Effect on nitrate leaching of plant N uptake in the perimeter of a urine patch

Abstract

Intensification of New Zealand’s grazed pasture systems over the past decade has brought focus upon the negative impacts of agriculture on the environment. The important issue of nitrate leaching losses is currently driving policy and regulation that will significantly influence the way agricultural practices are carried out in the future. Urine patches are the leading cause of nitrate leaching losses, as urinary N concentrations far exceed plant N uptake capacity. Previous studies have suggested that the effective area of a urine patch is larger than the directly wetted area. However, the effect of this on nitrate leaching has not been studied. Therefore, the aim of this research was to investigate the effect on nitrate leaching of plant N uptake in the perimeter of a urine patch. A lysimeter study was conducted at Lincoln University using 28 lysimeters collected from a perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture on a Templeton silt loam soil. Of the lysimeters, 14 represented the wetted area of a urine patch (300 mm diameter) and 14 represented a urine patch and perimeter grass (500 mm diameter). Within these two groups 7 lysimeters had 0.8 L of cattle applied to their central 300 mm diameter area, equivalent to an N loading rate of 700 kg N/ha. The remaining 7 control lysimeters were treated with 0.8L water applied to the central 300 mm diameter area. Natural rainfall was supplemented with simulated rainfall applied, first at the 75th and then at the 90th percentile of historical average rainfall for the area. Drainage water was collected once to twice per week, or as required following a significant rainfall event and the leachate was analysed to measure the NO3- - N concentration. Results showed a 45% reduction in leaching losses from urine patches when perimeter grass was included. However, plant N uptake, dry matter production and botanical composition results indicate that the primary cause of this reduction was not plant N uptake in the perimeter of the urine patch, but differences in soil moisture. Due to the design of the sprinklers used there was an error in irrigation application leading to higher application and drainage volumes from the 300 mm diameter lysimeters than the 500 mm diameter lysimeters. In order to account for this, results were presented on a NO3- - (kg/ha) loss per 100 mm of drainage basis. However, the unequal irrigation amounts had a profound effect on pasture growth, therefore it cannot be determined whether plant N uptake in the perimeter of the urine patch has an effect on nitrate leaching. It is recommended that this trial be repeated using a re-designed, lysimeter diameter-specific irrigation sprinkler system.