Continental-scale patterns of extracellular enzyme activity in the
subsoil: an overlooked reservoir of microbial activity
Abstract
Stabilization of microbial-derived products such as extracellular
enzymes (EE) has gained attention as a possibly important mechanism
leading to the persistence of soil organic carbon (SOC). While the
controls on EE activities and their stabilization in the surface soil
are reasonably well-understood, how these activities change with soil
depth and possibly diverge from those at the soil surface due to
distinct physical, chemical, and biotic conditions remains unclear. We
assessed EE activity to a depth of 1 m (10 cm increments) in 19 soil
profiles across the Critical Zone Observatory Network, which represents
a wide range of climates, soil orders, and vegetation types. Activities
of four carbon (C)-acquiring enzymes (α-glucosidase, β-glucosidase,
β-xylosidase, and cellobiohydrolase), two nitrogen (N)-acquiring enzymes
(N-acetylglucosaminidase and leucine aminopeptidase), and one phosphorus
(P)-acquiring enzyme (acid phosphatase) were measured fluorometrically
along with SOC, total N, Olsen P, pH, clay concentration, and
phospholipid fatty acids, which we used to characterize the microbial
community composition and biomass (MB). For all EEs, activities per gram
soil correlated positively with MB and SOC; all of which decreased
logarithmically with depth (p < 0.05). Across all sites, over
half of the potential soil EE activities per gram soil consistently
occurred below 20 cm for all measured EEs. Activities per unit MB or SOC
were substantially higher at depth (soils below 20 cm accounted for 80%
of whole-profile EE activity), suggesting an accumulation of stabilized
(i.e., mineral sorbed) EEs in subsoil horizons. The pronounced enzyme
stabilization in subsurface horizons was corroborated by mixed-effects
models that showed a significant, positive relationship between clay
concentration and MB-normalized EE activities in the subsoil.
Furthermore, the negative relationships between soil C, N, and P and C-,
N-, and P-acquiring EEs found in the surface soil decoupled at 20 cm,
which could have also been caused by EE stabilization. This suggesting
that EEs do not reflect soil nutrient availabilities at depth. Taken
together, our results suggest that deeper soil horizons hold a
significant reservoir of EEs, and that the controls of subsoil EEs
differ from their surface soil counterparts.