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.