Contributions to nitrogen leaching and pasture uptake by autumn-applied dairy effluent and ammonium fertilizer labeled with 15N isotope

The objective of this study was to compare the N leaching loss and pasture N uptake from autumn-applied dairy shed effluent and ammonium fertilizer (NH4Cl) labeled with N-15, using intact soil lysimeters (80 cm diameter, 120 cm depth). The soil used was a sandy loam, and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The DSE and NH4Cl were applied twice annually in autumn (May) and late spring (November), each at 200 kg N ha(-1). The N applied in May 1996 was labeled with N-15. The lysimeters were either spray or flood irrigated during the summer. The autumn-applied DSE resulted in lower N leaching losses compared with NH4Cl. However, the N applied in the autumn had a higher potential for leaching than N applied in late spring. Between 4.5-8.1% of the N-15-labeled mineral N in the DSE and 15.1-18.8% of the N-15-labeled NH4Cl applied in the autumn were leached within a year of application. Of the annual N leaching losses in the DSE treatments (16.0-26.9 kg N ha(-1)), a fifth (20.3-22.9%) was from the mineral N fraction of the DSE applied in the autumn, with the remaining larger proportion from the organic fraction of the DSE, soil N and N applied in spring. In the NH4Cl treatments, more than half (53.8-64.8%) of the annual N leaching loss (55.9-57.6 kg N ha(-1)) was derived from the autumn-applied NH4Cl. DSE was as effective as NH4Cl in stimulating pasture production. Since only 4.4-4.5% of the annual herbage N uptake in the DSE treatment and 12.3-13.3% in the NH4Cl treatment were derived from the autumn-applied mineral N, large proportions of the annual herbage N uptake must have been derived from the N applied in spring, the organic N fraction in the DSE, soil N and N fixed by clover. The recoveries of N-15 in the herbage were similar between the DSE and the NH4Cl treatments, but those in the leachate were over 50% less from the DSE than from the NH4Cl treatment. The lower leaching loss of N-15 in the DSE treatment was attributed to the stimulated microbial activities and increased immobilization following the application of DSE.