Nitrate leaching following cultivation of contrasting temporary grasslands

1. 437 Nitrate leaching following cultivation of contrasting temporary grasslands Jørgen Eriksen & Karen Søegaard Danish Institute of Agricultural Sciences, PO Box 50, 8830 Tjele, Denmark 3. N balance of grasslands 1. Introduction In grazed grassland considerable build-up of N takes place. The extent depends on fertilization, feeding, stocking density, time of grazing and botanical composition of the sward. Consequently, the ploughing of grassland is followed by a large increase in the mineralisation of N. We studied residual effects of temporary grassland on plant production and nitrate leaching. Grassland managements in 1994-96 were either cut leys or grazing with cows, in both pure grass and grass-clover. Cows were fed two levels of N in supplements. Following ploughing of the six sward types was in 1997 spring barley and in 1998 spring wheat. Leaching losses after ploughing were estimated from the concen-tration of nitrate in ceramic suction cups installed at a depth of 1 m. • The N surplus increased in the order: cut - grazed low N - grazed high N. The surplus was higher in grazed pure grass than in grazed grass-clover. 2. Precrop effects Cereal yields following the six grassland systems were compared to yields after cereals: 1st year (spring barley) 4. Nitrate leaching 2nd year (spring wheat) • Overall, leaching losses from pure grass and grass-clover were similar. • The differences in the N balance were only expressed in pure grass. • Fertilizer application resulted in considerably increased nitrate leaching. • Fertilizer-N to cereals following grassland influenced nitrate leaching more than the history of the grassland. • Leaching losses and N-uptake did not match the N surplus. • Precrop effects of grassland were generally huge and the need for supplementary fertilizer low • 1st year precrop effect was about 25 kg N for cut pure grass, 100 kg N for grazed pure grass and >115 kg N ha-1 for grass-clover swards. • 2nd year precrop effect was <0 kg N for cut pure grass, 40 kg N for grazed pure grass and about 60 kg N ha-1 for grass-clover swards.