Extracellular enzyme activity in soils from a temperate forest climate manipulation study demonstrate significant seasonal trends

Background/Question/Methods

Climate change is predicted to alter temperature and precipitation regimes, however responses of the soil microbial community to such changes remain poorly understood.  Microorganisms are important drivers of soil carbon cycling, in large part due to the activity of the enzymes that microbes produce to break down polymers in the soil.  Increases in temperature and moisture availability may alleviate limitations to potential enzyme activity, at least in the short term, with acclimation possible in the long term.  Our goal was to evaluate the impact of climate change on the potential enzyme activity of soil microorganisms.  Soils were collected from a climate change manipulation in Northern Michigan with both temperature (+4-5 °C) and moisture (+30% over ambient precipitation) treatments in a fully factorial blocked design.  Soils were collected monthly from May to October 2011.  Hydrolytic extracellular enzyme activities measured included cellobiodisase, β-glucosidase, N-acetylglucosaminidase and acid phosphatase.  Assays for two oxidative enzymes, phenol oxidase and peroxidase, were also performed.  Samples from all field plots were incubated at 15 °C and 20 °C in order to measure the temperature sensitivity of potential enzyme activity.  The amount of microbial biomass carbon and nitrogen were measured using chloroform fumigation extractions.

Results/Conclusions

Significant seasonal trends were apparent in all potential enzyme activity rates and temperature sensitivities.  This may be due to a drought, which began in July and peaked early in September, when potential enzyme activities were lowest.  Treatment had a significant impact on all enzyme activity rates except peroxidase activity.  The impact of the warming treatment was evaluated by comparing incubations at 20 °C for the heated treatments to the incubations performed at 15 °C for the unheated treatments.  As expected based on traditional enzyme kinetics, most potential enzyme activities were higher when the assay was performed at a higher incubation temperature.  Moisture addition treatments in the field increased potential acid phosphatase activity, regardless of temperature treatment.  N-acetylglucosaminidase activity was consistently highest in the treatment with combined temperature and moisture manipulations.  The temperature sensitivity data indicate that there has been no short-term acclimation of potential activity to changes in temperature or moisture availability in the field.  The amount of microbial biomass did not differ significantly between treatments, but also showed significant seasonal trends with biomass peaking in the middle of the growing season.  The results of this study demonstrate clear seasonality in the functions of the soil microbial community.