COMPARISONS OF METHODS FOR MEASURING THE LEACHING OF MINERAL NITROGENFROM ARABLE LAND

Comparisons were made between 1988 and 1991 to evaluate three methods of estimating the leaching of mineral nitrogen (N) from unstructured f reely draining sandy loam and loamy sand soils. The studies compared t he drainage patterns and quantities of N (almost exclusively nitrate) leached from monolith lysimeters with those estimated from ceramic suc tion cups and soil core extracts. The latter two methods gave direct m easurements of the mineral N concentrations in drainage. but required an estimate of the drainage volume calculated from meteorological obse rvations and evapotranspiration equations to give total N leached. A b romide tracer was also used to confirm conclusions from nitrate leachi ng studies. There was a delay in the onset of drainage from free drain ing lysimeters because they lack the subsoil matric potential of field soils. However, total annual drainage measured by lysimeters or calcu lated from meteorological observations was similar, providing that ret urn to field capacity was correctly identified in the field soil. Duri ng the first year there were discrepancies between methods which were attributed to soil disturbance during lysimeter and/or ceramic cup ins tallation. In the second and third years of the experiment, estimates of N leaching losses using the lysimeters and ceramic cups were in goo d agreement. Nitrate concentrations in soil solution at a depth of 130 cm measured from soil core extracts were smaller than found by the ot her methods during the second year and the peak concentrations were si gnificantly different (P<0.05). However, total overwinter N leached wa s not significantly different. Thus, while lysimeters and cups can be used to quantify leaching losses on unstructured, free draining soils if used correctly, the use of soil core extracts is questionable.