Fluxes of agricultural nitrogen and metolachlor metabolites are highly correlated in a first order stream in Maryland, USA

Clifford Rice; W Dean Hively; Gregory W McCarty; Cathleen J Hapeman

doi:10.1016/j.scitotenv.2020.136590

Fluxes of agricultural nitrogen and metolachlor metabolites are highly correlated in a first order stream in Maryland, USA

Sci Total Environ. 2020 May 10:716:136590. doi: 10.1016/j.scitotenv.2020.136590. Epub 2020 Jan 8.

Authors

Clifford Rice¹, W Dean Hively², Gregory W McCarty³, Cathleen J Hapeman³

Affiliations

¹ US Department of Agriculture (USDA), Agricultural Research Service (ARS), Beltsville Agricultural Research Center (BARC), 10300 Baltimore Avenue, Beltsville, MD 20705, USA. Electronic address: clifford.rice@usda.gov.
² United States Geological Survey (USGS), Lower Mississippi-Gulf Water Science Center, 10300 Baltimore Avenue, Beltsville, Maryland, 20705 USA.
³ US Department of Agriculture (USDA), Agricultural Research Service (ARS), Beltsville Agricultural Research Center (BARC), 10300 Baltimore Avenue, Beltsville, MD 20705, USA.

PMID: 32044477
DOI: 10.1016/j.scitotenv.2020.136590

Abstract

Nitrogen pollution in watersheds containing significant cropland area is generally problematic. Conservation practices intended to reduce nitrate-N (NO₃^--N) export from watersheds are being implemented by many regions without necessary tools to assess effectiveness of abatement. A commonly used herbicide metolachlor degrades in the vadose zone of croplands to form two metabolites (metolachlor ethane sulfonic acid (MESA) and metolachlor oxanilic acid (MOXA)) which are both highly soluble in soils. Study of metabolite fates in a first order watershed provided evidence that transport of these metabolites to stream water is highly correlated to transport of agricultural NO₃^--N that also forms in the cropland vadose zone. Linear models describing the relationships of stream flux of MESA and MOXA to NO₃^--N flux generated goodness of fit values of 0.93 and 0.81, respectively. These findings support a conclusion that both MESA and MOXA act as excellent transport analogs of NO₃^--N and become strongly correlated to agricultural NO₃^--N leaching from the cropland vadose zone. Moreover, their use as conservative tracers in agricultural watersheds can provide valuable information concerning movement and fate of agricultural nitrogen at watershed scales of observation.

Keywords: Agriculture; MESA; MOXA; Metolachlor metabolites; Nitrate transport.