Soil extracellular enzymes (EEs) are the principal agents of organic matter decomposition and nutrient mineralization, yet the vast majority of studies examining EE activities have been confined to only the upper soil layers (< 20 cm depth). While the controls of EE activity in the topsoil are reasonably well-understood, we know little of how these enzyme activities change with soil depth. The controls and subsequent patterns of EE activity at depth may diverge from those at the soil surface due to changing physical, chemical, and nutritional conditions in subsurface soils. We assessed EE activity to a depth of 1 m at 10 cm increments in 20 soil pits across the Critical Zone Observatory Network, which represents a wide range of hydrogeological provinces, soil orders, and biomes. Activities of four carbon (C)-degrading enzymes (α-glucosidase, β-glucosidase, β-xylosidase, and cellobiohydrolase), two nitrogen (N)-mineralizing enzymes (N-acetylglucosaminidase [NAG] and leucine aminopeptidase [LAP]), and one phosphorus (P)-mineralizing enzyme (acid phosphatase) were measured fluorometrically using 4-Methylumbelliferone (MUB)- and 7-Amino-4-methylcoumarin (MUC)-linked substrates. We used mixed effects models to investigate the effects of depth, soil order, and their interaction on EE activity and enzyme stoichiometry.
For all EEs, activities decreased exponentially with depth (p < 0.05). However, over half of potential soil EE activity occurred below 20 cm for all measured EEs. Although the effect of depth on EE activities depended on soil order for certain EEs, no generalizable patterns emerged across all EEs assessed. Depth showed varied effects on enzyme stoichiometry, which serves as a relative index of nutrient demand. Across all sites, we found no significant effect of depth on C:N or C:P enzyme stoichiometry; however, there was a significantly negative correlation between depth and N:P enzymatic activity (p < 0.001). This result suggests that P bioavailability decreases relative to N as soil depth increases across a diverse range in soils. Overall, our results suggest that assessing EE activities only in the upper 20 cm misses a large proportion of the total soil EE activity and that enzymatic relationships that are robust in the surface soil layers (e.g., 1:1 N:P enzyme stoichiometry) may change in deeper soil layers. Characterization of microbial activity with soil depth is critical in developing a more in-depth understanding of soil microbial ecology and microbial mediated biogeochemical processes.