Nitrate leaching and pasture production from different nitrogen sources on a shallow stoney soil under flood-irrigated dairy pasture

Abstract

The leaching of nitrate (NO₃^–) in intensive agricultural production systems, e.g. dairy pastures, is a major environmental concern in many countries. In this lysimeter study we determined the amount of NO₃^– leached following the application of urea, dairy effluent, urine returns, and pasture renovation to a freedraining Lismore stony silt loam (Udic Haplustept loamy skeletal) growing a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture. The study showed that NO₃^–-N leaching losses ranged from 112 to 162 kg N/ha per year, depending on the amount and forms of N applied and pasture conditions. Nitrate leaching under the urine patches was the main contributor to the N leaching loss in a grazed paddock. Nitrate leaching losses were lower for urine applied in the spring (29% of N applied) than for urine applied in the autumn (38–58%). The application of urea or dairy effluent only contributed a small proportion to the total NO₃^– leaching loss in a grazed paddock. Pasture renovation by direct-drilling may also have caused an increase in NO₃^– leaching (c. 31 kg N/ha) in the first year. Modelled annual average NO₃^–-N concentrations in the mixed recharge water in the acquifer were significantly lower than those measured under the rooting zone due to dilution effects by recharge water from other sources (3.9 v. 13–27 mg N/L). Herbage nitrogen offtake and dry matter yield were higher in the urine treatments than in the non-urine treatments.

groundwater, denitrification, mineralisation, grazing, forage.