Biogeochemical Hotspots in Forested Landscapes: The Role of Vernal Pools in Denitrification and Organic Matter Processing

Quantifying spatial and temporal heterogeneity in ecosystem processes presents a challenge for conserving ecosystem function across landscapes. In particular, many ecosystems contain small features that play larger roles in ecosystem processes than their size would indicate; thus, they may represent "hotspots" of activity relative to their surroundings. Biogeochemical hotspots are characterized as small features within a landscape that show comparatively high chemical reaction rates. In northeastern forests in North America, vernal pools are abundant, small features that typically fill in spring with snow melt and precipitation and dry by the end of summer. Ephemeral flooding alters soil moisture and the depth of the soil's oxic/anoxic boundary, which may affect biogeochemical processes. We studied the effects of vernal pools on leaf-litter decomposition rates, soil enzyme activity, and denitrification in vernal pools to assess whether they function as biogeochemical hotspots. Our results indicate that seasonal inundation enhanced leaf-litter decomposition, denitrification, and enzyme activity in vernal pools relative to adjacent forest sites. Leaves in seasonally flooded areas decomposed faster than leaves in terra firme forest sites. Flooding also influenced the C, N, and P stoichiometry of decomposing leaf litter and explained the variance in microbial extracellular enzyme activity for phosphatase, beta-d-glucosidase, and beta-N-acetylglucosaminidase. Additionally, denitrification rates were enhanced by seasonal flooding across all of the study pools. Collectively, these data suggest that vernal pool ecosystems may function as hotspots of leaf-litter decomposition and denitrification and play a significant role in decomposition and nutrient dynamics relative to their size.