Watershed Functional Traits from Bedrock-to-Canopy Regulate the Retention and Release of Nitrogen within Mountainous Watersheds
Nicholas Bouskill1* (firstname.lastname@example.org), Michelle Newcomer1, Rosemary Carroll2,3, Baptiste Dafflon1, Nicola Falco1, Zelalem Mekonnen1, Patrick Sorensen1, Tetsu Tokunaga1, Jiamin Wan1, Haruko Wainwright4, Helen Weierbach1, Yuxin Wu1, Qing Zhu1, Eoin Brodie1, Kenneth Williams1,3
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2Desert Research Institute, Reno, NV; 3Rocky Mountain Biological Laboratory, Gothic, CO; 4Massachusetts Institute of Technology, Cambridge, MA
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 δ15NNO3 and δ18ONO3 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– (δ15NNO3 and δ18ONO3) 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.