Friday, August 7, 2009

PS 79-39: Winter snowpack effects on biogeochemical processes in a restored tallgrass prairie

Laura M. Carpenter, David W.P. Manning, Erin C. Seybold, Grace M. Wilkinson, and John D. Schade. St. Olaf College


Correlations between winter soil biogeochemistry and microbial activity are highly relevant in light of climate change predictions. Warming is likely to influence precipitation in many areas and may alter the magnitude of snow accumulation.  Knowledge of these relationships in northern tallgrass prairies will be applicable for further studies on the effects of climate change in other northern ecosystems. We assessed the effects of snow cover on soil microbial activity and spatial heterogeneity of nutrients in restored tallgrass prairie plots in central Minnesota to determine whether snow cover influences soil resource pools and rates of nitrogen cycling. Snow fences were erected and snow was removed from inside plots in January 2009. Snow was allowed to drift outside. Soil cores were taken in March (pre-thaw) and April (post-thaw) and analyzed for NO3 and NH4 concentrations to estimate rates of mineralization and nitrification in situ during freeze-thaw cycles and in laboratory incubations; CO2 flux and snow density were measured in situ. We hypothesized increased winter nutrient cycling rates due to insulating effects of snow, which could lead to a larger nutrient pool early in the growing season.   


CO2 flux was significantly higher in snow-covered soil than exposed soil in March, indicating that microbial respiration increased in soils insulated by snow. Conversely, laboratory nitrification and mineralization rates did not respond to snow removal treatment. This result implies microbes are metabolizing carbon-rich substrates under snow. Size and composition of resource pools varied between March and April, with lower NO3:NH4 ratios in March and higher in April.  Changes in NO3:NH4 should be accompanied by higher nitrification rates from March to April, however, laboratory results showed no significant increase in nitrification rates between sample dates, although in situ estimates display positive nitrification in all plots. Field mineralization rates from March to April were calculated from soil extractions and found to be negative, signifying immobilization of N. This also suggests microbes are relying on carbon-rich substrates in winter months, leading to N limitation of heterotrophic production. Other studies show similar results suggesting nitrogen retention in soil microbes resulting from respiration of substrates with a high C:N ratio under snow. Therefore, changing precipitation in response to global warming, and effects on snow accumulation, may have important implications for microbial activity and coupled C and N cycling.