N Cycling and Retention through Plant and Soil Pools in a Bottomland Hardwood Forest with Elevated N Deposition

N deposition is a major concern in ecosystems across the globe, with potential impacts on biodiversity, primary production, and biogeochemical processes. Despite some progress in reducing N deposition in the US, many areas still receive high levels of deposition from anthropogenic sources. Bottomland hardwood forests (BHFs) along the Texas Gulf coast occur in close proximity to reactive N (N_r) emission sources such as petroleum refineries and croplands. We quantified N deposition using ion exchange resin collectors, and soil and plant N pools and their δ¹⁵N values to assess the effect of deposition from a refinery and other non-point sources on N cycling and retention in a BHF. We aimed to: (i) estimate the amount and chemical composition of atmospheric inorganic-N deposition at 3 sites increasingly distant from the refinery, (ii) investigate the historic trend of plant N uptake in response to 8 decades of elevated deposition using tree-ring %N and δ¹⁵N values, and (iii) understand the underlying processes by which N is partitioned through this ecosystem by comparing δ¹⁵N values of deposition, plant tissues, and soils. Overall, N deposition across the landscape was about 3.5x higher than the national average, highest near the refinery, and dominated by NH₄-N. Tree ring δ¹⁵N values were distinctly lower in trees closest to the refinery, indicative of an anthropogenic source. Tree-ring %N and basal area index trends were strongly correlated and both increased dramatically with refinery emissions, suggesting a fertilizing effect due to N deposition. The higher proportion of NH₄-N in deposition was strongly reflected in the soil inorganic N pool. Soil δ¹⁵N values were affected by NH₄-N concentration, suggesting the control of deposited NH₄-N over soil N cycling. Despite exposure to anthropogenic N deposition for several decades, trees in this region did not show signs of N saturation such as a decline in N absorption or growth over time, indicating N retention capacity of these BHFs. Although long-term input and uptake of ¹⁵N-depleted inorganic N can accelerate N losses from the system and deteriorate environmental conditions, results suggest that these BHFs are currently strong sinks for N_r that remove large quantities of anthropogenic N emissions.