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

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”.