A lysimeter study of the impact of cow urine, dairy shed euent, and nitrogen fertiliser on nitrate leaching

Abstract

The effect of cow urine, dairy shed effluent (DE), and urea fertiliser on nitrate leaching was determined using undisturbed soil lysimeters (500 mm diameter by 700 mm deep) with ryegrass (Lolium perenne)–white clover (Trifolium repens) pasture. Cow urine was applied to the lysimeters, at rates of 0 and 1000 kg N/ha in May 1996. Urea and DE were applied to urine-applied and non-urine-applied lysimeters at rates of 0, 200, and 400 kg N/ha in 4 split equal applications in May, August, and November 1996 and February 1997. Natural rainfall was supplemented with simulated rainfall in winter and spring (May–October) to achieve the 75th percentile of winter–spring rainfall records in the region. Flood irrigation was applied 6 times during summer–autumn (November–April) at 100 mm per application, which is a typical practice used by dairy farmers in the region. Drainage water was collected and analysed for nitrate, nitrite, bromide (added tracer), and ammonium.

Over the first year of the experiment (May 1996–April 1997), 12% of the urine-N applied was lost through leaching, mainly in nitrate form. When urine (1000 kg N/ha) was applied in combination with DE (200 kg N/ha) and urea (200 kg N/ha), the mineral N leaching loss increased to 14% of the total N applied. The annual average nitrate concentrations in the drainage from the lysimeters that received urine alone, or urine+DE and/or urea, were 33–57 mg N/L, with a mean peak concentration of 110 mg N/L. These nitrate concentrations were significantly higher than in those treatments that did not receive urine (1–5 mg N/L).

Because, on average, about 25% of the area of a grazed dairy paddock receives urine per year, the field-scale leaching losses were calculated by taking into account the dilution effect of drainage water from non-urine patch areas of the paddock. The calculated annual paddock losses were 33–60 kg N/ha, and on average the annual paddock nitrate concentrations were 10–17 mg N/L. This demonstrates the importance of accounting for the dilution of nitrate in the leachate from non-urine patch areas of the paddock.

The annual average concentration from the treatment DE at 400 kg N/ha was significantly lower than that from the urea treatment at the same rate. This was probably because of the different chemical forms of N in each material, and needs to be taken into account when developing regional rules for land application of urea and effluents.