Crop recovery of labelled -15N urinary nitrogen following simulated winter forage grazing

Abstract

In Canterbury, New Zealand, sowing a spring brassica crop is a popular option for dairy cow wintering management providing feed at a time when reserves are low. However, the intensive winter forage grazing system has a high potential for nutrient loss, and gaseous and leaching losses of N increase disproportionately with stocking rate and increasing fertiliser-N use. Some studies have identified that the wintering component can represent as much as 60% of the dairy system‟s total annual N leaching but typically, from less than 15% of the dairy system area. Elevated nitrate concentrations in drinking water (>11.3mg NO3--N/L) may deem these unsafe for consumption by humans and livestock. Furthermore, elevated NO3--N concentrations in surface waters may cause excessive aquatic plant growth and algal blooms, causing eutrophication. Developing mitigation strategies and technologies to improve N cycle efficiency in soils is critical to minimising environmental damage, and ultimately improve the efficiency and sustainability of NZ agriculture. Winter forage grazing (WFC) systems represent a new challenge to reduce N losses from an intense event that occurs over the peak leaching period at times of minimal pasture growth. The use of a catch crop sowed following grazing and/or the application of the nitrification inhibitor, dicyandiamide (DCD), may offer a mitigation strategy and technology to reduce N leaching losses. We report the results of a field lysimeter study to measure the N balance of a winter application of labelled-15N urine (350 & 700 kg N/ha) after simulated WFG and its capture by late winter sowing of either oats (Avena sativa) or Italian ryegrass (Lolium multiflorum). We also measured the effect of a single application of DCD (20 kg/ha) on gaseous N2O and nitrate leaching losses, in particular. Results of the 15N balance for soil, plant, gases (N2O and N2) and leachate showed that leaching losses comprised about 30-34% of total-N applied in the urine and were similar for both rates of urine-N application and plant species. Applying DCD reduced N leaching loss by ~50% for the oats but less so for the ryegrass although establishment issues for the latter may have affected the result. Although leaching losses were relatively high, the late sowing of the oats and ryegrass meant significant leaching occurred prior to crop establishment. DCD application slowed the nitrification rate of the urine-N and enabled greater uptake of N by the crop when better growth conditions prevailed in the spring months.