Estimating the effect of winter cover crops on nitrogen leaching using cost-share enrollment data, satellite remote sensing, and Soil and Water Assessment Tool (SWAT) modeling

W.D Hively,S Lee,A.M Sadeghi,J Keppler,I.-Y Yeo,G.E Moglen,B.T Lamb,A Soroka,G.W Mccarty

doi:10.2489/jswc.75.3.362

Abstract

This study employed a novel combination of data (winter cover crop [WCC] cost-share enrollment records, satellite remote sensing of wintertime vegetation, and results of Soil and Water Assessment Tool [SWAT] water quality simulations) to estimate the environmental performance of WCC at the watershed scale, from 2008 through 2017, in the Tuckahoe Creek watershed, located within the Choptank River basin. The Choptank River is a tributary of the Chesapeake Bay, and, as a focus watershed for the USDA9s Conservation Effects Assessment Project, has been the subject of considerable study assessing linkages between land use and water quality. Farm enrollment data from the Maryland Agricultural Cost Share (MACS) program documented a large increase in the use of WCC within the Tuckahoe Creek watershed during the study period, rising from 27% of corn (Zea mays L.) fields and 9% of soybean (Glycine max L.) fields in 2008 to 89% of corn fields and 46% of soybean fields in 2016. Satellite remote sensing of wintertime ground cover detected increased wintertime vegetation following corn crops, in comparison to full season and double cropped soybean, consistent with patterns of cover crop implementation. Although interannual variation in climate strongly affected observed levels of vegetation, with warm winters resulting in increased vegetative cover, a 30-year analysis of wintertime greenness revealed significant increases in wintertime vegetation associated with increased adoption of WCC. The MACS WCC enrollment data were combined with output from the SWAT model, calibrated to streamflow and nutrient loading from the Upper Tuckahoe watershed, to estimate water quality impacts based on known distribution of cover crop species and planting dates (2008 to 2017). Results indicated a 25% overall 10-year reduction in nitrate (NO3−) leaching from cropland attributable to cover crop adoption, rising to an estimated 38% load reduction in 2016 when 64% of fields were planted to cover crops. Results suggest that increased environmental benefits would be achieved by shifting agronomic methods away from late-planted wheat (Triticum aestivum L.), which comprised 34.7% of all WCC planted between 2008 and 2017.

Highlights

Winter cover crops (WCC) have been identified as one of the most cost-effective conservation practices for reduction of agricultural nitrogen (N) pollution in the Chesapeake Bay watershed (McCarty et al 2008; Ator and Denver 2012)
The cover crops are typically terminated the following spring to release nutrients for the subsequent cash crop. Because they help to meet important environmental targets, cover crops are strongly promoted by agricultural conservation agencies in the Chesapeake Bay region, and their use on farms has rapidly increased over the past decade
From 2008 through 2017, 91,790 ha of WCC were planted in the Tuckahoe Creek watershed—33% in the early planting category, 28% in the standard planting category (October 1 to 14), and 39% in the late planting category (October 15 to mid-November).The large majority of planted cover crop was wheat (68.1%), followed by barley (16.1%), rye (7.2%), forage radish (4.4%), triticale (Triticale hexaploide Lart.; 1.7%), and canola (1.0%), as well as clover (Trifolium spp.)/wheat mix, legume/ cereal mix, ryegrass, and spring oats (Avena sativa)

Summary

Introduction

Winter cover crops (WCC) have been identified as one of the most cost-effective conservation practices for reduction of agricultural nitrogen (N) pollution in the Chesapeake Bay watershed (McCarty et al 2008; Ator and Denver 2012). The Maryland Agricultural Cost Share Program (MACS) subsidizes the costs of planting WCC with a primary goal of reducing nutrient and sediment loss from farmland.WCC such as rye (Secale cereale L.), barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), radish (Raphanus sativus L.), and rapeseed (Brassica napus L.) are planted in the fall following the harvest of summer row crops such as corn (Zea mays L.), soybean (Glycine max L.), and vegetables As they grow, the WCC take up N that would otherwise be vulnerable to leaching over the winter, protect the soil from raindrop impact and erosion, and promote soil health (Hively et al 2009; Sharma et al 2018). Various publications detailing the linkages between landscape and water quality outcomes were generated, including such diverse subjects as fate and transport of nutrients and trace organic contaminants in 15 monitored subwatersheds (McCarty et al 2008; Hively et al 2011; Nino De Guzman et al 2012; McCarty et al 2014) as well as in the Choptank River estuary (Whitall et al 2010); the effects of current and prior converted wetland areas on nutrient loading (Denver et al 2014; Hunt et al 2014; Sharifi et al 2016; Li et al 2017; Lee et al 2017a, 2018a); use of remote sensing to map water use, wetland characteristics, and wetland connectivity to stream networks (Lang et al 2012a, 2012b; Fenstermacher et al 2014; Huang et al 2014; Sun et al 2017; Jones et al 2018;Yeo et al 2019a, 2019b); the use of remote sensing to map crop residue and tillage intensity (Hively et al 2018, 2019; Quemada et al 2018); modeling of nutrient transport,WCC, and climate change impacts on water quality using the Soil and Water Assessment Tool (SWAT; Yeo et al 2014; Lee et al 2016, 2017b, 2017c, 2018b, 2018c, 2018d); and the use of remote sensing to measure WCC conservation performance at the landscape scale (Hively et al 2009, 2015; Hunt et al 2010, 2011; Prabhakara et al 2015)

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Conclusion