A lysimeter study of the fate of15N‐labelled nitrogen in cow urine with or without farm dairy effluent in a grazed dairy pasture soil under flood irrigation

Abstract

Nitrate (NO 3 ^– ) leaching from cow urine patches is considered to be a major contributor to the overall NO 3 ^– leaching loss in grazed dairy pastures. Farm dairy effluent (DE) is usually applied to grazed pastures to recycle nutrients. The objective of this study was to determine the fate of cow urine nitrogen (N), labelled with ¹⁵N, applied to soil monolith lysimeters with or without DE application. The soil was a Templeton fine sandy loam (Haplustepts), and the pasture was perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Cow urine was applied in the autumn at 1000 kg N ha^–1 either alone or with DE (400 kg N ha^–1 yr^–1 split into four applications). The lysimeters were flood irrigated. One year after application, 6.4–9.1% of the ¹⁵N‐labelled urine N was lost by leaching, 29.3–38.8% was removed in the cut pasture, 45.7–47.5% remained in the soil and plant roots, and 15N‐labelled urine N in the pasture top compared with the Urine alone treatment. The amount of N unaccounted for was slightly higher in the Urine + DE (13.9%) treatment than in the Urine alone treatment (5.3%), probably due to increased denitrification as a result of organic carbon inputs from the DE. The urine N application significantly suppressed the clover component of the pasture, and this has implications for N budgeting on grazed dairy pastures. The amount of ¹⁵N lost by leaching from the Urine alone treatment confirms that this is a major contributor to the overall effect of dairying on drainage water quality.