Loss of nitrogen and phosphorus in tile drainage as influenced by urea application and grazing animals

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Abstract
The effects of urea fertiliser and grazing dairy cattle on the amounts of N and P forms transported in tile drainage from permanent pasture were investigated. In 4 weeks after an application of urea (60 kg N/ha) in July 1975 the increased loss of nitrate (NO3) and total nitrogen (TN) in tile drainage accounted for 2.0 and 2.3% respectively, of the applied fertiliser N. Grazing resulted in a dramatic increase in the concentrations of dissolved inorganic P (DIP) and particulate P (1.5- and 40-fold increases respectively). Concentrations of both NO3 and TN increased 5-fold immediately after grazing, although this effect was less sustained than that caused by urea application. Grazing was of comparable importance to urea application of the loss of NO3 and TN in tile drainage. The greater increase in the loss of particulate P (46.9 g/ha/4 weeks), compared to that of DIP (23.1 g/ha/4 weeks), resulted from a 50% increase in the amount of sediment carried. A 17% reduction in the amount of water discharged after grazing is attributed to the destruction of drainage channels and surface “pugging”.