Nitrate leaching from cut grassland as affected by the substitution of slurry with nitrogen mineral fertilizer on two soil types

A field experiment was conducted to find out whether there is any difference in risk of N leaching to groundwater when cattle slurry and/or mineral fertilizer-N was applied to cut grassland. The experiment was carried out over two consecutive years on two sites (one with a relatively wet sandy soil and one with a relatively dry sandy soil). Treatments were mineral fertilizer-N at annual rates of 0–510 kg N ha⁻¹ year⁻¹ and combinations of sod-injected cattle slurry (85, 170, 250 and 335 kg N ha⁻¹ year⁻¹) and mineral fertilizer-N (289, 238, 190 and 139 kg N ha⁻¹ year⁻¹). Yield responses indicated that in the short run, 0·44–0·88 (average 0·60) of the slurry-N was as available as mineral fertilizer-N. The total N input from mineral fertilizer and slurry was a worse predictor of nitrate leaching (     0·11) than the N surplus (i.e. the difference between total N input and harvested N) (     0·60). The effective N surplus, based on the difference between the summed inputs of the plant-available N and harvested N, proved to be the best indicator of leaching (     0·86). Annual N application rates of up to 340 kg plant-available N ha⁻¹ complied with the target nitrate concentration in groundwater of 11·3 mg N L⁻¹ set by the European Union in both years on the wet sandy soil, whereas on the dry sandy soil none of the treatments did.