Soil sedimentation and quality within the roadside ditches of an agricultural watershed

Roadside ditches are an integral component to the >6.3 million km of roadsides in the U.S. and act as drainageways for millions of hectares of watershed runoff. Our study of six roadside ditches in Lime Creek watershed characterized soil nutrients and heavy metal patterns as well as quantified the physical and hydrological properties of ditch soils. At all ditch sites, we identified significant sedimentation of silt-sized particles, total nitrogen, and soil carbon in shallow roadside ditch soils. A post-settlement surface soil horizon significantly higher in silt content was observed compared to the underlying subsoil and parent material. Although accumulation of several heavy metals was measured in ditch soils, significant variability was not observed within the ditch environment. Most of the heavy metal concentrations were found to be either similar to or lower than state-wide averages. Higher levels of calcium near the roads were likely due to annual use of road deicers. Overall, we estimated that 42 Mg/ha of total carbon and 5 Mg/ha of total nitrogen are being stored in agricultural ditch soils, which is similar to that of surrounding agricultural land in terms of total carbon storage, but much higher than estimates of total nitrogen storage. Our study of six roadside ditches in an eastern Iowa watershed documented the soil chemistry, morphology, and sediment accumulation that occurred since ditch construction. Further research is needed to develop a better understanding of how the soil and water conditions in the ditches related to the watershed areas that feed them.

Agricultural conservation planning framework: 1. Developing multipractice watershed planning scenarios and assessing nutrient reduction potential

Spatial data on soils, land use, and topography, combined with knowledge of conservation effectiveness, can be used to identify alternatives to reduce nutrient discharge from small (hydrologic unit code [HUC]12) watersheds. Databases comprising soil attributes, agricultural land use, and light detection and ranging-derived elevation models were developed for two glaciated midwestern HUC12 watersheds: Iowa's Beaver Creek watershed has an older dissected landscape, and Lime Creek in Illinois is young and less dissected. Subsurface drainage is common in both watersheds. We identified locations for conservation practices, including in-field practices (grassed waterways), edge-of-field practices (nutrient-removal wetlands, saturated buffers), and drainage-water management, by applying terrain analyses, geographic criteria, and cross-classifications to field- and watershed-scale geographic data. Cover crops were randomly distributed to fields without geographic prioritization. A set of alternative planning scenarios was developed to represent a variety of extents of implementation among these practices. The scenarios were assessed for nutrient reduction potential using a spreadsheet approach to calculate the average nutrient-removal efficiency required among the practices included in each scenario to achieve a 40% NO-N reduction. Results were evaluated in the context of the Iowa Nutrient Reduction Strategy, which reviewed nutrient-removal efficiencies of practices and established the 40% NO-N reduction as Iowa's target for Gulf of Mexico hypoxia mitigation by agriculture. In both test watersheds, planning scenarios that could potentially achieve the targeted NO-N reduction but remove <5% of cropland from production were identified. Cover crops and nutrient removal wetlands were common to these scenarios. This approach provides an interim technology to assist local watershed planning and could provide planning scenarios to evaluate using watershed simulation models. A set of ArcGIS tools is being released to enable transfer of this mapping technology.

Letting wet spots be wet: restoring natural bioreactors in the dissected glacial landscape

In this paper, we argue that there is tremendous potential for nitrate-N reductions to occur throughout the Corn Belt region of the USA if we simply let naturally occurring wet spots on the landscape be wet. Geologic and hydrologic data gathered in the Walnut Creek watershed located in south-central Iowa provides compelling evidence that substantial nutrient-processing capacity exists in this dissected glacial landscape. Self-similarity of stratigraphy, sedimentology and hydrology observed at all spatial scales in the watershed suggests that Holocene alluvial fill deposits provide a natural bioreactor for denitrification of upland groundwater nitrate-N; the occurrence of such deposits can be mapped to identify potential nitrogen sinks across the landscape. This approach to identifying potential nitrogen sinks is geology focused and extends potential locations for nutrient processing upstream into the headwater catchments of individual fields.