Print PageMicrobes substantially control many biogeochemical processes in semiarid systems, including carbon and nitrogen fixation and carbon mineralization. Bacteria and fungi are osmotrophs that release enzymes into the environment to generate assimilable carbon and nutrients from organic particles. These enzymes are also the first agents to respond to pulses of soil moisture. The capacity to stabilize extracellular enzymes on soil particles preserves the utility of these nutrient-generating agents during extended dry periods. Enzyme stability can relate to environmental conditions and increase with clay mineral and humic compound concentrations. To better understand microbial response to precipitation variability, our objective was to determine the stability of extracellular enzymes under various monsoon precipitation regimes. During summer 2010, soil enzyme activity was measured in a rainfall manipulation study within a mixed-grass semiarid grassland in New Mexico, USA. Plots received either one large rain event or three evenly spaced small rain events per month. Before and after the first rain of each month, soil samples from the rhizosphere and from interspaces between plants were collected and analyzed for activity of four hydrolases; beta-glucosidase, beta-N-acetylglucosaminidase, leucine aminopeptidase, and alkaline phosphatase.
Results/Conclusions
The capacity of soil to stabilize enzymes during dry intervals was assessed by calculating the response ratio based on activity measured at the beginning and end of each large drought. Response ratios <1 indicated loss of activity while ratios >1 indicated net increases in activity. Early in the growing season, enzyme activity was fairly stable over droughts in both watering treatments with response ratios from 0.6 to 1.3. Late in the season stability across droughts declined, resulting in more variable response ratios from 0.7 to 3.7. Beta-glucosidase activity became more dynamic in interspaces between plants, indicating losses of plant carbon inputs. Compared to beta-glucosidase, beta-N-acetylglucosaminidase was more dynamic in interspaces and in rhizospheres of grasses receiving frequent, small precipitation, indicating greater processing of nitrogen from microbial cell wall material. Nitrogen generated by small precipitation events, especially late in the season, was presumably unavailable to plants. Overall, enzyme stability was greatest in the rhizospheres of grasses experiencing less frequent, larger rain events that supported plant growth. Enzymes were potentially stabilized on increased cell wall organic matter in these regions. In general, enzyme activity was related to the spatial distribution of grass roots, seasonal variation in soil carbon inputs, and precipitation variation.
