Evaluating Strategic Retention of Artificial Drainage Flows for Nitrate-N Reduction under Waikato Conditions

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Report: TR 2006/45
Author: Greg Barkle, Aqualinc Research Ltd

Abstract

This project investigates a mitigation strategy of retaining drainage flows in the soil profile during the autumn period to reduce nitrate-N leaching from artificial drainage under dairy farming in the Waikato.

A literature review of field drainage experiments in New Zealand confirmed that the highest nitrate-N concentrations occur in early autumn and decline through to spring. This trend in declining nitrate-N concentrations is interrupted if nitrate-N generating process such as fertilizer or grazing occurs. Such processes increase the size of the potential leachable nitrate-N pool.

Based on published data, it is estimated that the maximum potential nitrate-N that could be removed by this mitigation strategy would be 10% of the annual nitrate-N exported through the artificial drainage system. While it is difficult to estimate what fraction of this potential would be realised in the field, it is unlikely to be more than 50%. This would indicate that only about 5% of the nitrate-N exported from artificial drainage would be removed by implementing a strategy of strategic retention of drainage waters. As nitrate-N concentrations in drainage waters are related to fertiliser application, grazing patterns, soil water characteristics, and weather patterns, without field trials this estimate should be treated as an indicative value only.

While this strategy of strategic retention of drainage flow can be implemented relatively easily on-farm, it still requires considerable input of resources for farmer education and promotion before it is likely to be widely adopted. The implementation of the strategy also somewhat increases the risk of short-term flooding but, with good management, this risk should be minimal.

The reduction in nitrate-N exported has been estimated to be limited. It is therefore an unlikely candidate for Environment Waikato to promote as an important drainage management strategy. Regardless, it does have a place amongst the tools that could be considered as best management strategies for farmers wanting to reduce nitrate-N export from artificial drainage systems.

Evaluating Strategic Retention of Artificial Drainage Flows for Nitrate-N Reduction under Waikato Conditions
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