Nitrogen and phosphorus losses in mole and tile drainage from a cattle‐grazed pasture in eastern Southland

Abstract

An experimental system for monitoring drainage outflows from mole‐ and tile‐drained plots is described, and nitrogen (N) and phosphorus (P) losses in drainage are reported for Year 1 of a 4‐year study examining nutrient losses in drainage from a pasture in Southland. Twelve plots (0.09 ha), grazed by non‐lactating dairy stock, were artificially drained by installing a mole and tile drainage network. A metering station was used to monitor drainage flow rate from six of these plots using a V‐notch weir and a shaft encoder system. Drainage water samples were collected on a flow proportional basis using either an automated water sampler triggered by the flow monitoring system, or by manual collection during daylight hours. The amount of nitrate‐N lost in drainage water in this first year of study was 25 kg N ha–1, resulting in a volume‐averaged nitrate‐N concentration of 6.9 mg N litre–1. Although this is a significant loss of potentially plant available N, the average nitrate‐N concentration of the drainage water was below the 11.3 mg N litre–1 standard adopted by the New Zealand Ministry of Health for acceptable nitrate levels in drinking water. Mean dissolved reactive P and total P concentrations in drainage waters were 23 and 74 μg P litre–1, respectively. Analysis of forms of P showed 61 % of the total P lost in the drainage was in the form of particulate P, which may reflect the recent introduction of mole and tile drainage to this site.