Soil and stream corridor biogeochemistry of nitrate and sulfate and the influence of hydrologic connectivity in an agricultural system, Judith River Watershed, Montana

Abstract

Human activities across landscapes alter physical and chemical properties of soil, thereby influencing the movement and chemical composition of soil water. Soil hydrologic and biogeochemical processes thus mediate how land management influences the quality of water that passes through soil in route to groundwater aquifers and streams. Riparian areas are particularly important in mitigating the consequences of land use for the quality of water exported from watersheds. Soils, sediments, and shallow aquifers within riparian areas of stream corridors provide low oxygen environments favoring microbial transformation of solutes for energy and carbon. Despite the limited areal extent compared to the rest of the landscape, this unique biogeochemical character is disproportionately important in determining inorganic solute export, particularly in agricultural systems. For this dissertation, I investigated sources and fate of soil water in an area managed primarily for production of cattle and non-irrigated wheat within the semi-arid Northern Great Plains of North America. I explored patterns in solute concentrations and isotopic compositions across stream corridors draining cultivated soils to infer dominant hydrologic transport and biogeochemical pathways influencing solute loading to ground and surface waters. I investigated the influence of agricultural practices and soil weathering on biogeochemical processes influencing solutes, to answer the overarching research question: How do upland soils and stream corridors influence water and solute loading from upland aquifers to stream channels in a semi-arid landscape managed for agricultural production? Results show that soils are important mixing reservoirs for seasonally variable sources of precipitation, and that water movement through soil transports nitrate and sulfate from cultivated soils. Stream corridors receiving these inorganic solutes from upland groundwaters facilitate biogeochemical pathways of production, transformation, and irreversible loss. Changes in the isotopic composition of these solutes relative to changes in their mass abundance inform gross production and loss in stream corridors at the catchment scale, revealing both internal production of sulfate and/or nitrate and more substantial nitrate loss than indicated by net changes between uplands and streams. Geomorphic constraints on hydrologic connectivity and the arrangement of riparian soils and sediments determine how stream corridors mitigate the consequences of land use on downstream water quality.