Nitrate leaching losses during pasture renewal - Effects of treading, urine, forages and tillage.

Pasture renewal is a key component of intensive temperate grassland farming. This practice is performed to improve pasture yields, but it may increase nitrate (NO) leaching losses, which can impact on water quality. Farmers face many choices when renewing pasture, however, there is limited information to guide decisions to reduce leaching losses. An experiment was established to study how different management practices and grazing affect biomass production and NO leaching during pasture renewal on a heavy soil. Long-term pasture was either re-sown into ryegrass (Lolium perenne) in autumn (GG) or into forage rape (Brassica napus) followed by ryegrass in spring (GCG). Rape was established following ploughing or direct-drilling. Grazing was simulated in winter, whereby split plots ±urine (600 kg N ha) and ±treading were established. Nitrate concentrations at 1 m depth were measured with suction cups and drainage predicted using a crop model. Estimated NO leaching losses at 1 m depth ranged from 16 to 38 kg N ha, with little difference between GCG and GG. However, the risk of future leaching was much greater below GCG plots. At the end of the study, soil NO between 0.6 and 1 m ranged from 28 to 130 kg N ha in GCG plots and 1-28 kg N ha in the GG plots. Timing of leaching differed between renewal systems, reflecting the differences in plant N uptake and fallow period. Overall, there was no difference in dry matter production between the two systems. Treading resulted in greater compaction, especially in tilled plots and reduced NO leaching by c. 40% - this can be attributed to increased denitrification. Our study demonstrates the complex nature of management and environmental factors and their interaction during pasture renewal. We show that management practices affect the risk and timing of N leaching. Practical implications for farmers are discussed.