Convective transport of enzymes through soil columns

Plant roots and soil microorganisms produce hydrolytic extracellular enzymes to acquire nutrients via transformation of organic matter. Microorganisms inhabit hydraulically active pores, being attached to their surfaces or to organic and mineral colloids in soil solution. Therefore, diffusion of enzymes due to Brownian motion is constrained by their interactions with the surfaces of soil particles dispersed in the solution. It is generally unknown to which extent the extracellular enzymes are associated with solid and liquid soil phases and whether enzyme motility is affected by the movement of colloids occurring in soil solution. Therefore, the goal of our study was to quantify enzyme transport in soils with contrasting properties. Transport of ß-glucosidase, acid-phosphatase, xylosidase and cellobiohydrolase was studied in undisturbed non-sterile columns of soils with three contrasting textures: sandy, sandy loam and loam. The colloids, microorganisms, and enzymes inherent for each soil were applied via soil suspensions to the tops of the undisturbed columns. The suspensions were prepared by dispersing 1 g of each soil in 100 ml of de-ionized water, followed by 30 min sedimentation. Approximately 2.5 pore volumes of the applied suspensions were passed through the columns with continuous collection of the effluent from the bottom of the columns. The effluent was analyzed for colloid contents and enzyme activities before and after removal of soil particles of size 1-10 μm by centrifugation. From 7 to 49% of applied colloids recovered from the columns with higher colloid retention capacity in finer textured soils. The enzyme activity and colloid content were the highest in the first portions of the effluent and decreased as more suspension passed the columns, suggesting presence of enzymes and colloids in soil pores readily available for convective transport. Removal of soil particles of size 1-10 μm from the effluents by centrifugation reduced enzyme activity by factors 2-5, which was much larger than reduction in the enzyme solutions free of colloids centrifuged at the same settings (24- 30%). Our results indicated that most enzymes are present and transported through soil pores convectively while attached to soil colloids. 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.