Nematode dissemination by water leached in soil : case study of Radopholus similis (Cobb) Thorne on nitisol under simulated rainfall

New crop systems have been developed in the French West Indies that combine fallow or rotation crops with nematode-free vitro-plants to avoid the massive application of nematicides in banana fields. In these new banana fields, recontamination by the burrowing nematode Radopholus similis can then happen either by run-off or irrigation water. To understand these phenomena, we studied the vertical dissemination of R. similis by water leaching using soil cylinders and rainfall simulations. Steel drums were used to collect cylinders of soil (around 14.3 dm(3)) with a backhoe. The soil cylinders were placed in an aspersion chamber, saturated with water, and a R. similis suspension was placed on the soil surface. Afterwards, rainfalls ranging from 12 to 540 mm were simulated. Nematodes were then extracted from different soil layers (0-5 to 20-25 cm depth) using either (i) a Seinhorst elutriator followed by a Baermann funnel or (ii) centrifugation-flotation combined with Meldola blue staining. Results showed that dissemination of R. similis at the scale of the decimetre in nitisoil is limited: less than 8% of the applied nematodes reached layers deeper than 10 cm after exceptional rainfalls that represent several times the poral volume of the soil. Dissemination below 25 cm depth seemed to be limited to very few individuals, from 0.1 to 0.2%. Among leached nematodes, the percentage of non-active and dead nematodes increased significantly with depth. It seems that R. similis have developed a behaviour to escape leaching. These findings question the generally accepted idea that water dissemination of plant-parasitic nematodes is mainly a passive phenomenon

Nematode dispersion by runoff water : case study of Radopholus similis (Cobb) Thorne on nitisol under humid tropical conditions

To minimize application of nernaticides in banana fields, crop systems have been developed in the French West Indies that combine fallow or rotation crops and nematode-free in vitro plants. After two to four years, populations of the burrowing nematode Radopholus similis have developed enough to cause economic losses, leading banana growers to use nematicides. To understand how banana fields are recontaminated, we studied the dissemination of R. similis by water flow. At a 1-m scale, we analyzed the dispersion of R. similis under a rainfall simulator: we isolated a 1-m(2) study plot, placed a R. similis suspension on the upstream soil surface, and simulated a 60 mm/h rainfall for 72 min. We collected soil samples every 10 cm downstream after 12 min of rainfall, and subsequently at 20-min intervals, and extracted the nematodes using a Seinhorst elutriator and then a Baermarm funnel. our results showed that the nematode dissemination follows an inverse exponential law, and depends more on soil moisture at the beginning of rainfall than on the length of rainfall: in fresh soil, 69-80% of the R. similis recovered were found less than 10 cm downstream from the nematode inoculation line, whereas in wetted soil, 76-85% of the recovered individuals were collected in the outlet tub located downstream from the apparatus. This passive dissemination model partially explains the distance covered by individual nematodes but not the low percentage of motile nematodes recovered in the outlet tub (10% and 36% in fresh and wet soils) compared to the percentage of motile nematodes found in the soil (80% and 84% in fresh and wet soils). Indeed, water runoff is likely to disseminate R. similis over long distances only when soil moisture is close to field capacity

Modelling runoff at the plot scale taking into account rainfall partitioning by vegetation : application to stemflow of banana (Musa spp.) plant

Rainfall partitioning by vegetation modifies the intensity of rainwater reaching the ground, which affects runoff generation. Incident rainfall is intercepted by the plant canopy and then redistributed into throughfall and stemflow. Rainfall intensities at the soil surface are therefore not spatially uniform, generating local variations of runoff production that are disregarded in runoff models. The aim of this paper was to model runoff at the plot scale, accounting for rainfall partitioning by vegetation in the case of plants concentrating rainwater at the plant foot and promoting stemflow. We developed a lumped modelling approach, including a stemflow function that divided the plot into two compartments: one compartment including stemflow and the related water pathways and one compartment for the rest of the plot. This stemflow function was coupled with a production function and a transfer function to simulate a flood hydrograph using the MHYDAS model. Calibrated parameters were a 'stemflow coefficient', which compartmented the plot; the saturated hydraulic conductivity (Ks), which controls infiltration and runoff; and the two parameters of the diffusive wave equation. We tested our model on a banana plot of 3000 m(2) on permeable Andosol (mean Ks=75 mm h(-1)) under tropical rainfalls, in Guadeloupe (FWI). Runoff simulations without and with the stemflow function were performed and compared to 18 flood events from 10 to 140 rainfall mm depth. Modelling results showed that the stemflow function improved the calibration of hydrographs according to the error criteria on volume and on peakflow, to the Nash and Sutcliffe coefficient, and to the root mean square error. This was particularly the case for low flows observed during residual rainfall, for which the stemflow function allowed runoff to be simulated for rainfall intensities lower than the Ks measured at the soil surface. This approach also allowed us to take into account the experimental data, without needing to calibrate the runoff volume on Ks parameter. Finally, the results suggest a rainwater redistribution module should be included in distributed runoff models at a larger scale of the catchment

Mechanisms of magnesium isotope fractionation in volcanic soil weathering sequences, Guadeloupe

Magnesium (Mg) stable isotopes are increasingly used as a weathering proxy in soils and rivers, but the impact of the mineralogy of secondary phases on isotope fractionation remains obscure. A better understanding of the behaviour of Mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone. Here we investigate isotopic variations in δ26Mg in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from Guadeloupe formed under contrasting climatic conditions. Soils formed in drier conditions (low precipitation) contain smectite, whereas soils formed under wet conditions (high rainfall) are characterized by halloysite and Fe-oxides or kaolinite.All clay fractions have Mg isotopic compositions (δ26Mg -0.41‰ to -0.10‰) similar to or heavier than their parent andesite (δ26Mg -0.47‰) supporting the preferential incorporation of heavy Mg isotopes in secondary Mg-bearing clay minerals with the first direct measurements on clay fractions. Soils with lighter Mg isotope compositions have greater quantities of exchangeable Mg. The data support a contribution from sea spray to the exchangeable Mg pool correlated to the soil weathering degree. This study highlights for the first time that the soil δ26Mg not only depend on δ26Mg of the parent rock, and on any fractionation that might occur, but also on the Mg retention on the exchange complex, which could in turn be controlled by external inputs such as sea spray. © 2012 Elsevier B.V

Silicon isotopes and the tracing of desilication in volcanic soil weathering sequences, Guadeloupe

Silicon (Si) stable isotopes have the potential to become a useful weathering proxy, given that light Si isotopes are preferentially incorporated into secondary clay minerals. Here we investigate how Si depletion in soils and associated clay mineralogy influence the Si isotope fractionation associated with clay mineral formation. We report δ 30Si compositions in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from Guadeloupe that were formed under contrasting climatic conditions. Strongly desilicated soils containing kaolinite that formed in wet areas (high precipitation) are compared with less desilicated soils containing smectite formed in drier conditions (low precipitation). Clay fractions are isotopically lighter than the parent andesite (δ 30Si-0.23‰), and increasingly lighter with Si depletion in soils, which supports the view that the Si isotope composition in secondary clay fractions is controlled by the degree of soil desilication. It is shown that the Si isotope fractionation factor between the parent silicate material and the secondary clay minerals is smaller for Si-rich secondary clay minerals such as smectite and larger for Si-poor secondary clay minerals such as kaolinite. This study provides new insights to better define Si isotopes as a proxy for environmental conditions for clay neoformation. © 2012 Elsevier B.V