Nitrogen Budget and Topographic Controls on Nitrous Oxide in a Shale-Based Watershed

The high spatial and temporal variabilities of nitrous oxide (N₂O) emissions from the soil surface have made it difficult to predict flux patterns at the ecosystem scale, leading to imbalances in nitrogen (N) budgets at all scales. Our research sought to quantify topographic controls on the sources or sinks of N₂O in the soil profile to improve our ability to predict soil-atmosphere N₂O fluxes and their contribution to watershed N budgets. We monitored surface-to-atmosphere N₂O fluxes for 2 years in the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania. Topographically convergent flow path locations had significantly higher surface N₂O flux rates than nonconvergent flow path locations in the summer, but not other seasons. Overall, N₂O fluxes were a large percentage (~19%) of total ecosystem N losses, and nearly twice as large as stream N export. Surface N₂O fluxes were better correlated with concentrations of O₂, N₂O, and NO₃ ⁻ in shallow soil layers (<30 cm) than deeper soils. Following decades of anthropogenic atmospheric deposition and additional N from shale weathering, watershed N inputs (~8 kgN ha⁻¹ yr⁻¹) are greater than outputs (~3.7 kgN ha⁻¹ yr⁻¹). Our research revealed patterns of N cycling that are distinct from many other watersheds that have been extensively studied to understand N saturation; despite showing no other symptoms of N saturation, the watershed had high upland N₂O losses, especially in convergent flow paths during summer. High upland N gas losses may be a mechanism that maintains N limitation to biota in the Shale Hills catchment.