Whether enzyme activity is the same in different soil pores

Soil microorganisms preferably occupy intermediate-sized pores, which are the arena for most biochemical reactions due to high nutrient contents and beneficial air and water regimes in these pores. Extracellular enzymes produced by microorganisms for organic matter transformations are assumed to reside in the same pores. However, there is a lack of direct experimental evidence of enzymatic activity being associated with pores of particular sizes. In this study we measured activity of ß-glucosidase in soil pores Ø<14 μm, 14<Ø<164 μm and Ø<164 μm. Undisturbed soil cores (5 cm Ø, 5 cm height) were taken at continuous sorghum (G2), switchgrass (G5) and restored prairie (G10) treatments of KBS Great Lakes Bioenergy Research Center's Biofuel Cropping System Experiment. Soil cores were drained at 500 kPa, and undisturbed subsamples (0.8 cm Ø, 1 cm height) were taken from them and placed at ceramic plates connected to a vacuum system. The same quantities of 4-Methylumbelliferyl β-D-glucoside were applied to soil subsamples at vacuum corresponded to saturation of the three studied groups of pores and kept in soil for 30 min. Produced 4-methylumbelliferone (MUF) was then extracted from the soil. The results demonstrated that the enzyme activity increased in all groups of pores in the order G2<G5<G10, but was significantly different only between G2 and G10 treatments for Ø<14 μm pores (p<0.05). The enzyme activity was lower in Ø<14 μm pores than in the 14<Ø<164 and Ø<164 μm pores (p<0.05), with only numerically higher activity in Ø<164 μm compared to the 14<Ø<164 μm pores. Interconnectivity of pores drained at different matrix pressures likely results in an overestimation of the enzyme activity associated with Ø<14 μm pores and an underestimation of that associated with 14<Ø<164 μm pores, hence the observed differences among the pore sizes are likely smaller than the actual pore effects. Thus, the results support our earlier observations and confirm that the activity of extracellular enzymes is higher within intermediate-sized pores than within fine pores. Support for this research was provided by the USDA NIFA Program (Award # 2019-67019-29361), by the NSF LTER Program (DEB 1027253) at the Kellogg Biological Station, by USDA NC1187 project, by the Great Lakes Bioenergy Research Center, U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under Award Number DE-SC0018409.