A Lysimeter Study to Investigate the Effect of Dairy Effluent and Urea on Cattle Urine n Losses, Plant Uptake and Soil Retention

Loss of nitrate (NO₃ ^?) from grazing land is a major cause of surface and groundwater contamination. These losses increase when N sources such as fertilizer are applied to grazing land. The objectives of this work were to (1) study the impact of dairy effluent (DE) or urea on N losses and plant uptake when DE or urea was applied with and without cattle urine and; (2) determine the effect of organic C rich DE on the fate of urine N. The experiment was conducted using lysimeters that contained Templeton sandy loam soil extracted from a pasture in New Zealand. Application of DE resulted in significantly less (p < 0.05) NO₃ ^? leaching compared with urea in the first year, but not in the second year. Differences between years could be attributed to the comparatively lower C:N ratio of applied DE in the second year, causing relatively greater N mineralization and greater NO₃ ^? leaching. Differences could also be due to cumulative effects of DE (first year applied) on second year NO₃ ^? leaching. Total annual pasture N uptake was similar for DE and urea treatments. During the first year, the average NO₃ ^? concentration was lower when DE was combined with urine compared to urine alone, but not in the second year. The combination of DE with urine resulted in significantly greater (p < 0.01) annual pasture N uptake compared with the urine alone treatment in both years. Urine plus urea resulted in the greatest leaching losses in both years, but its impact on pasture N uptake was mixed. The total leaching loss of N from urine plus DE (90 kg N ha^?1) was similar to urine alone (77 kg N ha^?1) in the second year. Likewise, the annual percentage of ¹⁵N recovered in the leachate from urine plus DE (9%) was not significantly different from urine alone (6%). However, ¹⁵N recoveries revealed that the contribution of urine N to NO₃ ^? leaching was greater when urine was combined with DE (98.8%) compared to urine alone (83%). The greater NO₃ ^? leaching from urine when combined with DE could be a result of greater nitrification due to the low C:N ratio of DE. Additionally, the annual percentage of urine N uptake by the pasture from urine plus DE (29%) was significantly less than from urine alone (39%) (p < 0.01). The application of organic C rich DE had no significant effect on soil N retention or denitrification when combined with urine.