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

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, β-D- glucosidase, and β-N-acetylglucosaminidase. Additionally, denitrification rates were enhanced by seasonal flooding across all of the study pools. Collectively, these data suggest that vernal pool eco- systems may function as hotspots of leaf-litter decomposition and denitrification and play a significant role in decomposition and nutrient dynamics relative to their size

Study of the Fate of Nutrients in Chronically Acidified and Nitrogen-Enriched Watersheds: How Abiotic Sorption, Microbial Denitrification, and Consumer-Resource Stoichiometry Are Altered

Chronic acid and nitrogen (N) deposition have become major global issues as more countries become industrialized and emissions increase. The negative impacts of excess N and acidification are well known, but their combined effects on nutrient cycling in streams are less well studied. In this research, I aimed to understand how N and acid deposition alter the cycling of phosphorus (P), how the dominant microbial process of N removal is impacted, and how resource stoichiometry and acidification affect a primary consumer. Excess N deposition can induce limitation of other nutrients such as P, and acidification of watersheds can increase abiotic sorption of P by aluminum (Al) and iron (Fe) hydroxide, which can limit its bioavailability. Thus, uptake of added P was enhanced in streams receiving elevated deposition. Leaves provided a significant abiotic sink for Al, Fe, and P, suggesting that fresh leaf inputs in the Fall may play a large role in the regulation of P availability. Microbial enzyme activity indicated that excess P bound to Al on leaves may not be available for microbial use which may lead to negative effects on stream productivity. To determine the ability of microbes to remove excess N from the landscape, denitrification rates were determined across watersheds and a landscape gradient. Denitrification was positively correlated with stream nitrate concentration, but few differences were found between substrate type. Microbes possessed the potential to remove more N, but similarities between reference and treated conditions within each region suggest that N deposition is not significantly stimulating N removal, possibly due to the concurrent negative impacts of acidification. Changes in resource stoichiometry and limited capacity for excess N removal in affected watersheds led to my interest in studying the response of consumers to these stressors. Increased nutrient excretion was evident when macroinvertebrate larvae were fed P-enriched leaves. In combination with little to no growth, the results imply that nutrients are being rapidly recycled and exported but are not being assimilated, particularly in acidified water. Although the negative effects of N and acid deposition are well-studied, this research elucidates some of their combined impacts on important ecosystem processes responsible for efficient nutrient cycling