Sorption of the herbicide terbuthylazine in two New Zealand forest soils amended with biosolids and biochars

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. 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 (700A degrees C), and a biochar from pyrolysis with a lower temperature (approximately 350A degrees 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. 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. 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.