Field-scale nutrient loss assessment following cover crop and manure rate change

Eutrophication due to elevated nitrogen (N) and phosphorus (P) loss from croplands remains one of the most pressing water quality issues throughout the world. Understanding the effect of implementing conservation management practices is critical for meeting nutrient reduction goals as well as informing conservation programs and policies. A before-after-control-impact (BACI) analysis was used to evaluate the individual and combined effect of cover crops and manure application rate on discharge and nutrient loss using six water years (WY2014-WY2019) of measured data across four distinct drainage zones (1X-NCC; 1X-CC; 2X-NCC; 2X-CC) within an Ohio, USA, crop production field. White mustard significantly reduced mean monthly nitrate (NO₃^--N) concentration regardless of manure application rate (i.e., 65 m³ ha^-1 and 130 m³ ha^-1). However, neither the use of white mustard, doubling manure rate, or the combination of the two had a significant impact on mean monthly drainage discharge, dissolved-reactive P (DRP), or total P (TP) loss. Seasonal analysis confirmed that NO₃^--N concentration in the cover crop zones was signficantly less in fall, winter, and spring. However, significant increases in spring discharge, NO₃^--N, DRP, and TP loads as well as TP concentration were noted with cover crop and greater manure rate treatments. These findings confirm that cover crops have a reducing effect on NO₃^--N concentration but may not have any effect on addressing P concerns. Further research is warranted; however, this study highlights that the resource concern (e.g., N or P) should be considered prior to implementing cover crops as a conservation management practice.