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, βxylosidase, 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 1

Depth dependence of climatic controls on soil microbial community activity and composition

“…It is becoming increasingly evident that microorganisms in the subsoil play important roles in soil C and nutrient cycling 5,25,29,31 , yet our understanding on the controls of microbial communities in the subsoil is limited. Here, we not only show that microbial community composition changes with depth, but also that the impact of climate, which strongly controls the surface soil microbial community composition, decreases in subsoil horizons 7,8,61,62 .…”

“…This suggests that while EE activity on a MBC or SOC basis is positively correlated with MAT and MAP 6 , this may not be the case in deeper soils. It is possible that greater clay stabilization of EE activities at depth may have reduced the in uence of climate on microbial activity because higher microbial activities would re ect soil mineralogy rather than biology 5,72,73 . Hence, our results provide another line of evidence for the diminished effect of climate on subsoil microbial communities.…”

“…Microbial community composition varies considerably throughout the soil pro le 1 likely due to edaphic factors that change with depth, including organic carbon (C) availability, nutrients, pH, and texture [2][3][4][5] . However, most studies on subsoil microbial communities do not span large ecological gradients, preventing a complete understanding of large-scale drivers of subsoil microbial communities.…”

“…Understanding the controls on microbial communities at depth is particularly important because they are relatively understudied 27 and they play a signi cant role in decomposing the ~1,400 Pg of soil organic carbon (SOC) stored below 20 cm 28 . For instance, over half of extracellular enzyme activity in the upper meter of the soil pro le occurs below 20-cm 5 . Furthermore, when warmed, subsoil respiration can account for over 40% of the increase in CO 2 emissions from the whole soil pro le 29 .…”

Identifying Mechanisms for Successful Ecological Restoration with Salvaged Topsoil in Coastal Sage Scrub Communities

“…This suggests that inoculum size may play a role in affecting microbial activity, as well as potentially influencing fungal:bacterial abundance, along with associated ecosystem functioning. Previous studies examining bacteria and soil depth have shown that microbial functions, including microbial biomass normalized potential extracellular enzyme activity [87], and the proportion of Gram + bacteria [82] in the microbial community increased with depth. Although our study extended the soil profile to include heightened topsoil thickness, rather than extending deeper into soil horizons, our findings suggest that adding topsoil could have consequences for functions driven by microbial processes, with associated implications on both aboveground and belowground ecosystems.…”