Quantifying and reducing nitrogen leaching under intensive vegetable production in Horowhenua : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand

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Massey University
Figures 5.2 and 5.17 are reproduced with permission.
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Vegetable growers often need to apply relatively large quantities of nitrogen (N) fertiliser to successfully grow their crops, and this can lead to large losses of N via runoff and/or leaching to receiving water. Growers are under increasing pressure to reduce N leaching. Therefore, practices that increase N use efficiency and mitigate N losses are receiving increasing attention from growers, regulators, and scientists both in New Zealand and worldwide. For instance, high N levels have been observed in the Arawhata stream and Lake Horowhenua in the Arawhata catchment, where most of the fresh vegetables consumed in the lower North Island of New Zealand are grown. There is currently little research data available which quantifies N leaching from different practices of vegetable production systems in New Zealand, including for the Arawhata catchment. Therefore, the first objective of this study was to identify the likely range of N leaching rates under vegetable production in the Arawhata catchment. Nitrate-N in soil and N losses were measured under a series of crops at two field trial sites which were established on two local farms: a site with a potato-onion rotation (Potatoes site) and a site under a beetroot-Pak choi rotation (Green vegetables site). Soil nitrate-N was sampled at depths of 0-30 and 30-60 cm at the Potatoes site, and at 0-20 cm at the Green vegetables site. Mean soil nitrate-N in the topsoil (0-20 and 0-30 cm) ranged between 9.3 and 18.3 ppm, while mean soil nitrate-N in the subsoil (30-60 cm) ranged between 7.3 to 9.6 ppm. N leaching was measured using suction cups and lysimeters. Mean nitrate-N concentrations in soil water ranged between 16.9 to 61.9 mg L⁻¹, and N leaching ranged from 95 to 225 kg N ha⁻¹. These field measurements of N leaching were used to calibrate and validate the APSIM Next Generation model. APSIM simulates N dynamics, including leaching, under crops at the paddock scale. APSIM was then used to investigate N leaching rates under a wider range of climate conditions (31 years), the effect of soil type on leaching, and to quantify the ability of different mitigation strategies to reduce N losses under intensive vegetable farming in the region. The mitigation scenarios at the Potatoes site included: potato harvest at maturity and ryegrass cultivation during the drainage season, and potato harvest at maturity and cultivation of winter catch crops. The mitigation scenarios at the Green vegetables site included: potential of drainage management, a range of alternative cover crops and catch crops instead of ryegrass, and other crop rotations. These mitigation scenarios resulted in an N leaching reduction of 7 to 47% in the Potatoes site, and of 6 to 52% in the Green vegetables site. The final objective of this research was to evaluate the impact of field-scale mitigation measures on the Arawhata stream by coupling the N losses simulated in APSIM with a catchment-scale model (SWAT+). This simulation exercise suggested that by considering crop rotations that use less N fertiliser and frequently grow cover crops, N loads in the Arawhata stream could be reduced by 17% to 21%.
nitrate losses, water quality, diffuse pollution, vegetable farming, mitigation, APSIM, crop rotations, cover crop, N fertiliser, SWAT