Depth and topographic controls on microbial activity in a recently burned sub-alpine catchment

Microbial communities influence and are influenced by environmental conditions that, together with the extracellular enzymes produced by soil microorganisms, control the rate of decomposition of organic matter in soil. Here, we aim to characterize the interaction of landscape position and depth on potential enzyme activities in a recently burned forest catchment. To accomplish this, we first characterized the heterogeneity of environmental properties, including topography, depth, and soil geochemistry, in order to delineate landscape position and depth controls on potential enzyme activities. To account for the impact of recent wildfire on extracellular enzyme activities (EEA), we delineated surface (0–5 cm) and deeper (5–40 cm) soils to understand how fire (which disproportionally impacts the surface) alters the relationship between EEA and the environmental covariates. We excavated 22 soil pits to 40 cm and measured potential activities of seven hydrolytic enzymes involved in carbon (C) (α-glucosidase [AG], β-1,4-glucosidase [BG], β–D-cellobiohydrolase, [CB] and β-xylosidase [XYL]), nitrogen (N) (β-1,4,N-acetylglucosaminidase, [NAG] and leucine-aminopeptidase [LAP]) and phosphorus (P) acquisition (acid phosphatase [PHOS]) across a subalpine catchment. Fire resulted in decreased BG, CB and NAG activity in surface (0–2 cm) soils. Fire altered N and P acquisition strategies with depth suggesting potential nutrient scavenging or increased internal microbial cycling with depth as a response to fire. Digital soil mapping demonstrated consistently higher potential enzyme activities in the convergent zones of the catchment, which were primarily correlated with higher soil moisture, clay content, and vegetative cover as quantified through normalized difference vegetation index (NDVI). Integrating remotely sensed measures of topography with the identification of drivers of microbial C, N, and P cycling can help inform how millimeter-scale processes influence and feedback to patterns at a catchment scale.