Figure 4 : Conceptual model of changing controls on extracellular enzyme activity (EEA) between surface soil and subsoil. Solid lines represent fluxes and dashed lines represent moderating controls. Boxes represent pools or concentrations, and other shapes represent moderating variables. Blue parameters represent microbial parameters, and green boxes represent edaphic variables such as substrate (including carbon [C] and nutrients) and clay concentrations. The differences in the size of boxes between the surface and subsoil represent the relative size of the pool, and differences in the thickness of arrows between the surface and subsoil represent the hypothesized relative magnitude of the flux or control. A portion of the substrate pool is available to microbial biomass (MB) and is moderated by clay concentration and active EEA. Substrate availability moderates substrate demand. Bacterial biomass, fungal biomass, and substrate demand influence active EEA. Additionally, our conceptual model incorporates stabilized EEA (i.e., EEs sorbed onto clay particles), which is primarily influenced by clay concentrations. At depth, the impact of clay on substrate availability and stabilized EEA increases, while the absolute impact of substrates and microbial properties (i.e., microbial biomass and substrate demand) decreases.
SUPPLEMENTAL TABLES
Table S1 : Proportion (and standard error, n = 19) of extracellular enzyme activity (kg-1 soil) below 20 cm across the three main soil orders represented in the study. Different superscript letters represent significant differences among soil orders for each enzyme (α = 0.05). Key: AG = α-glucosidase, BG = β-glucosidase, CB = cellobiohydrolase, XYL = β-xylosidase, LAP = leucine aminopeptidase, NAG = N-acetylglucosamine, AP = acid phosphatase.