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Wastewater irrigation in a forested ecosystem: nutrient leaching

Gujja N. Magesan, Loretta Garrett, Gerty Gielen, Stephen Pearce, Carolyn Anderson, Hailong Wang and John Lavery

Forest Research, Private Bag 3020, Rotorua, New Zealand. Email: gujja.magesan@forestresearch.co.nz

Abstract

Land application of treated municipal wastewater is becoming common in New Zealand because of government regulations, Maori cultural beliefs, and economic and environmental benefits. Forested land has become the main contender for such application because of restrictions on other land uses.

A large lysimeter research facility (LLRF) was constructed in Whakarewarewa Forest, Rotorua, New Zealand, to develop an understanding of wastewater irrigation issues in forest ecosystems. The facility had 36 plots (21 volcanic soil and 15 sand dune soil) each 5 x 5m, and with 9 trees in each plot. Secondary and tertiary effluents were irrigated over a 3-year period at 0, 30 and 60 mm/week. This paper reports nutrient leaching, especially nitrate and phosphate, from two different soil types under various irrigation regimes.

In general, for both soils, nitrate leaching from secondary wastewater irrigated plots was greater than from the tertiary wastewater irrigated and the control plots. As expected, with higher irrigation rates more nitrate leaching occurred. Also, more nitrate was leached from the volcanic ash soil than from the sandy soil which could be due to contributions from the native volcanic soil. Generally, wastewater irrigation did not result in phosphate leaching from the volcanic soil. In the sandy soil, although P concentrations were initially low, they increased with time, with concentrations up to 4 g m-3.

The final results of the leaching of these nutrients in relation to soil storage and plant uptake will be presented.

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