Environmental fate of terbuthylazine and hexazinone in a New Zealand planted forest Pumice soil

Herbicide use is an important tool for primary production in New Zealand, including forestry, and can significantly increase crop growth and economic yield. However, increasing concerns around the off-site impacts of herbicides in the environment has emphasised the need to understand their fate in soil and water. In the New Zealand forestry sector terbuthylazine and hexazinone are widely used for weed control, yet little is known about their environmental fate in New Zealand forests. A catchment scale (12.5 ha) experimental site was established in Pumice soil, recently harvested and replanted with Pinta radiata (D. Don), in a plantation forest in the central North Island, New Zealand. Forest litter (fresh litter (L), and partly and well decomposed litter (FH); LFH < 10 cm diameter), that included harvest residues, and mineral soil were monitored for terbuthylazine and hexazinone traces to depth (0-10, 10-50, 50100 cm) after two operational aerial herbicide spray application events in 2012 and 2013. In order to determine the movement and persistence of each herbicide, the amount of terbuthylazine and hexazinone were measured periodically following the initial spraying event. There was no drainage of soil water past 1 m depth within the first 5 months after the first spray application in 2012 due to a regional drought. This resulted in limited movement of both terbuthylazine and hexazinone down the soil profile. During the 2013 monitoring period, three drainage events occurred after spray application, 30 mm of drainage occurred at 7-8 days, 32 mm occurred at 36-37 days and 24 mm occurred 170 days after spray application. Even with drainage both terbuthylazine and hexazinone were located predominantly in the LFH and top-soil (0-10 cm) layers with only small amounts detected at the lowest monitoring depth (50100 cm). The presence of the LFH layer (which included harvest residues from the previous crop), high in organic rich matter strongly retained terbuthylazine and to a lesser extent hexazinone. Both terbuthylazine and hexazinone had appreciably shorter half-lives than reported in the literature. Reported half-lives were up to 10 days for terbuthylazine and up to 18 days for hexazinone. The first month after spray application was found to pose the greatest potential risk of movement off-site. After this period, potential risks were low. Organic matter management practices that influence the amount and distribution of forest floor and harvest residue retention dearly play an important role in determining the environmental fate of both terbuthylazine and hexazinone. These practices should be considered in further risk based management decision making. (C) 2014 Elsevier B.V. All rights reserved.