Browsing by Subject "Transfer pathways"

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Rhizosphere processes as determinants for glyphosate damage of non-target plants
(2010) Bott, Sebastian; Neumann, Günter
Due to low production costs and high herbicidal efficiency, glyphosate is the most widely used wide-spectrum herbicide. Glyphosate acts as a non-selective, total herbicide by inhibiting the biosynthesis of aromatic amino acids. Apart from glyphosate drift contamination, risks of glyphosate toxicity to crop plants and other non-target organisms are generally considered as marginal, because glyphosate is almost instantaneously inactivated by adsorption to the soil matrix and rapid microbial/chemical degradation in the soil solution. However, in the recent past, an increasing number of yet unexplained observations on significant damage of crop plants have been reported in the literature and by farmers, suggesting gaps in the risk assessment, with respect to the fate glyphosate in the rhizosphere and the interaction with rhizosphere processes. According to these observations, the aim of present study was a systematic evaluation of potential rhizosphere effects of glyphosate, including direct toxicity, risks of re-mobilisation by fertiliser application, potential role of pathogens and allelopathic compounds, and interactions with micronutrients, both in glyphosate-sensitive and transgenic glyphosate-resistant crops. A series of field trials in reduced soil tillage cropping systems as well as green-house experiments on soils with contrasting properties with sunflower, winter wheat and soybean, consistently revealed a close clausal relationship between crop damage and (a) short waiting times between glyphosate application on target weeds and subsequent sowing of crops and (b) the density and speed of decay of glyphosate-treated weeds. The results suggested that damage of crop plants is induced by a rhizosphere transfer of glyphosate from weeds to subsequently sown crops. This transfer might take place by contact contamination due to exudation of glyphosate from living roots of treated weeds and/or release during decomposition of the root residues. A comparison between phytotoxic effects of glyphosate and aminomethylphosphonic acid (AMPA) as major metabolite of glyphosate in soils, revealed high toxicity in case of root exposure to glyphosate, but not to AMPA. By contrast, a significant decline of germination was induced by seed exposure to AMPA, while germination was not affected by glyphosate treatments. The observed differences in sensitivity to glyphosate and AMPA in different stages of plant development may explain variable symptoms of crop damage under field conditions, ranging from growth depressions and chlorosis to reduced field emergence. The results of the present study further suggest that risks for crop damage associated with rhizosphere transfer of glyphosate are additionally influenced by a range of environmental factors, such as growth season (spring or fall application), temperature, soil moisture, redox potential of soils and soil microbial activity. These factors might shorten or prolongate the time window for crop damage of glyphosate contact contamination in the rhizosphere under field conditions. Model experiments investigating the sensitivity of different plant species to glyphosate root exposure, revealed significant differences between winter wheat, maize and soybean in terms of glyphosate-induced plant damage but also in their ability for recovery from glyphosate damage suggesting marked genotypic differences in the expression of damage symptoms also under field conditions. In agreement with previous investigations, results of the present study indicated a rapid inactivation of glyphosate by adsorption to the soil matrix. Glyphosate adsorption in soils seem to be mainly mediated by the phosphonate group of the molecule in a way similar to the adsorption of inorganic phosphate. Accordingly glyphosate re-mobilisation is possible via ligand exchange by phosphate application. The results of the present study have demonstrated for the first time that depending on soil properties also the application of fertiliser phosphate is able to re-mobilise glyphosate in sufficient quantities to mediate crop damage in pot experiments. This finding suggest, that re-mobilisation of glyphosate potentially by fertiliser P or root-induced chemical modifications for P and Fe mobilisation needs to be considered as additional potential rhizosphere pathway for glyphosate damage to non-target plants. Field trials and model experiments under soil and hydroponic conditions consistently revealed a significantly impaired nutritional status of glyphosate-sensitive but also glyphosate-resistant crops. However, depending on the culture conditions different mineral nutrients were affected by the glyphosate treatments and plant damage was not related with a certain nutrient deficiency. These findings suggest that damaged root growth, induced by glyphosate toxicity, rather than specific interactions with certain mineral nutrients are responsible for the observed impairment of nutrient acquisition. In conclusion, results of the present study highlight that risks for crop damage associated with glyphosate toxicity in the rhizosphere can be substantial and is influenced by factors such as waiting time after herbicide application, weed density, cropping systems, fertilizer management, genotypic differences, and probably also environmental factors including temperature, soil moisture, and soil microbial activity. The independency between these factors is so far not entirely clear but should be investigated in future studies. Nevertheless, results of present study suggest that risks could be minimized by simple management tools such as the consideration of waiting times between application of glyphosate and sowing of crops particularly in case of high weed densities and alternation of herbicides to reduce not only risk for remobilization of glyphosate but also problems associated to the selection of glyphosate-resistant weeds.