Report: TR 2003/15

Author: Craig Burgess, Lincoln Environmental

Abstract

As a result of increasing concerns about nitrogen (N) leaching from soils with low water holding capacity under irrigated dairying and possible contamination of receiving waters, a trial to investigate N leaching from Pumice Soils (Whenuaroa Series) was established on an irrigated commercial dairy farm at Reporoa in September 1998. The trial (which ceased in September 2002) investigated N leaching and drainage volumes over four years under four different treatments:

1. Non-irrigated dairy farming (NonIrr)
2. Dairy farming with effluent irrigation (Eff)
3. Dairy farming with water irrigation (Irr)
4. Dairy farming with water and effluent irrigation (IrrEff).

To obtain representative data from the trial farm, six barrel lysimeters (200 mm dia. x 350 mm deep) were installed in three replicate plots of each treatment, giving a total of 72 lysimeters. Leachate from barrel lysimeters was collected monthly and analysed for N (NO₃-N, NH₄-N and org. N) concentrations. To elucidate the N pathways, pasture N uptake and N inputs (fertiliser, and effluent N) were measured. Meteorological parameters allowing PET estimation, rainfall, soil moisture and soil temperature were also measured on farm. Ground water and surface water samples were also taken and analysed for mineral N concentrations.

Management of the trial farm changed over the four years of the trial, with intensification in stocking rate and fertiliser inputs from 1998 to 2002, and also changes in grazing patterns. Drainage volumes and N leaching were influenced by drier than average annual meteorological conditions over the majority of the trial. However, results generally showed that N leaching was more related to pasture N use efficiency in the different treatments, than to the total volume of water draining through the soil.

Annual data showed that despite the Irr treatment having the greatest drainage volume, it did not have the greatest amount of N leaching. Instead, the Eff treatment showed the greatest N leaching losses, which occurred predominantly as NH₄-N and org.N in the month after dairy farm effluent applications. Despite similar N loading to the Eff treatment, less N was leached in the IrrEff treatment, as water irrigation maintained pasture growth and N uptake in dry periods, resulting in more efficient use of the N applied. The Irr and NonIrr treatments also had similar N loading, however, drainage volume from the Irr treatment was much greater. Despite this, there was no significant difference in N leaching between Irr and NonIrr treatments. In the water irrigated treatments (Irr and IrrEff) the constant pasture growth and N uptake throughout dry periods is thought to have prevented a build-up of N in the soil profile. In contrast, a build-up of N is likely to have occurred in the non water-irrigated treatments (NonIrr and Eff) during dry periods. This excess N was subsequently leached below the root zone when an autumn drainage flush occurred.

Data from the final year of the trial showed that a substantial increase in mineral N fertiliser inputs across all treatments did not result in an increase in pasture growth and N uptake, resulting in increased N leaching from all treatments compared to the previous three years of the trial.

In general results indicated that the nutrient budget of the Overseer decision support model (developed by AgResearch) is useful in determining N leaching trends under different farm management practices.

Reporoa Nitrogen Leaching Trial 1998-2002: Final [PDF, 1.5 MB]