AN IN-SITU MINI LYSIMETER WITH A REMOVABLE ION-EXCHANGE RESIN TRAP FOR MEASURING NUTRIENT LOSSES BY LEACHING FROM GRAZED PASTURES

This study describes the construction, installation and evaluation of an in situ mini-lysimeter with a removable ion exchange resin trap for measuring nutrient losses by leaching from grazed pastures. The resin trap efficiently removed solutes from simulated drainage water at a f low rate of 14 mm h-1. Over 88% of each of the solutes was removed fro m synthetic nutrient solution containing 1.65 mM nitrate-N, 1.65 mM am monium-N, 0.25 mM sulfate-S (SO4(2-)-S) and 0.6 mM potassium. In a fur ther test of the system, sulfate leached in simulated rainstorm events from two undisturbed soil cores, taken from legume based pastures of contrasting superphosphate (SSP) fertilizer history following 495 mm o f simulated rainfall, was all recovered using the resin trap. Seven ti mes more SO4(2-) (21.2 kg S ha-1) was leached and recovered from the r esin trap of the core collected from the high fertility (HF, 375 kg SS P ha-1 year-1) site than from the low fertility (LF, 125 kg SSP ha-1 y ear-1) site (3.1 kg S ha-1). As part of the field evaluation of the te chnique, lysimeters with resin traps were placed in the field at four sites (8 lysimeters/site) contrasting in fertilizer history, landslope , and dung and urine return. Two additional lysimeters with drainage c ollection reservoirs (vessels) and eight soil solution samplers were p laced on each site to collect drainage water and soil solution. The am ount of SO4(2-) present in drainage water was more closely related (1: 1, R2 = 0.861) to the amount of SO4(2-) collected by the resin traps o ver a period of 9 months than estimates made using soil solution sampl ers (1:1, R2 = 0.829). The advantages of the resin trap technique over alternative methods for estimating SO4-S leaching losses from field s oils are discussed, as are applications of the technique for studying nutrient losses and cycling in grazed pastures.