Litter decomposition and nutrient turnover in temperate forests is partially controlled by the microbial production of extracellular enzymes (exoenzymes) in soil. In particular, those enzymes involved in lignocellulose and chitin breakdown, and the liberation of organically bound N and P, play an important role in belowground C cycling and nutrient dynamics in these ecosystems. Nevertheless, seasonal patterns and environmental controls on enzyme activity are poorly understood. Because of high production costs and potential turnover of these enzymes, we expected activities to be low during cold or snow-covered months in response to low microbial activity, and that warm-season activity would be controlled by soil organic matter quality and water availability. To test this, we established six long-term study plots in an old-growth Beech-Maple forest in NE Ohio. Plots were a maximum of 150 m apart. Composite soil cores from each plot were collected monthly between November 2006 and November 2007, and the potential activity of 6 exoenzymes was measured for each composite sample, as well as soil C and N pools. Soil temperature and herbaceous ground cover data was also collected.
Results/Conclusions
Exoenzyme activity was not significantly different across season or month, and activity was not correlated with soil temperature. Over the sampling period, mean activities were 132.3 (± 12.6) nmol g soil-1 hr-1 for cellulase enzymes (i.e. β-glucosidase, α-glucosidase, β-xylosidase, and cellobiohydrolase), 172.9 (± 33.6) for chitinase (i.e.N-acetylglucosaminidase), and 2181.4 (± 193.7) for phosphatase. Plot location did have a significant effect on all enzymes except α-glucosidase, but the effect was driven by variability in one plot. Only β-glucosidase activity was significantly positively correlated (p = 0.002) with soil moisture content. With the exception of cellobiohydrolase, all measured enzymes were positively correlated (p < 0.05) with soil C and N content; however, none were correlated with the C/N of soil organic matter. Percent herbaceous ground cover was not correlated with enzyme activity, but all enzymes were positively correlated (p < 0.05) with richness of the understory plant community. Our results imply that, 1) temperature and snow cover has less of an effect on soil microbial activity and exoenzyme production than previously thought, and/or 2) that these enzymes are highly conserved and stabilized in soil organic matter during periods of low microbial activity. Future work needs to address the role of winter processes on temperate forest biogeochemistry and the role of understory herbs in mediating microbial processes.
