Bridging fields and floodplains: hydrologic connection, nitrate reduction, and water storage across landscapes managed for dryland cereal production in central Montana

Abstract

In the Judith River Watershed of Central Montana, chronically high and increasing nitrate concentrations in shallow groundwater have motivated decades of research. This work has quantified nitrate leaching from dryland agricultural soils into adjacent uncultivated stream corridors. In this thesis I investigated both the influence of riparian substrate on nitrate attenuation patterns within these stream corridors and soil water storage patterns as a function of crop residue management, a control on nitrate leaching into the groundwaters. High-nitrate terrace groundwaters flowing into stream corridors are subject to nitrate attenuation, as abundant organic matter and hydric soils support anaerobic biogeochemical activity. We hypothesized that variation in the sedimentary substrate hosting riparian aquifers controlled these biogeochemical conditions and flow dynamics, with finer textured substrate increasing nitrate transformation. We tested this hypothesis in a 700-m reach of a stream corridor draining the Moccasin Terrace (near Moccasin, Montana USA), simulating groundwater dynamics, constrained by geophysical surveys, to assess how substrate interactions control nitrate attenuation. We explored how the observed plume and associated nitrate concentrations were consistent with finer textured substrate regulating nitrate attenuation, and coarser textured substrate regulating water movement. In the upland agricultural soils that supply nitrate to stream corridors, management practices that promote water use efficiency, such as increasing the height of the standing crop stubble following harvest, support continuous cropping rotations which limits nitrate leaching loss. We hypothesized that increased stubble height would entrap more snow during the winter, providing additional soil moisture during spring melt, and reduce evaporative losses of shallow soil water. We used a time series of soil water isotope measurements to quantify soil water storage and loss processes relative to seasonal temperature, snow retention and melt, and pulse crop growth. Our observations suggest that tall stubble enhances snow entrapment and limits evaporative soil water losses, but the magnitude of the effect depends on meteorological conditions of the water year. Taken as a whole, my work demonstrates how distinct controls on water balance and flux in these agricultural landscapes dictate outcomes of nutrient use inefficiencies enhanced or imparted by both landscape character and human management.