The effect of time of winter urine deposition on mineral nitrogen leaching and implications for grazed forage crops

Grazing kale or swedes in winter can generate a disproportionately large part of a farm’s nitrogen (N) leaching. We tested if (a) N leaching is lower if crops are grazed towards the end of winter because of less drainage after grazing, and (b) if process inhibitors targeted at decreasing N leaching only need to treat the early grazing because of this differential risk. Small plots treated with artificial urine (600 kg N ha-1) with or without dicyandiamide (DCD) and instrumented with porous ceramic cups, were used to simulate grazing of a winter forage crop in early June or late July. The same experiment was established in an adjacent paddock of ryegrasss/white clover. The sites were on a Taupo pumice soil near Mangakino, Central Plateau. Mineral N (Nmin) leached by the end of the first winter was remarkably similar for pasture and ex-forage crop and was affected by ‘grazing’ time: c. 185 vs. 60 kg N ha-1 for early and late urine applications respectively (400-450 mm drainage), i.e. a 58% reduction. Nitrogen leaching was incomplete after the first drainage season, with 170 kg N ha-1 (early urine) or 300 kg N ha-1 (late urine) remaining in the soil (0-600 mm) in spring at the ex-forage crop site. These effects were not as large in the pasture (149 vs. 198 kg N ha-1), presumably because of the added effect of pasture N uptake removing some of the soil Nmin. A chicory crop established after the forage crop or the pasture could not fully utilise this residual soil Nmin in the following spring/summer and there was an additional 54-118 kg N ha-1 leached in the following winter. This negated some of the benefit seen in year 1 but, as a sum of both years, late grazing decreased N leaching by 35-40%. Evidence of a DCD effect was mixed. Porous cup sampling was unable to detect a DCD effect on N concentrations but soil measurements at the end of drainage in year 1 on the ex-forage crop site showed more soil Nmin present after DCD was applied to the early urine, supporting our hypothesis. The results show that on this Taupo pumice soil not all mineral N is leached in the first winter, even from an early June grazing, and that it is important to utilise as much of the residual soil Nmin as possible to avoid it leaching in the following year. This is particularly challenging under summer dry conditions.