semi-arid Sagebrush-Steppe ecosystem to address the possibility of increased nutrient release due to freeze-thaw cycles by use of infrared heat lamps to create eight replicates each of three heating regimes; warming, snowmelt, and a control. For the warming treatment heat lamps are on continuously over the plots to melt the snow and slightly warm the soil. For the snowmelt treatment, the heat lamps are turned on only to melt the snow, and then are turned off to allow the soil to experience ambient air temperatures. Control plots are subjected to natural environmental conditions with normal snowpack thicknesses. In spite of increase plant growth in warming and snowmelt treatments, no significance difference was found in nitrate and ammonium pool sizes, net mineralization, and net nitrification rates over the winter months. Differences in microbial biomass, extractable phosphate, and gross N-cycling rate measurements taken by ¹⁵N-isotope pool dilution will be discussed. These preliminary results suggest that initial increases in plant growth due to warming and reduced snowpack treatments are not due to increased N availability.