Herbicides applied in the cropping industries in Queensland are regularly detected in waters flowing to the Great Barrier Reef (GBR) and Reef Plan (Anon, 2013) sets targets for the reduction in loads of 'priority' herbicides by 2018. Progress towards these targets is assessed through the Paddock to Reef Integrated Monitoring, Modelling and Reporting Program (P2R) which includes modelling from farm scale to end of system loads. The ability to accurately predict the fate of herbicides is an important aspect to assessing the environmental risks of application and making informed land management decisions, however, there are currently limited data available to inform prediction of the fate of herbicides in a tropical environment. The objective of this study was to provide measured half-lives for herbicides on Queensland cropping soils and crop residue to improve modelling of herbicide fate in the GBR catchment.
Half-lives of commonly applied herbicides in cane, grains and grazing industries in Queensland have been compared in a controlled environment on common cropping soils. Cropping soils were collected from the tilled layer (0-0.1 m) from nine sites in Queensland to represent a range in textural properties and pH. Cane crop residues (trash) were also collected. Soils were kept moist using capillary rise from a hanging water column and were placed in a temperature controlled glasshouse. Following herbicide application, surface samples were collected over a period of 100 days to track dissipation of the herbicides. Half-lives have been calculated for fourteen herbicides including those considered to be high 'priority' under Reef Plan (Anon, 2013) (atrazine, ametryn, diuron, hexazinone and tebuthiuron) and alternative residual products which are being used with increasing frequency in Queensland (isoxaflutole, imazapic, pendimethalin, trifloxysulfuron, S-metolachlor, metribuzin). Three commonly applied knockdown herbicides, 2,4-D, glyphosate and paraquat, were also included in the study.
Half-lives were found to range from 12 days for atrazine on the Ferrosol (WAK) to >744 days for paraquat on a heavy Vertosol (MAK). The half-life values were comparable to international database and literature values from tropical field studies for well-studied herbicides including atrazine and ametryn. However, half-lives for several herbicides that have not previously been reported under local conditions (imazapic, pendimethalin, trifloxysulfuron) were on average 2.5-6 fold faster than international database values (PPDB, University of Hertfordshire). For example, the mean half-life of imazapic was 37 days (ranging from 20-81) which is approximately 6 fold shorter than reported in the PPDB. In contrast, the degradation rate of isoxaflutole was >10 fold slower than PPDB values. Half-lives for all herbicides on cane residues were slower than has been previously reported, which may be due to the effects of herbicide washoff in field studies and to limited photodegradation in the current study.
Half-lives varied by less than a factor of 2 between soils for the herbicides atrazine, 2,4-D and isoxaflutole whereas half-lives for diuron varied by a factor of 14. Total organic carbon was found to be a significant explanatory variable for predicting degradation rates for many of the herbicides in this study. Further analysis of the data is required to investigate the relationship between measured half-lives and soil properties to extend prediction of spatial variability in herbicide degradation for all soils in the GBR catchment.
Half-lives of herbicides in the controlled environment were compared to dissipation measured on several of these same soils in field trials. The lack of consistent trends between herbicide degradation rates observed in the field and those observed in the controlled glasshouse environment highlights the difficulty in isolating factors contributing to dissipation in a field scenario. Values derived in the current study can be considered to provide conservative
estimates for the tested soil temperature and moisture conditions since loss processes including leaching, runoff, plant uptake and to some degree photodegradation have been excluded. Further, the half-lives derived in this study relate to the surface 2.5 cm of soil which is the depth relevant to extraction of herbicides into runoff. These values are are more appropriate for use in agricultural soil water balance models where losses due to degradation are considered as a separate process to loss through runoff/leaching/plant uptake. Adoption of the half-life values from this study in the paddock to reef modelling program will improve prediction of herbicide losses under various management scenarios and therefore improve the prediction of loads of herbicides entering the GBR lagoon.