Dispersal of Weed Seeds by Erosion and Flow Processes in Upland Fields

Abstract This study examined the impact of runoff water and soil erosion on the dispersal of weed seeds in small cultivated catchments. Water samples and soil samples were collected from eight hydrological stations in the Houay Pano headwater catchment, near Lak Sip village in northern Laos, during the first big event of the rainy season in 2005 (7 th June, 2005). Samples were also taken from the topsoil near each station before the beginning of the rainy season. The number of viable seeds was determined for each sample using the simple seedling-emergence method. The rainstorm generated an outflow of approximately 4,000 kg of suspended material including some 970,000 viable weed seeds, transported by runoff water. Most of the seeds that left the catchment (an area of 64.6 ha) were troublesome composite weeds and grasses. The same rainfall event also generated a sediment load of 1,100 kg of soil that was trapped in the tank of the outlet (drained from 8.4 ha). This bedload carried 43,500 viable seeds. The study of the mi-cro-catchments revealed that fields that were recently planted with rice, following local slash-and-burn practices, had a total seed flow of 61,000 viable seeds per hectare. Land covered by well-developed fallow vegetation, on the other hand, produced a seed flow of only approximately 500 viable seeds per hectare. It was also found that the riparian area and riverbank were the major sources of weed seeds in the stream during rainstorms. This study showed that a large amount of weed seeds are transported out of the catch-ment and are very likely to pollute the downstream cultivated lands. In the Houay Pano catchment, weed seed dispersal during rainstorms could be greatly reduced by management of the riparian area.