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Insecticide dynamics in the soil environment of a tropical lychee plantation – A case study from Northern Thailand –

Adverse side-effects of pesticide use are not only controlled by the acute toxicity of the applied substances, but also by their persistence and mobility. Both are hardly investigated for humid tropical climates. The scope of my study was (i) to monitor background concentrations of pesticides in an intensively cultivated Northern Thailand watershed and (ii) to quantify soil-related pathways of pesticide dissipation in a representative lychee plantation under realistic agricultural practice. Therefore, baseflow samples of river-water were analysed for commonly used pesticides. In addition, water and pesticide fluxes were monitored on profile scale during two rainy seasons with a tension-controlled, high-resolution soil solution sampling device and conventional wick-lysimeters, while surface runoff was collected with metal troughs. Pesticide residues in soil were assessed after sequential extractions. Riverine pesticide concentrations were above European threshold values even in baseflow, so that I found it relevant to study the pathways of pesticide translocation from the plot into ground and surface water in greater detail. A single manual application of various organochlorine and organophosphorous insecticides in the first study year revealed that up to 1% of the applied amount can be leached into 55 cm soil depth over night by preferential flow. Under saturated flow conditions, the preferential flow pathways within the Acrisol were so numerous that sampling devices with a diameter of 9 cm pretended homogeneous flow. Two independent diversity indices showed that, under unsaturated conditions, the flux became increasingly heterogeneous, probably because one flow pathway after the other was "switched off" so that water flux concentrated on distinct fingers. The pesticide half-lives in the studied Acrisol were among the shortest ever reported (1.4 – 7.2 days, malathion and chlorpyrifos), because the humid tropical climate rather than microbial adaptations promoted both abiotic (leaching, volatilization) and biotic (microbial decay) dissipation processes. Besides climate, also the ground vegetation of the orchard probably enhanced the rate of dissipation, because pesticides on plant surfaces volatilize faster and are more exposed to photodecomposition than pesticides on soil surfaces. Despite this rapid dissipation, all substances except mevinphos (completely miscible with water) accumulated in soil after five repeated applications in the second study year. Using a conventional and a new sorption coefficient (KOC(app) and MAR, the latter calculated from methanol- and acetone:ethylacetate:water-extractable fractions of pesticides), I showed that the accumulation went along with aging processes, which were most evident for endosulfan. The ground cover and the exceptionally high infiltration capacity of the soil effectively reduced the total amount of surface runoff. Nevertheless pesticide concentrations in surface runoff clearly exceeded tabulated toxicity data for vertebrate and invertebrate aquatic test species. Therefore, I cannot rule out adverse effects on aquatic biota, and the use of pesticides in Northern Thailand fruit cropping requires technical optimization before this form of land-use system can be considered sustainable.

 

Keywords: sustainable land-use , fruit cropping , pesticides , Southeast Asia , environmental fate

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