NITRATE LEACHING FROM GRAZED GRASSLAND LYSIMETERS - EFFECTS OF FERTILIZER INPUT, FIELD DRAINAGE, AGE OF SWARD AND PATTERNS OF WEATHER

Drained and undrained grassland lysimeter plots were established in 19 82 on a clay loam of the Hallsworth series at a long-term experimental site in south-west England. The plots were continuously grazed by bee f cattle, and received fertilizer at either 200 or 400 kg N ha-1 per a nnum to the existing permanent sward, or at 400 kg N ha-1 to a new swa rd, reseeded to perennial ryegrass following cultivation. Drainage wat er was monitored at V-notch weirs and sampled daily for the analysis o f nitrate-N. Seven years of data are presented (five years for the res eeded swards). On the drained plots a large proportion of the rainfall was routed preferentially down large pores to the mole drains, whilst on the undrained plots, drainage was mainly by surface runoff. The av erage quantities of nitrate N leached per year were 38.5, 133.8 and 55 .7 kg ha-1 from the old sward that received 200 and 400 kg N ha-1, and from the reseed that received 400 kg N ha-1 fertilizer, respectively. Ploughing and reseeding resulted in a two-fold reduction in leaching, except during the first winter after ploughing, and twice as much lea ching occurred after a hot, dry summer as after a cool, wet one. Nitra te concentrations in drainage from either drained or undrained plots w ere rather insensitive to rainfall intensity, such that concentration was a good predictor of nitrate load for a given drainage volume. The drainage volume determined the proportion of the leachable N that rema ined in the soil after the winter drainage period. Initial (peak) conc entrations of nitrate N ranged, on average, from 55 mg dm-3 for the dr ained old sward that received 400 kg N ha-1 fertilizer, to 12 mg dm-3 for the undrained sward at 200 kg N ha-1 fertilizer input. Concentrati ons of nitrate N in drainage from similar, unfertilized plots rarely e xceeded 1 mg dm-3. The results suggest that manipulating the nitrate s upply can lessen leaching and that the route of water through soil to the watercourse determines the maximum nitrate concentration for a giv en load.