ABSTRACT:

Dryland cereal production in the Northern Great Plains requires efficient water use. Historically, the management practice of summer fallow, in which arable land is left idle and weeds are controlled chemically, has been used as a means of mitigating drought induced crop failure. However, summer fallow can also create economic inefficiencies, increase soil erosion, and facilitate nutrient loss to groundwater. Alternative management practices that promote more effective soil water retention offer a potential solution by promoting beneficial conditions, supporting continuous cropping. Maximizing dryland cereal crop stubble using alternative harvest methods (e.g., “stripper header” technology) is known to improve snow capture and reduce evaporative losses, but impacts of these practices on the isotopic composition of soil water remain unclear. At two sites in central and southwest Montana, we combined meteorological monitoring, snowpack analysis, and multiple measures of soil water content and isotopic composition to assess these relationships. In southwest Montana during the winter of 2022-2023, we observed significantly more snowpack and snow water equivalent (SWE) with increased stubble height until late spring, when a large storm eliminated differences. At the central Montana site, we used soil water isotopic composition (δ18O, δ2H values) to quantify snow contributions and evaporative loss. At this site, during the low-snow winter of 2023-2024, we observed mixed results of snow accumulation, with more accumulation in taller stubble in January during a small snowfall event, but equal snow accumulation in February, following a separate event. The greater snow accumulation in taller stubble during the January snowfall event led to soil water δ18O and δ2H values (0-10 cm) more closely resembling the isotopic composition of the snow in tall versus short stubble, reflecting an addition of 2.3 cm of snow-derived water into the tall stubble soil profile. During summer sampling events, increased evaporation altered soil water δ18O and δ2H values, with a stronger evaporative signature at 10 cm depth and 1.2 cm greater water loss across the sampled soil profile (0-30 cm) in short stubble compared to tall stubble following a short period of reduced precipitation and warmer temperatures. Overall, our results demonstrate variability in snow accumulation in tall stubble as a function of residue management and snow delivery and show how soil water isotopic composition reflects gains and losses of soil water – caused by stubble height management - in dryland agricultural settings of central Montana.

ABSTRACT:

In agricultural landscapes, increasing groundwater nitrate concentrations are common and reflect leaching from cultivated soils, often into adjacent riparian zones within stream corridors. High nitrate concentrations may be attenuated in riparian groundwater, where abundant organic matter and saturated anoxic soils and sediments (collectively “substrate”) support denitrifying activity. Variable substrate and the resulting residence time distribution in shallow groundwater can drive redox status and net nitrate removal, yet can be challenging to simulate in detail. This study explores how spatial variation in the texture of riparian aquifer substrate may influence groundwater residence times and biogeochemical behaviour of a riparian aquifer subject to chronic nitrate loading from non-irrigated wheat production in the semiarid Northern Great Plains. This is addressed using a novel combination of physicochemical measurements, geophysical observation of a groundwater tracer injection, and a simplified groundwater mixing model analysis. Higher and more variable nitrate concentrations were documented in wells completed in coarser substrates, suggesting generally shorter residence times compared to finer substrates, which exhibited lower nitrate concentrations suggestive of longer residence times. Therefore, we hypothesised that net nitrate consumption could be captured with a simple simulation approach using (a) the proportion of finer-textured riparian aquifer substrate to quantify redoximorphic processes that result in net nitrate consumption and (b) the proportion of coarser riparian substrate to quantify the groundwater residence time distribution. We tested this hypothesis by first exploring spatial patterns in groundwater chemistry and hydraulic characteristics at 16 shallow (< 1.5 m) wells and then by directly observing residence time of solutes in groundwater flow, using high frequency monitoring of groundwater specific conductivity and time-lapse electrical resistivity tomography imaging of a cross-section of a riparian groundwater flow path. Mixing models informed by geophysical imaging and tracer breakthrough constrain the potential influence of exchanges between fine and coarse substrates on the net nitrate transformation occurring along riparian groundwater flow paths. Dual-textured groundwater mixing model simulations illustrate how the proportion of coarse textured material may dictate the total amount of flow through riparian substrate while the proportion of flow through the finer-textured material that mixes with flow through coarse material may dictate the extent of net nitrate consumption processes. This work leverages novel geophysical observations to contribute a simple bimodal approach exploring how hydrologic complexity in riparian subsurface flow systems may influence the overall potential of riparian groundwaters to process nutrients before watershed export.