Decomposition of organic matter in soils is driven by microbially produced extracellular enzymes (exoenzymes). Exoenzyme activities are regularly assayed, but the physical location of enzyme activity within the soil matrix is not often known, and is a growing area of interest. The physical locations of decomposition are important for our mechanistic understanding of decomposition and for understanding how exoenzyme producers take up decomposition products. Exoenzymes may float freely in soil pore water, be membrane-bound to live or dead microbial cells, or bound to minerals, large compounds such as tannins, or organic matter particles. Here we test the hypothesis that substantial exoenzyme activity is driven by exoenzymes that are bound to soil components rather than free-floating in the soil pore water. To test this hypothesis, we ran a β-glucosidase activity assay on three components of soils from an arctic tussock tundra ecosystem near Barrow, Alaska. The three components were soil pore water collected via lysimetry, subsamples of whole soil cores homogenized in a blender with a buffer solution, and the same blended buffer solution where the larger (> ~0.1 mm) organic and mineral components were filtered out using a coarse gauze.
We observed extremely low β-glucosidase activities in soil pore water (< 0.01 nmol g soil-1 hr-1) suggesting that exoenzymes are not floating freely in soil pore water. In soil core samples, we observed overall higher rates, and a substantial difference between total soil (26.2 ± 9.8 nmol g soil-1 hr-1 ) and the gauze-filtered fraction (6.6 ± 1.8 nmol g soil-1 hr-1). The higher rates in the total soil suggest that a substantial fraction—but not all—of the β-glucosidases are associated with larger solid components of the soil. These results indicate that it is possible to use chemical separation techniques to isolate soil exoenzymes at least to some extent and that substantial differences in activity rates are likely to be observed. The blending process used may even lyse cells, meaning that some "exo" enzymes may in fact be intracellular. The association of higher activity rates with the coarse unfiltered fraction suggests substantial association between exoenzyme activity and the soil matrix or substrates. These binding associations may help the microbial producers of these enzymes to gain first access to decomposition products.