Monitoring Nitrate Leaching from Submerged Drains

ABSTRACT Monitoring nitrate N (NO3–N) leaching is important in order to judge the effect that agricultural practices have on the quality of ground water and surface water. Measuring drain discharge rates and NO3–N concentrations circumvents the problem of spatial variability encountered by other methods used to quantify NO3–N leaching in the field. A new flow‐proportional drainage water sampling method for submerged drains has been developed to monitor NO3–N leaching. Both low and high discharge rates can be measured accurately, and are automatically compensated for fluctuations in ditch‐water levels. The total amount of NO3–N leached was 10.6 kg N ha−1 for a tile‐drained silt‐loam soil during the 114‐d monitoring period. The NO3–N concentrations fluctuated between 5 mg L−1 at deep ground water levels and 15 mg L−1 at shallow levels, due to variations in water flow. A flow‐proportional drainage water sampling method is required to measure NO3–N leaching accurately under these conditions. Errors of up to 43% may occur when NO3–N concentrations in the drainage water are only measured at intervals of 30 d and when the precipitation excess is used to estimate cumulative NO3–N leaching. Measurements of NO3–N concentrations in ground water cannot be used to accurately estimate NO3–N leaching in drained soils.