Detection of manure-derived organic compounds in rivers draining agricultural areas of intensive manure spreading
Résumé
It is essential to have suitable tools able to trace the fate of manure organic matter in the environment to assess whether manure disposal on the soils of catchments could affect the organic quality of rivers. Sterol compounds mainly expressed as C29 + 28/C27 and 5ß/C27 ratios have been shown to be specific molecular tracers of pig, dairy and poultry manures in soils. The objective of this study was to measure C29 + 28/C27 and 5ß/C27 ratios in five Brittany rivers (Elorn, Yar, Léguer, Min Ran and Couesnon) draining agricultural catchments receiving massive annual inputs of pig, poultry and dairy manures and compare these ratios with ratios published for enriched soils and manure samples. The particulate organic fractions from the studied rivers yielded steroid signatures typical of animal manures. More specifically, a stanol compound diagnostic of pig slurry the 5ß-stanol known as coprostanol was found to be very widespread, with particularly high concentrations in one of the rivers (Elorn). The C29 + 28/C27 and 5ß/C27 ratios of the particulate fractions of the rivers were compared with ratios measured directly in pig, dairy and poultry manure samples, as well as with the breeding activities on river catchments. These comparisons show that the steroid profiles of the five investigated rivers correlate closely with the types of manure (i.e. pig, poultry or dairy) spread on soils in their catchments. For instance, the C29 + 28/C27 and 5ß/C27 ratios in the Elorn river are similar to the values typical of pig slurry (e.g. 5ß/C27 > 4); compared with other catchments, the soils in this area receive by far the largest amount of pig slurry. By contrast, the Yar river drains a catchment receiving only poultry and dairy manures, and its soils exhibit C29 + 28/C27 and 5ß/C27 ratios similar to those of dairy and poultry manures (e.g. 5ß/C27 1). Thus, this study indicates that the organic quality of rivers is modified in catchments where there is intense manure spreading on soils. It also provides evidence that rivers draining areas receiving different manure types may exhibit differences in the long-term evolution of their OM content. Indeed, two of the investigated river catchments receive dominantly dairy and poultry manure, and exhibit clear long-term upward trends in OM. On the other hand, one catchment receiving high proportions of pig slurry clearly shows a long-term downward trend in OM contents. A survey of the literature shows that the relative amount of OM and N received by soils in agricultural catchments could be the key parameter in determining the direction of the long-term OM trend of the river, rather than the absolute amount and/or type of manure that is applied to the soil. In any case, the present study suggests that sterol/stanol compounds may be of diagnostic value in determining whether a stream or a river is undergoing contamination by manure-derived organic matter.