Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/15/2010
Publication Date: 9/10/2010
Citation: Zhou, X., Helmers, M.J., Asbjornsen, H., Kolka, R., Tomer, M.D. 2010. Perennial filter strips reduce nitrate levels in soil and shallow groundwater after grassland-to-cropland conversion. Journal of Environmental Quality. 39:2006-2015. Interpretive Summary: Lands that have been in the Conservation Reserve Program are being returned to production as agriculture responds to increasing demand for food and biofuel. Returning these lands to crop production may negatively impact water quality. This study tracked nitrate in subsurface waters in a set of small watersheds that were converted from at least 10 years of brome grass cover to corn and soybean production. Several of the watersheds were treated by seeding prairie filter strips, located in varying slope positions and proportional amounts of cover (0-20%). The filter strips are expected to reduce erosion losses on these highly erodible slopes. However, we also found that increases in subsurface nitrate were mitigated by the prairie strips, particularly at the lower slope positions in the watersheds. Increases in nitrate occurred in all watersheds, but were comparatively smaller (< 2 ppm nitrate-N increase) where prairie strips were included than where they were not included (> 6 ppm nitrate-N increase). This information is useful to producers and policy makers who are interested in implementing practices that can mitigate water quality impacts of returning conservation reserve lands to agricultural production.
Technical Abstract: Many croplands planted to perennial grasses under the Conservation Reserve Program are being returned to crop production, and with potential consequences for water quality. The objective of this study was to quantify the impact of grassland-to-cropland conversion on nitrate-nitrogen (NO3-N) concentrations in soil and shallow groundwater, and assess the potential for prairie vegetation planted as perennial filter strips (PFS) to mitigate increases in NO3-N levels. The study, conducted at the Neal Smith National Wildlife Refuge (NSNWR) in central Iowa, consisted of a balanced incomplete block design with 12 watersheds and four watershed-scale treatments having different proportions and topographic positions of PFS planted in native prairie grasses: 100% rowcrop, 10% PFS (toeslope position), 10% PFS (distributed on toe and as contour strips), and 20% PFS (distributed on toe and as contour strips). All treatments were established in the fall of 2006 on watersheds that were under bromegrass cover for at least 10 years. Non-perennial areas were maintained under a no-till two-year corn-soybean rotation since spring of 2007. Suction lysimeter and shallow groundwater wells located at upslope and toeslope positions were sampled monthly during the growing season to determine NO3-N concentration from 2005 to 2008. The results indicated significant increases in NO3-N concentration in soil and groundwater following grassland-to-cropland conversion. NO3-N levels in the vadose zone and groundwater under PFS were lower compared to 100% cropland, with the most significant differences occurring at the toeslope position. During the years following conversion, PFS mitigated increases in subsurface nitrate, but long-term monitoring is needed to account for potential effects of climatic variation, filter strip development, and nutrient dynamics on observed patternsobserve and understand the full response to land use conversion.