Preferential flow, nitrogen transformations and N-15 balance under urine-affected areas of irrigated and non-irrigated clover-based pastures

Urine-affected areas can lead to considerable losses of N by leaching, ammonia volatilisation and denitrification from dairy pastures in the southeast of South Australia. Potable groundwater supplies are considered to have become contaminated by nitrate as a result of leaching from these leguminous pastures. Dairy cow urine, labelled with N-15 urea, was applied to micro-plots and mini-lysimeters installed in two adjacent irrigated (white clover-rye grass) and non-irrigated (subterranean clover-annual grasses) paddocks of a dairy farm on four occasions representing different seasonal conditions. These experiments allowed measurement of nitrogen transformations, recovery of N-15 in the pasture and soil, and leaching below various depths. Gaseous losses were calculated from the nitrogen balance. The results of the four experiments showed that within a day of urine application up to 40% of the applied urinary-N was leached below a depth of 150 mm as a result of macropore flow in the irrigated paddock, and up to 24% in the non-irrigated one. After application to the irrigated paddock 17% of the urinary-N moved immediately below 300 mm but only 2% below the 450-mm depth. The urinary-N remaining in the soil was converted from urea to ammonium within a day regardless of season. Within the first 7 days of application six times more nitrate was produced in summer than in winter. This has obvious implications for leaching potential. Leaching of N-15 from the top 150 mm of soil, following urine applications in all seasons, was between 41% and 62% of the applied N-15 in the irrigated paddock and 25-51% in the non-irrigated paddock. However, leaching losses measured at depths of 300 or 450 mm were smaller by a factor of 2-4. The leaching loss of N-15 applied in spring in both paddocks was 41% below 150 mm and 12% below 450 mm. Recovery of N-15 from the soil-plant system in the 450-mm-deep lysimeters was similar to 60% of that applied. Estimated ammonia volatilisation was similar to 9% of applied N-15 With no paddock or season effect. No denitrification was evident in summer nor in the non-irrigated paddock in winter but 12% of the applied N-15 was lost due to denitrification following winter application to the irrigated paddock. Estimated N-15 loss due to denitrification from urine applied in spring was similar to 13% of that applied and no difference was found between paddocks. The combination of mini-lysimeters, micro-plots and N-15 measurements enabled the nitrogen budget to be determined during four periods throughout the year.