Raphaël Achard

Long-term pollution by chlordecone of tropical volcanic soils in the French West Indies: A simple leaching model accounts for current residue

Chlordecone was applied between 1972 and 1993 in banana fields of the French West Indies. This resulted in long-term pollution of soils and contamination of waters, aquatic biota, and crops. To assess pollution level and duration according to soil type, WISORCH, a leaching model based on first-order desorption kinetics, was developed and run. Its input parameters are soil organic carbon content (SOC) and SOC/water partitioning coefficient (K(oc)). It accounts for current chlordecone soil contents and drainage water concentrations. The model was valid for andosol, which indicates that neither physico-chemical nor microbial degradation occurred. Dilution by previous deep tillages makes soil scrapping unrealistic. Lixiviation appeared the main way to reduce pollution. Besides the SOC and rainfall increases, K(oc) increased from nitisol to ferralsol and then andosol while lixiviation efficiency decreased. Consequently, pollution is bound to last for several decades for nitisol, centuries for ferralsol, and half a millennium for andosol.

Determination of soil content in chlordecone (organochlorine pesticide) using near infrared reflectance spectroscopy (NIRS).

Chlordecone is a toxic organochlorine insecticide that was used in banana plantations until 1993 in the French West Indies. This study aimed at assessing the potential of near infrared reflectance spectroscopy (NIRS) for determining chlordecone content in Andosols, Nitisols and Ferralsols from Martinique. Using partial least square regression, chlordecone content conventionally determined through gas chromatography-mass spectrometry could be correctly predicted by NIRS (Q(2) = 0.75, R(2) = 0.82 for the total set), especially for samples with chlordecone content <12 mg kg(-1) or when the sample set was rather homogeneous (Q(2) = 0.91, R(2) = 0.82 for the Andosols). Conventional measures and NIRS predictions were poorly correlated for chlordecone content >12 mg kg(-1), nevertheless ca. 80% samples were correctly predicted when the set was divided into three or four classes of chlordecone content. Thus NIRS could be considered a time- and cost-effective method for characterising soil contamination by chlordecone.

Decision support tool for soil sampling of heterogeneous pesticide (chlordecone) pollution

When field pollution is heterogeneous due to localized pesticide application, as is the case of chlordecone (CLD), the mean level of pollution is difficult to assess. Our objective was to design a decision support tool to optimize soil sampling. We analyzed the CLD heterogeneity of soil content at 0–30- and 30–60-cm depth. This was done within and between nine plots (0.4 to 1.8xa0ha) on andosol and ferralsol. We determined that 20 pooled subsamples per plot were a satisfactory compromise with respect to both cost and accuracy. Globally, CLD content was greater for andosols and the upper soil horizon (0–30xa0cm). Soil organic carbon cannot account for CLD intra-field variability. Cropping systems and tillage practices influence the CLD content and distribution; that is CLD pollution was higher under intensive banana cropping systems and, while upper soil horizon was more polluted than the lower one with shallow tillage (<40xa0cm), deeper tillage led to a homogenization and a dilution of the pollution in the soil profile. The decision tool we proposed compiles and organizes these results to better assess CLD soil pollution in terms of sampling depth, distance, and unit at field scale. It accounts for sampling objectives, farming practices (cropping system, tillage), type of soil, and topographical characteristics (slope) to design a relevant sampling plan. This decision support tool is also adaptable to other types of heterogeneous agricultural pollution at field level.

Cation exchange capacity and aluminum–calcium–magnesium binding in roots of bananas cultivated in soils and in nutrient solutions

Bananas (Musa spp.), as many other crop species, accumulate aluminum (Al) in roots when grown in nutrient solution containing Al ions. Aluminum can compete with calcium (Ca) and magnesium (Mg) on the root exchange sites, which has been reported as a possible cause for Al toxicity to the plant. We measured the cation exchange capacity of roots (CECR) of 5 banana cultivars (Grande Naine, Agbagba, Obino lEwaï, Igitsiri, and Kayinja) and determined the composition of the root exchange sites in nutrient solutions with and without Al, and in two tropical soils. Aluminum, Ca, and Mg were extracted using a sequential procedure involving 0.01 M copper sulphate for Cu-extractable cations, 0.1 M hydrochloric acid and ash solubilization in nitric acid. The mean CECR values of the 5 cultivars were similar in the 2 soils and in the nutrient solution without Al (CECR=23.0 cmolc kg− 1), but significantly larger for bananas grown in nutrient solution with Al (CECR=32.6 cmolc kg− 1). The most Al-sensitive cultivar (Kayinja) had larger CECR than the others. Calcium was the dominant Cu-extractable cation in all cases. In the roots grown in nutrient solution with Al and in the two soils, Al amounted to 15–35% of the sum of Cu-extractable cations, but most Al was not extracted with Cu. The Cu-extractable Mg was drastically reduced in nutrient solution with Al, whereas the Cu-extractable Ca was little affected. Therefore we suggest that the Al/Mg ratio on root exchange sites could be a better indicator of Al toxicity than the Al/Ca ratio.