Contributions to nitrogen leaching and pasture uptake by autumn-applied dairy effluent and ammonium fertilizer labeled with N-15 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 (N H4Cl) 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). T he DSE and NH4Cl were applied twice annually in autumn (May) and late sprin g (November), each at 200 kg N ha(-1). The N applied in May 1996 was labele d 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 NH 4Cl. However, the N applied in the autumn had a higher potential for leachi ng than N applied in late spring. Between 4.5-8.1% of the N-15-labeled mine ral N in the DSE and 15.1-18.8% of the N-15-labeled NH4Cl applied in the au tumn were leached within a year of application. Of the annual N leaching lo sses in the DSE treatments (16.0-26.9 kg N ha(-1)), a fifth (20.3-22.9%) wa s from the mineral N fraction of the DSE applied in the autumn, with the re maining 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%) o f the annual N leaching loss (55.9-57.6 kg N ha(-1)) was derived from the a utumn-applied NH4Cl. DSE was as effective as NH4Cl in stimulating pasture p roduction. Since only 4.4-4.5% of the annual herbage N uptake in the DSE tr eatment 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 ha ve 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 w ere similar between the DSE and the NH4Cl treatments, but those in the leac hate were over 50% less from the DSE than from the NH4Cl treatment. The low er leaching loss of N-15 in the DSE treatment was attributed to the stimula ted microbial activities and increased immobilization following the applica tion of DSE.