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

Abstract

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.

Soil CO2 and O2 Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

“…To account for the different diffusivities of the gases, all plots of CO 2 vs. O 2 in this manuscript contain a theoretical relationship line with a slope of –0.76 and an x intercept of 20.95 (concentration of O 2 in atmospheric air). Plotting O 2 on the x axis and CO 2 on the y axis is a departure from the convention in some past publications (Kim et al, 2017; Weitzman and Kaye, 2018), but we show it this way to make a direct comparison to ARQ. Plotted in this way, the absolute value of the slope of the regression line for CO 2 vs. O 2 yields an estimate of the prevailing ARQ at the site.…”

“…For example, Hodges et al (2019) found significant potential for iron respiration in subsurface soils during summer in a forested catchment in the piedmont of South Carolina; the authors attributed this Fe reduction to soil moisture at depth limiting O 2 diffusion in the clayey B horizon during periods of high root respiration. At Shale Hills, Weitzman and Kaye (2018) documented subsurface N 2 O production, another pathway of anaerobic respiration that could generate the observed high CO 2 (relative to O 2 ) at LRMS. Furthermore, other researchers at Shale Hills found relatively high Fe (26.7 ± 3.0 μmol L −1 ) and Mn (34.8 ± 3.2 μmol L −1 ) in surface waters in early October and comparatively low concentrations in April through June (Herndon et al, 2018).…”

“…For example, researchers in high‐N systems recorded a decrease in soil RQ (respiration quotient, CO 2 produced for O 2 consumed), which they attributed to nitrification (Müller et al, 2004; Tsutsui et al, 2015). However, the effect of nitrification on ARQ is likely negligible at the SSHCZO due to the low potential net nitrification rates found in these watersheds (Weitzman and Kaye, 2018).…”

Observing and Simulating Spatial Variations of Forest Carbon Stocks in Complex Terrain

“…Biome‐BGC's default N deposition rate is 0.0001 kgN m −2 . The observed N deposition (Wet and Dry Inorganic N + Dissolved Organic N) rate is reported to be 0.00071 kgN m −2 (Weitzman & Kaye, ). Biome‐BGC's default N fixation rate is 0.0004 kgN m −2 .…”

“…Shi et al (, ) and Baldwin et al () have shown predictable patterns of soil moisture, which are closely related to soil physical properties and topographic features. Weitzman and Kaye () calculated the N budget in the watershed and indicated that the watershed is N limited even given the relatively high N deposition that exists at present. Smith et al () and Orr () documented the spatial differences in aboveground and belowground C stocks, respectively, associated with complex terrain.…”