N and P leaching and microbial contamination from intensively managed pasture and cut sward on sandy soil in Finland

The aim of this study was to find out if nutrient leaching from grassland represents a threat to groundwater in areas that are subject to freezing and extensive snow cover. The effects of grazing and cutting on groundwater quality were compared using field lysimeters during a 3-year experiment at North Savo Research Station (63°10′N, 27°18′E), Finland. One-third of the annual precipitation at this site (578 mm per year) falls as snow. Snowmelt produces over 100 mm of water flow and there is a substantial risk of nutrient leaching. The soil at the site was a medium textured Dystric Regosol, consisting mainly of fine sand. The lysimeters used in this study were 100 m2 and 1.8 m deep, suitable for grazing studies. The watering point for cows was located on one of the lysimeters. The area was fertilized with N at 220 kg ha−1 per year. After two summers of grazing and cutting the area was treated with glyphosate and in the following spring it was ploughed and resown to grass. The NO3-N concentrations were low and there were no differences among treatments during the first 2 years of the trial. After glyphosate application nitrate leaching increased and the average concentrations of NO3-N from cut, grazed and watering point lysimeters were 3.4, 10.2 and 21.8 mg l−1, respectively. The amounts of leached NO3-N after glyphosate treatment were 10, 43 and 139 kg ha−1 per year, respectively. Phosphorus leaching was negligible. In the first year the numbers of heterotrophic micro-organisms in leachate from grazed and cut lysimeters were similar. In the second year leachate from grazed lysimeters contained more heterotrophic micro-organisms than that from cut plots (geometric means 44 000 and 4600 CFU ml−1, respectively). Moreover, the leachate from the grazed plots and the watering point occasionally contained low numbers of enteric micro-organisms, faecal coliforms and tyrobutyricum clostridia. The study indicated that cutting and grazing did not represent a risk to groundwater quality during grass cover years, but renovation of pasture caused a significant N leaching to groundwater.