Herbicides are applied in most banana plantations to reduce competition with weeds. There are currently seven registered herbicides used in the Australian banana industry, with different modes of action, e.g. pre-emergence (pendimethalin), selective post emergence (haloxyfop or fluazifop), broadspectrum systemic activity (glyphosate) and broad-spectrum knock-down activity (glufosinate, paraquat and diquat). The broad-spectrum knockdown herbicides are most commonly used once banana plantations are established. There is speculation that herbicides reduce soil functions, which potentially undermines the productivity and resilience of Australian banana plantations. Furthermore, in the wet-tropics region of Queensland, where the majority of bananas are grown, there is a need to demonstrate that agrochemicals are not impacting the Great Barrier Reef. To validate and improve the environmental credibility of the banana industry it is important to demonstrate the impacts of herbicides on soil functions through their influence on soil organisms. However, there are few and often conflicting reports about herbicides impacts on soil organisms and biological functions. Therefore, it is necessary to quantify changes in soil biological communities following the application of herbicides and to determine if biological remediation is a viable option.
Investigations concluded that a single application of all registered herbicides applied at recommended registered rates had minimal impact on soil microbial communities. However, there are still questions about the impacts of herbicides following multiple applications, which is the common scenario in banana plantations. Furthermore, if herbicides are applied above recommended rates, there was greater reduction in soil biological functions. The investigations found the herbicides had a temporary impact, one week after application on soil organisms that utilize organic acids. The knockdown herbicides like glyphosate and glufosinate tended to increase bacterial activity during their decomposition following application to the soil at recommended rates. However, glufosinate, paraquat and diquat could also reduce the capacity of soil organisms to utilize organic acids, such as malic, oxalic and citric acids.
The chemical degradation pathway of the water soluble herbicides (glyphosate, glufosinate, paraquat and diquat) proved to be difficult to determine. However, it was possible to isolate soil bacteria that were chemically attracted (chemotaxis) to these compounds that could potentially move through soil water. When presented with a choice of carbon compounds only a population of Gemmatimonadaceae bacteria moved toward diquat in preference to glucose, indicating its potential as a bioremediation organism. The chemical degradation of the water insoluble herbicides (pendimethalin, haloxyfop, and fluazifop) could be determined and was measured. They were compared with two environmentally problematic herbicides not used in the banana industry, atrazine and diuron. Haloxfop and fluazifop exhibited typical degradation patterns over 60 day incubation. However, pendimethalin did not demonstrate a typical degradation pattern, indicating a potential to accumulate in soils. Unlike the water soluble herbicides, the water insoluble compounds were not found to significantly alter the microbial community compared to a control. The production of a draft herbicide risk tool and dissemination to banana grower groups will allow banana growers to assess the risks of herbicide application on soil organisms and their potential functions in the soil. Furthermore, the knowledge and research capacity to understand herbicide interactions with soil biology in banana plantation were outcomes from this project through two student theses and two prepared scientific manuscripts.
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