Nitrogen leaching from gorse to Lake Rotorua and the losses during a gorse to Pinus Radiata forest conversion

Abstract

Eutrophication has been the main cause of concern for Lake Rotorua water quality and is attributed to excessive quantities of phosphorous and nitrogen entering the lake. Magesan and Wang (2008) suggest that gorse maybe a significant contributor to nitrate leaching to groundwater and subsequently to Lake Rotorua. Large areas of gorse (Ulex europaeus), have been assessed to determine possible diffuse contributions. One suggestion, proposed to convert gorse infested areas to pine forest thereby changing the land-use to a ground cover that has a reduced nitrogen-loss. Using seven treatments at Whakapoungakau Land Blocks (near Rotorua airport), this research aimed to; (a) identify the best way to manage gorse conversion in terms of nitrogen losses to ground water; (b) measure the effectiveness of a nitrogen inhibitor in reducing the nitrogen leaching ‘spike’ that may occur during conversion; and (c) develop Best Management Practice (BMP) to reduce nitrate leaching. Each plot has four drainage flux meters to capture and measure leachate. The study commenced October 2010. Water samples were collected monthly and analysed for nitrate and nitrite, ammonium, dissolved organic nitrogen and total nitrogen. This trial estimates that gorse in the catchment loses nitrogen to ground water at a leaching rate between 35.8 kg NOxN ha-1 yr-1 and 59.7 kg NOxN ha-1 yr-1, which could contribute between 30.9 and 51.6 tonne of mobile nitrogen per year to ground water loadings. This is approximately 5.5% to 9.4% of nitrogen known to be entering Lake Rotorua annually. Converting all of the gorse in the Rotorua catchment to pine forests would reduce N loss loadings to ground water from between 3.9 Tonne and 97.4 Tonne, indicating that the developments of BMP are critical. The nitrogen inhibitor when used in cleared plots, was demonstrated to be effective, however exacerbated losses in crushed gorse. The BMP for converting gorse to pine forest should focus on roller crushed gorse. Losses to ground water may be improved if done in late spring or early summer. This thesis concludes that gorse is a significant contributor of nitrogen to groundwater, and if conversion is not managed correctly; losses may be unnecessarily increased.