Watershed Functional Traits from Bedrock-to-Canopy Regulate the Retention and Release of Nitrogen within Mountainous Watersheds

Authors

Nicholas Bouskill¹* (njbouskill@lbl.gov), Michelle Newcomer¹, Rosemary Carroll^2,3, Baptiste Dafflon¹, Nicola Falco¹, Zelalem Mekonnen¹, Patrick Sorensen¹, Tetsu Tokunaga¹, Jiamin Wan¹, Haruko Wainwright⁴, Helen Weierbach¹, Yuxin Wu¹, Qing Zhu¹, Eoin Brodie¹, Kenneth Williams^1,3

Institutions

¹Lawrence Berkeley National Laboratory, Berkeley, CA; ²Desert Research Institute, Reno, NV; ³Rocky Mountain Biological Laboratory, Gothic, CO; ⁴Massachusetts Institute of Technology, Cambridge, MA

URLs

• https://watershed.lbl.gov/

Abstract

Mountainous watersheds are characterized by extreme variance in functional traits such as vegetation, topography, lithology, and geomorphology, which together play critical roles in determining nitrogen (N) retention and release. This research parses out the role different watershed traits play in cycling N at the hillslope and the catchment scale.

At the hillslope scale, researchers worked along a montane hillslope underlain by N-rich marine shale and quantified organic and inorganic nitrogen pools in soils, porewater, vegetation, microbial biomass, and derived from shale weathering products. The major sinks for N along the hillslope include vegetation (~13 kg m^-2, roots + shoots) and soils (~3.5 kg m^-2: 0.68–6.8 kg m^-2). While a portion of the mobile nitrogen, such as nitrate (NO₃^–), translocates from the hillslope to floodplain soils and the groundwater, a significant fraction is lost in situ through gaseous pathways as the water table ascends and recedes through hillslope soils. Nitrate stable isotope systematics show the concomitant enrichment of both δ¹⁵N_NO3 and δ¹⁸O_NO3 within the weathering zone of the hillslope, accompanying a drop in NO₃^– concentrations. Moreover, the accumulation of nitrous oxide (N₂O), simultaneous with isotopic enrichment, suggest denitrification is likely responsible for gaseous NO₃^– loss. Interestingly, N₂O fluxes at the soil surface are negligible (-3.7±2.4 g m^-2 yr^-1), indicating a strong sink for N₂O (to N₂) between the weathering zone and atmosphere.

At the catchment scale, researchers compared two representative but contrasting catchments that differ markedly in total NO₃^– export. Coal Creek is underlain by granitic rock and sandstone, with a conifer-dominated land cover, while seven kilometers away, the East River has a diverse vegetation cover, sinuous floodplains, and is underlain by N-rich marine shale. However, the N-cycles of these catchments differ starkly. The stable isotope ratios of NO₃^– (δ¹⁵N_NO3 and δ¹⁸O_NO3) demonstrated that conifer-dominated Coal Creek retained nearly all (~97 %) atmospherically deposited NO₃^–. By contrast, the East River showed stronger biogeochemical processing of NO₃^– prior to export. The conservative N-cycle within Coal Creek is likely due to the abundance of conifer trees and a smaller riparian region, retaining more NO₃^– overall and reduced processing prior to export. The East River catchment is a strong hotspot for nitrogen cycling, with NO₃^– reduction at the soil-saprolite interface and within the floodplain, buffering the export of NO₃^–.

These studies aim to highlight the value of integrating isotope systematics to link watershed functional traits to mechanisms of watershed element retention and release—an endeavor that becomes more important in the face of changing climate.