Urine patch and fertiliser N interaction: Effects of fertiliser rate and season of urine application on nitrate leaching and pasture N uptake

Abstract

Urine patches are the primary source of N loss from pastoral systems due to the high N loading that occurs over a relatively small area. However, few studies have sought to determine the effect of concurrently deposited urine and fertiliser on the fate of N in pastoral systems, even though the application of fertiliser soon after grazing is commonly practised, while no studies have examined seasonal effects of any interaction. The objective of this study was therefore, to understand how the combination of fertiliser-N and urine affected fertiliser-associated NO3- leaching losses and plant uptake of N. A two year lysimeter study was undertaken with urine (800 kg N ha-1) applied in either autumn or spring. Urea fertiliser enriched with 15N was applied to these lysimeters at rates equivalent to 200 or 400 kg N ha-1 per year according to the standard regional practice. Urine and fertiliser at the 400 kg N ha-1 rate increased total NO3- leaching by up to 58 kg ha-1 (P < 0.001), from urine applied in either autumn or spring. Fertiliser applied at 200 kg N ha-1 did not increase N leaching from urine patches. Fertiliser 15N recovery in drainage was <2.2% and was not affected by fertiliser rate. Pasture uptake accounted for up to 52% of the fertiliser 15N recovery and this increased with increasing fertiliser rates, even in the presence of urine. Recovery of fertiliser 15N in the soil at the end of the experiment averaged 22% with the majority of this in the top 10 cm soil. These results indicate that the potential for leaching of fertiliser N, applied to a urine patch, is low, and that avoiding fertiliser application over urine patches, reduces leaching losses of fertiliser-N by <2%, which is minimal in terms of total N loss mitigation. However, at high fertiliser application rates to urine patches (i.e. 400 kg N ha-1), the total N leaching from non-fertiliser (non 15N-enriched) sources can increase. Further work is required to quantify these effects at the paddock scale. The results also show that NO3- leaching losses were greater from autumn applied urine compared to spring applied urine by up to 306 kg NO3--N ha-1.