Short-term nitrogen balance in urine-treated areas of pasture on a yellow earth in the subhumid tropics of Queensland

Abstract

The recovery of nitrogen from cattle urine applied to a legume-grass pasture on a yellow earth at Lansdown Pasture Research Station in north-eastern Queensland was measured over periods of 8-28 d in March, July/August and October/November. Urea labelled with 15N was used as a tracer for urine nitrogen. The weather in all three experiments was dry; cumulative evaporation from an open pan was 6-25 times rainfall. The total apparent loss of 15N from the soil-plant system was 18% after 8 d in March, 32% after 28 d in July/August, and 16% after 28 d in October/November. Losses of 15N were most rapid during the first 1-2 d after application. Most of the recovered 15N was in the soil. Plant components contained only 13.5% of the applied 15N in March, 2.3% in August and 3.8% in November. Further, most of the 15N in the soil was present as mineral nitrogen, mostly exchangeable ammonium. The proportions of soil 15N in this form were 0.65 in March, 0.85 in August, and 0.83 in November. Wetting the soil resulted in poorer recovery. Thus, a single application of 50 mm of water before urine application in July increased the apparent loss of 15N after 28 d from 32 to 44%. A daily application of 5 mm water after urine application increased the loss to 50%. Apparent loss in unwatered plots where all herbage and litter had been removed was 47%.