A review of methods for nitrate leaching measurement

Nitrate leaching is responsible for the main environmental impact of nitrogen fertiliser application on water quality, however its measurement is difficult. Direct methods require the measurement of drainage rate and its nitrate concentration. Direct measurement of drainage flow is only possible by using lysimeters, but normally it is estimated from water balance calculations, or from field measurements of hydraulic water gradients and hydraulic conductivity. However, hydraulic conductivity is highly variable and measurements in the field are very laborious, so this has stimulated the development and use of empirical estimation methods. Under cropping, the main difficulty in estimating drainage from water balance measurements stems from the difficulties of obtaining accurate values of evapotranspiration. A chloride balance method for measuring drainage can be a good choice under certain circumstances. There are different ways of sampling the draining soil solution and one of the most frequent is porous suction cups, however, questions about their validity still remain. Pan samplers are also used, but having to determine their collection efficiency is one of their main drawbacks. The more recently developed capillary passive samplers still need further testing.