Application of carbon additives to reduce nitrogen leaching from cattle urine patches on pasture

A lysimeter experiment evaluated the use of sawdust or sucrose to decrease nitrogen (N) leaching from newly deposited urine patches to a perennial ryegrass/white clover pasture on sandy loam soil. Cow urine (equivalent to 500 kg N ha-1), labelled with 15N-urea, was applied to the surface of lysimeters and sawdust (9 t ha-1) or sucrose (3, 12 or 24 t ha-1) was applied immediately afterwards. Sawdust did not decrease mineral N leaching compared with urine-only (82 kg N ha-1). Sucrose applied at 12 or 24 t ha-1 decreased mineral N leaching by 27 or 66%, compared to urine-only. Pasture dry matter yields were decreased in all sucrose treatments (range 16-29% over 247 days, but 15-73% over the first 70 days) compared with urine-only. Total recovery of 15N in pasture and soil was similar across treatments (average 66%) but the 15N partitioning between soil and pasture differed; soil recovery was 19% in the urine-only and urine + sawdust treatments, and 27-51% in the sucrose treatments, increasing with increasing rate of sucrose. In the farm situation, the risk of yield loss and the large carbon applications required may limit the practical application of this N mitigation method.