Comparison of lysimeters and porous ceramic cups for measuring nitrate leaching in different soil types

Environmental concerns over nitrate levels in surface and ground water have led to increased efforts to measure nitrate leaching from farmland. This study compared two methods of measuring leaching (lysimeters versus ceramic suction cups) using 12 soil monolith lysimeters (70 cm deep) containing four replicates of three contrasting New Zealand soils: Gorge silt loam, Mataura sandy loam (both stone-free soils) and Lismore stony silt loam. The ceramic suction cups were installed at 35 cm and 60 cm depths. Urea solution was surface applied at 250 kg N ha(-1), followed by regular weekly irrigation. Nitrate leaching loss was calculated by combining the volume of drainage water collected from each lysimeter with the nitrate concentration in either the lysimeter drainage solution or the solution from the suction cups. Cup samples were taken twice per irrigation/drainage sequence: during irrigation and then immediately after irrigation ceased. No evidence of any difference in nitrate concentration and in cumulative leaching was observed between the first and second cup sampling times. However, there was a large variation in individual values measured by the suction cup samples. There were differences between the cumulative leaching loss measured in the lysimeter drainage and values estimated from the cups in the Gorge and Lismore soils. The cumulative leaching loss, measured from the top cups, bottom cups and lysimeter drainage, was 64, 68 and 54 kg N ha(-1) respectively in the Gorge soil, 57, 68 and 62 kg N ha(-1) in the Mataura soil and 61, 103 and 99 kg N ha(-1) in the Lismore soil. It was concluded that suction cups were inappropriate for the determination of cumulative leaching in the structured Gorge soil and the Lismore topsoil, but ceramic cups could provide useful data on cumulative leaching in the Mataura sandy loam soil. It was hypothesised that preferential flow was likely to be the cause of the differences between the results obtained from suction cups and the lysimeter drainage, especially in the Lismore and possibly the Gorge soils, where the small sampler size was not able to capture a representative sample of the pore network responsible for the soil drainage.