title: Metolachlor metabolite (MESA) reveals agricultural nitrate-N fate and transport in Choptank River watershed.
abstract: Over 50% of streams in the Chesapeake Bay watershed have been rated as poor or very poor based on the index of biological integrity. The Choptank River estuary, a Bay tributary on the eastern shore, is one such waterway, where corn and soybean production in upland areas of the watershed contribute significant loads of nutrients and sediment to streams. We adopted a novel approach utilizing the relationship between the concentration of nitrate-N and the stable, water-soluble herbicide degradation product MESA {2-[2-ethyl-N-(1-methoxypropan-2-yl)-6-methylanilino]-2-oxoethanesulfonic acid} to distinguish between dilution and denitrification effects on the stream concentration of nitrate-N in agricultural subwatersheds. The ratio of mean nitrate-N concentration/(mean MESA concentration * 1000) for 15 subwatersheds was examined as a function of percent cropland on hydric soil. This inverse relationship (R(2)=0.65, p<0.001) takes into consideration not only dilution and denitrification of nitrate-N, but also the stream sampling bias of the croplands caused by extensive drainage ditch networks. MESA was also used to track nitrate-N concentrations within the estuary of the Choptank River. The relationship between nitrate-N and MESA concentrations in samples collected over three years was linear (0.95 ≤ R(2) ≤ 0.99) for all eight sampling dates except one where R(2)=0.90. This very strong correlation indicates that nitrate-N was conserved in much of the Choptank River estuary, that dilution alone is responsible for the changes in nitrate-N and MESA concentrations, and more importantly nitrate-N loads are not reduced in the estuary prior to entering the Chesapeake Bay. Thus, a critical need exists to minimize nutrient export from agricultural production fields and to identify specific conservation practices to address the hydrologic conditions within each subwatershed. In well drained areas, removal of residual N within the cropland is most critical, and practices such as cover crops which sequester the residual N should be strongly encouraged. In poorly drained areas where denitrification can occur, wetland restoration and controlled drained structures that minimize ditch flow should be used to maximize denitrification.
authors.affiliation: United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Henry A. Wallace Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, MD 20705, USA.
authors.forename: Gregory W
authors.initials: GW
authors.lastname: McCarty
authors.affiliation: United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Henry A. Wallace Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, MD 20705, USA. Electronic address: cathleen.hapeman@ars.usda.gov.
authors.forename: Cathleen J
authors.initials: CJ
authors.lastname: Hapeman
authors.affiliation: United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Henry A. Wallace Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, MD 20705, USA.
authors.forename: Clifford P
authors.initials: CP
authors.lastname: Rice
authors.affiliation: United States Geological Survey (USGS), Eastern Geographic Research Center, stationed at USDA-ARS, Beltsville, MD, USA.
authors.forename: W Dean
authors.initials: WD
authors.lastname: Hively
authors.affiliation: United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Henry A. Wallace Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, MD 20705, USA.
authors.forename: Laura L
authors.initials: LL
authors.lastname: McConnell
authors.affiliation: United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Henry A. Wallace Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, MD 20705, USA.
authors.forename: Ali M
authors.initials: AM
authors.lastname: Sadeghi
authors.affiliation: USDA Forest Service (USFS), Northern Research Station, stationed at USDA-ARS, Beltsville, MD, USA.
authors.forename: Megan W
authors.initials: MW
authors.lastname: Lang
authors.affiliation: National Oceanographic and Atmospheric Administration (NOAA), National Centers for Coastal Ocean Science, Center for Coastal Monitoring & Assessment, 1305 East West Highway, Silver Spring, MD 20910, USA.
authors.forename: David R
authors.initials: DR
authors.lastname: Whitall
authors.affiliation: United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Henry A. Wallace Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, MD 20705, USA.
authors.forename: Krystyna
authors.initials: K
authors.lastname: Bialek
authors.affiliation: United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Henry A. Wallace Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, MD 20705, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Downey
mesh_terms: D000081:Acetamides / Q000032:analysis*; D000383:Agriculture; D004784:Environmental Monitoring; D063366:Estuaries; D006540:Herbicides / Q000032:analysis*; D009566:Nitrates / Q000032:analysis*; D045483:Rivers / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D000081:Acetamides; D006540:Herbicides; D009566:Nitrates; D014874:Water Pollutants, Chemical; C051786:metolachlor
keywords: 2-[2-ethyl-N-(1-methoxypropan-2-yl)-6-methylanilino]-2-oxoethanesulfonic acid; APFO; ARS; Aerial Photography Field Office; Agricultural Research Service; CBP; Chesapeake Bay; Chesapeake Bay program; Choptank River; MDE; MESA; Maryland Department of the Environment; NAS; NRCS; National Academy of Science; Natural Resources Conservation Service; Nitrate-N; Poorly-drained upland; SSURGO; SWCS; Soil Survey Geographic; Soil and Water Conservation Society; TMDL; USDA; USEPA; United States Department of Agriculture; United States Environmental Protection Agency; Well-drained upland; total maximum daily load