Using satellite remote sensing to estimate winter cover crop nutrient uptake efficiency

Abstract

Winter cover crops are recognized as an important agricultural conservation practice for reducing nitrogen (N) losses to groundwater following the summer growing season. Accordingly, cost-share programs have been established to promote winter cover crops for water quality on farms throughout the Chesapeake Bay watershed. However, current estimates of cover crop nutrient uptake are largely calculated from plot-scale studies extrapolated to watershed-scale based solely on enrollment acreage. Remote sensing provides a tool for rapid estimation of cover crop biomass production on working farms throughout the landscape. This project combined cost-share program enrollment data with satellite imagery and on-farm sampling to evaluate cover crop N uptake on 136 fields within the Choptank River watershed, on Maryland's eastern shore. The Normalized Difference Vegetation Index was a successful predictor of aboveground biomass for fields with >210 kg ha⁻¹ (>187 lb ac⁻¹) of vegetation (corresponding to 4.2 kg ha⁻¹ [3.7 lb ac⁻¹] of plant N), below which the background reflectance of soils and crop residues obstructed the cover crop signal. Cover crops planted in the two weeks prior to the regional average first frost date (October 15) exhibited average fall aboveground N uptake rates of 18, 13, and 5 kg ha⁻¹ (16, 12, 4 lb ac⁻¹) for rye, barley, and wheat, respectively, corresponding to 1,260, 725, and 311 kg ha⁻¹ (1,124, 647, 277 lb ac⁻¹) of aboveground biomass, with associated cost-share implementation costs of $5.49, $7.60, and $19.77 kg⁻¹ N ($2.50, $3.46, and $8.99 lb⁻¹ N). Cover crops planted after October 15 exhibited significantly reduced biomass and nutrient uptake, with associated program costs of $15.44 to $20.59 kg⁻¹ N ($7.02 to $9.36 lb⁻¹ N). Agronomic factors influencing cover crop performance included species, planting date, planting method, and previous crop. Field sampling locations with >1,000 kg ha⁻¹ (>890 lb ac⁻¹) of springtime cover crop biomass exhibited greatly reduced soil nitrate (<3 mg kg⁻¹ [<3 ppm]) in comparison to fields with low cover crop biomass (up to 14 mg kg⁻¹ soil nitrate), indicating a target biomass threshold for maximum water quality impact. Additional sampling years will be necessary to account for cover crop response to climate variability. Combining remote sensing with farm program data can provide important information to scientists and regulators working to improve conservation programs. Results can be used to more effectively utilize scarce conservation resources and increase water quality protection.