Newswise — A warming climate and an increase in nitrogen to the soil from the atmosphere have demonstrated large effects on soil microbial activity and biomass. A new scientific study looks at these effects while taking into account seasonal variation. Their findings showed that warming and nitrogen increased microbial biomass, but the effects of nitrogen were only observed in the summer. This research demonstrates the importance of annual sampling.

The goal of experiment was to determine how warming and nitrogen addition affect soil microorganisms across seasons. Researchers at the University of Western Ontario and the University of Guelph conducted the study in a temperate old-field, funded by the Natural Sciences and Engineering Research Council of Canada. Specifically, they examined how the experimental treatments affected microbial biomass, fungal to bacterial ratios and the microbial enzyme activity. Results from the study were reported in the May-June issue of the Soil Science Society of America Journal, published by the Soil Science Society of America. Preliminary results of the research were also presented in Milwaukee, WI at the 93rd ESA Annual Meeting in August 2008. The finding that warming and nitrogen both increased microbial biomass, but that effects of nitrogen only occurred in the summer, highlight the importance of sampling across multiple time points in order to account for seasonal variation. The increase in microbial mass was attributed to warming effects from both winter and summer months, suggesting that climate changes impacts on winter can be felt next summer. Terrence Bell, primary author of the project, further emphasizes that soil research should focus more on seasonal trends. “Studies that aim to predict the effects of global change on soil biology and nutrient cycling must take an annual sampling approach. So many aspects of an ecosystem change between seasons, things such as temperature, nutrient availability, and microbial community composition, that it is impossible to determine the overall effects of a treatment outside the context of this seasonal change”. Soil microbial enzyme activity shifted dramatically between seasons, and was lower throughout the growing season than it was when sampled at near-freezing temperatures. The mechanism for this response is unknown, but may result from an increase in enzyme production by microbes as cold temperatures reduce their enzyme efficiency. Further studies are ongoing at the University of Western Ontario to determine how warming and nitrogen affect other aspects of the old-field ecosystem across seasons and between years. This type of long-term analysis will lead to much stronger predictions of how temperate ecosystems will respond to future changes in climate and nitrogen pollution. The full article is available for no charge for 30 days following the date of this summary. View the abstract at

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Soil Science Society of America Journal (May/June 2010)