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Background, aim, and scope Terbuthylazine is one of the most commonly used herbicides for vegetation management in forest plantations in New Zealand. Knowledge about the sorption of terbuthylazine on forest soils, especially the influence of coexisting organic amendments, remains obscure. In this study, we evaluated the effects of biosolids and biochars on the sorption of terbuthylazine to forest soils. Materials and methods Two pumice soils, including a forest landing site soil with low soil organic matter content and an organic carbon rich topsoil under standard forest management, were sampled from a 2-year-old replanted pine plantation. The soils were mixed with four organic amendments, including two thermally dried biosolids with one digested and the other undigested, a biochar produced from high temperature pyrolysis (700°C), and a biochar from pyrolysis with a lower temperature (approximately 350°C). A batch equilibration method was used to determine terbuthylazine adsorption-desorption in organic amendment-treated and untreated soils. Adsorption and desorption isotherms were described with the Freundlich equation.Results and discussion Adsorption and desorption isotherms in the soils with or without organic amendments were well described by the Freundlich model. The undigested or digested biosolids added to the topsoil had a negligible or limited effect on the adsorption to terbuthylazine. The addition of the other amendments to the two soils all enhanced the adsorption. The biochars displayed higher efficiency in improving soils' adsorption capacity to terbuthylazine than the biosolids. Among the organic amendments evaluated, the biochar obtained from high temperature pyrolysis demonstrated the most significant enhancement on adsorption with an enhancing factor of 63; whereas, the digested biosolids showed the weakest enhancement. Furthermore, terbuthylazine adsorbed by the digested biosolids appeared to be more easily desorbed than that by biochar treatments. Conclusions This work indicates that the addition of organic amendments to forest soils, particularly biochar to a soil with low native organic matter, may enhance soil sorption of terbuthylazine and thus reduce the possibility of the hydrophobic herbicide leaching to groundwater.

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Radiata pine (Pinusradiata D. Don) is the predominant species in plantation forests of Australia and New Zealand. Removal or suppression of noncrop vegetation is often carried out to enhance crop growth and survival by reducing competition for water, light, and nutrients. Other reasons for vegetation removal include microclimate modification, providing access into stands, and fire hazard reduction. Many studies have demonstrated large increases in crop growth from removing noncrop vegetation. Vegetation management can also mean introducing noncrop species to improve site quality, to suppress unwanted species, or to provide fodder for grazing. While herbicides are still the most widely used weed control tool, nonchemical methods are of increasing importance. The role of machinery is likely to increase in both countries as the use of fire for vegetation management and general site preparation declines. To calculate the long-term cost–benefit of vegetation management, a better understanding of the crop growth response to these treatments is essential. This requires experiments that focus on the role of both crop and noncrop species in hydrological and nutrient cycles. Personal computer based decision support systems are likely to play an increasing role as an aid to cost-effective vegetation management decision making.

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