Soil processes are often measured during the growing season due to accessibility and the high amount of biological activity above and within soils at that time, however belowground processes continue throughout the year. The shoulder seasons (fall and spring) and winter produce dynamic environmental conditions because of rapidly changing temperature and moisture conditions along with reduced vegetation inputs that pose challenges to soil microbial communities. Microorganisms are the primary contributors to decomposition and their enzyme activity is the rate limiting step in decomposition. Enzymes are sensitivity to environmental conditions, specific to their substrate, and indicate changes in microbial nutrient demand. We hypothesized that enzyme activity would be greatest during summer due to large vegetation inputs along with increased competition from plants for nitrogen and phosphorus. During the shoulder seasons we expected nitrogen and phosphorus acquiring enzymes to be maintained at a greater level than carbon acquiring enzymes because of reduced nutrient availability and fluctuating conditions resulting in physiological stress. We collected soils monthly from March 2016- February 2018 at a field site in Natural Bridge State Park, VA and assayed the activity of eight different enzymes involved in carbon, nitrogen, and phosphorus cycling.
Results/Conclusions
Activity was significantly greater for all enzymes during the summer compared to winter as anticipated (p < 0.05), likely due to the large microbial biomass supported by nutrient inputs and appropriate environmental conditions. Additionally, the activity levels in spring, fall, and winter were not different from one another. This lack of difference could be attributed to numerous factors, one of which being decreased enzyme turnover with lower temperatures or decreased nutrient demand because of a smaller microbial biomass. Looking more closely at seasonal dynamics, activity for most enzymes responded positively to increasing temperatures (p < 0.001) but surprisingly there was no effect of soil moisture. Temperature was also a significant predictor of nutrient demand with enzyme acquisition ratios indicating an increased nitrogen demand relative to carbon and phosphorus as temperature increased (p< 0.05) but no effect of moisture. These results indicate that shoulder seasons are an important moment for microorganisms where their nutrient demands for carbon, nitrogen, and phosphorus shift with temperature. A shift towards warmer temperatures with climate change could result in increased nitrogen competition between microorganisms and plants during shoulder seasons.