The impact of intensive dairy farming on the leaching losses of nitrogen and phosphorus from a mole and pipe drained soil

Abstract

While it is widely believed that intensive dairy farming is a major contributor to the increased nutrient loads in surface waters, there is little current research quantifying the magnitude of nutrient loss from dairy farms to waterways, particularly from artificial drainage. An experimental site has been established on a Pallic soil (Tokomaru silt loam) to measure the impacts of intensive dairying on the quality and quantity of drainage water exiting from an artificial drainage system. A key component of this study is the development and evaluation of a land-based treatment system for farm dairy effluent (deferred irrigation). The research site has eight replicated plots, each with an isolated mole and pipe drain network. All the plots are subjected to the farm's standard grazing management. Four of the plots receive fer tiliser according to the farm's fertiliser programme, while the other four plots receive applications of farm dairy effluent. Measurements of drainage flows during year one of this study showed that the average concentrations of total nitrogen (12.9 mg N/L) and total phosphorus (0.15 mg P/L) in drainage water for the winter of 2002 under standard dairy farming practices were all well above the levels necessary to prevent aquatic weed growth in fresh water bodies. Adherence to the scheduling criteria prescribed by the 'deferred irrigation' system prevented the direct loss of nutrients during irrigation of farm dairy effluent in the summer of 2001/2002. Summer applications of farm dairy effluent did not increase N loss in subsequent winter drainage. Effluent irrigation increased P loss during the subsequent winter drainage period by 0.52 kg total-P/ha (0.38 kg P/ha as DIP). However, this increase in total loss corresponds to less than 4% of the P (16 kg/ha) applied as effluent. Deferred irrigation proved to be a very successful tool for minimising nutrient losses from effluent irrigated areas in direct drainage of effluent at the time of irrigation and subsequent winter drainage. Dairy cattle grazing events also increased nutrient concentrations in drainage waters following grazing by approximately 5 mg total-N/L (nearly all in the nitrate form) and 0.1 mg total-P/L (nearly all in the DIP form). The effect of an application of urea in spring on nitrate-N concentrations in drainage water was minimal. Keywords: dairying, effluent irrigation, mole and pipe drainage, nitrogen, nutrient leaching phosphorus, water quality