Bedrock weathering contributes to subsurface reactive nitrogen and nitrous oxide emissions

Abstract

Atmospheric nitrous oxide contributes directly to global warming, yet models of the nitrogen cycle do not account for bedrock, the largest pool of terrestrial nitrogen, as a source of nitrous oxide. Although it is known that release rates of nitrogen from bedrock are large, there is an incomplete understanding of the connection between bedrock-hosted nitrogen and atmospheric nitrous oxide. Here, we quantify nitrogen fluxes and mass balances at a hillslope underlain by marine shale. We found that at this site bedrock weathering contributes 78% of the subsurface reactive nitrogen, while atmospheric sources (commonly regarded as the sole sources of reactive nitrogen in pristine environments) account for only the remaining 22%. About 56% of the total subsurface reactive nitrogen denitrifies, including 14% emitted as nitrous oxide. The remaining reactive nitrogen discharges in porewaters to a floodplain where additional denitrification likely occurs. We also found that the release of bedrock nitrogen occurs primarily within the zone of the seasonally fluctuating water table and suggest that the accumulation of nitrate in the vadoes zone, often attributed to fertilization and soil leaching, may also include contributions from weathered nitrogen-rich bedrock. Our hillslope study suggests that under oxygenated and moisture-rich conditions, weathering of deep, nitrogen-rich bedrock makes an important contribution to the nitrogen cycle.

Does Nitrate Enrichment Accelerate Organic Matter Turnover in Subterranean Estuaries?

“…Thus, during the prolonged drop in the piezometric head of the coastal aquifer, the freshwaterseawater is displaced inland and surface seawater intrudes de aquifer. There, seawater intrusion completely changes ionic strength (and hence desorption of solutes such as ammonium from the aquifer matrix), pH, DOM content, or oxygen levels within the aquifer and can significantly increase NO 3 − levels through processes such as DOM mineralization, nitrification, and matrix weathering (Slomp and Van Cappellen, 2004;Moore and Joye, 2021;Wan et al, 2021). With the recovery of the piezometric head, this salty groundwater will be displaced toward the seepage face.…”

Rock-Sourced Nitrogen in Semi-Arid, Shale-Derived California Soils

“…Rock-derived nitrogen (N) can supplement N availability in terrestrial ecosystems (Morford et al, 2011); its contribution to the global N budget has been estimated at 6-17% (Morford et al, 2016b;Houlton et al, 2018), and in some ecosystems inputs of rock N are comparable to the flux from atmospheric fixation (Morford et al, 2016a). However, analysis of rock-derived N at the pedon scale -i.e., the scale of the soil profile and underlying bedrock -has been limited to a few humid, forested montane sites underlain by shale (Wan et al, 2021) or relatively N-rich ammonia-bearing metamorphic rock (Morford et al, 2016a). Expanding our understanding of rock N fluxes to other biomes and rock types can help constrain the importance of rock N in the nutrient budgets of natural and managed ecosystems.…”

“…However, much of the N in sedimentary bedrock resides in organic forms that cannot be distinguished from surface-derived organic N using conventional extraction methods (Morford et al, 2016a). A multi-year, intensive pore-water and gas sampling study set in the humid and montane Upper Colorado River Basin used element proxy-to-N ratios and temporally resolved subsurface water flux rates to estimate the contribution to subsurface N from the weathering of marine shale (Wan et al, 2021). Wan et al (2021) are currently the only watershed scale rock-derived N field study that focuses on N in sedimentary lithology, thus it remains unclear how much rock-derived N is released from sedimentary parent materials in other climates.…”

“…A multi-year, intensive pore-water and gas sampling study set in the humid and montane Upper Colorado River Basin used element proxy-to-N ratios and temporally resolved subsurface water flux rates to estimate the contribution to subsurface N from the weathering of marine shale (Wan et al, 2021). Wan et al (2021) are currently the only watershed scale rock-derived N field study that focuses on N in sedimentary lithology, thus it remains unclear how much rock-derived N is released from sedimentary parent materials in other climates. For example, wetter climates have higher weathering rates than drier climates (White and Blum, 1995;Kump et al, 2000).…”

“…However, using this approach for N is complicated because N is also sourced from atmospheric N deposition and biotic and abiotic N fixation (Chapin et al, 2011). Proxy elements have been used to estimate rock-derived N fluxes in the absence of direct quantification of rock-derived N in soil (Houlton et al, 2018;Wan et al, 2021). However, more common proxies (e.g., Ca and Na) can precipitate as salts in drier climates and in regions affected by coastal salt deposition (Vengosh, 2003;Zamanian et al, 2016).…”