Comparison of initial wetting pattern, nutrient concentrations in soil solution and the fate of15N-labelled urine in sheep and cattle urine patch areas of pasture soil

Abstract

Three field experiments were carried out to compare cattle and sheep urine patches in relation to (i) initial wetting pattern and volume of soil affected, (ii) soil solution ionic composition and (iii) the fate of15N-labelled urine in the soil over the winter period. The distribution of Br− (used as a urine tracer) across the soil surface and down the profile was irregular in all the patches. The pasture area covered by Br− in the sheep patches was 0.04–0.06 m2 and Br− was detected to a depth of 150 mm. Cattle patches were significantly larger covering a surface area of 0.38–0.42 m2 and penetrating to a depth of 400 mm. The rapid downward movement of urine occurred through macropore flow but even so, over half of the applied Br− was detected in the 0–50 mm soil layer in both sheep and cattle patches. Due to the larger volume of urine added to the cattle patches (2000 mL for cattle and 200 mL for sheep) the effective application rate was about 5 L m−2 compared with 4 L m−2 for sheep. Concentrations of extractable mineral N and ionic concentrations in soil solution were higher in cattle than sheep patches particularly near the soil surface. In both sheep and cattle patches, urea was rapidly hydrolysed to NH 4 + and nitrification occurred between 14 and 29 days after urine application. Initially the major anions and cations in the soil solution were HCO 3 − , SO 4 = , Cl−, NH 4 + , Mg++, K+ and Na+, which were derived from the urine application. Ionic concentrations in the soil solution decreased appreciably over time due to plant uptake and possibly some leaching. As nitrification proceeded, NO 3 − became the dominant anion in soil solution and the major accompanying cation was Ca++. The fate of15N-labelled urine-urea was followed during a 5 month period beginning in late autumn. Greater leaching losses of NO 3 − occurred below cattle patches (equivalent to 60 kg N ha−1 below 300 mm and 37 kg N ha−1 below 600 mm) compared with sheep patches (10 kg N ha−1 below 300 mm and 1 kg N ha− below 600 mm). While 6% of the applied15N was leached the amount of N leached was equivalent to 11% of the applied urine-N in cattle patches. This suggests that there was significant immobilsation-mineralisation turnover in urine patch soil with the release of mineral N from native soil organic matter. In both sheep and cattle patches 60% of the15N was accounted for in plant uptake, remaining in the soil and leaching. About 40% of the applied N was therefore lost through gaseous emission.