Abstract

Quantification of nitrate (NO^-₃) leaching is fundamental to understanding the efficiency with which plants use soil-derived nitrogen (N). A deep sand located in the northern wheatbelt of Western Australia was maintained under a lupin (Lupinus angustifolius)-wheat (Triticum aestivum) and a subterranean clover (Trifolium subterraneum) based annual pasture-wheat rotation from 1994to 1996. Fluxes of water and NO^-₃ through, and beyond, the root-zone were examined. Drainage was calculated on a daily basis from measurements of rainfall, evapotranspiration, and the change in soil water content to a depth of 1·5 m. Evapotranspiration was estimated from Bowen ratio measurements,and soil water content was determined by time domain reflectrometry. Soil was sampled in layers to1·5 m at the onset of winter rains and analysed for NO^-₃ . Ceramic suction cups were installed at 0·25, 0·4, 0·6, 0·8, 1·0, 1·2, and 1·4 m to sample soil solution from June to mid August. The NO^-₃ leached from each layer was computed by multiplying the daily drainage through each layer by the estimated concentration of NO^-₃ within the layer. The estimated concentration of NO^-₃ in a layer was calculated by taking into account NO^-₃ either entering that layer through mineralisation and leachingor leaving the layer through plant uptake. Mineral N was added to the surface 0·2 m in accordance with measured rates of net N mineralisation, and daily N uptake was calculated from the measured above-ground plant N derived from soil N. Root sampling was undertaken to determine root lengthdensity under pastures, lupin, and wheat.

Cumulative drainage below 1·5 m was similar under wheat and lupin, and accounted for 214 mmfrom 11 May to 15 August 1995 and 114 mm from 2 July to 15 September 1996. The cumulative evapotranspiration (E_a) over these periods was 169 mm from a wheat crop in 1995, and 178 mm from a lupin crop in 1996. The amount of NO^-₃ in soil at the start of the growing season was afiected by previous crop, with a lower range following wheat (31-68 kg N/ha) than following legumes (40-106 kgN/ha). These large quantities of NO^-₃ in the soil at the break of the season contributed substantially to NO^-₃ leaching. Leaching of NO^-₃ below 1·5 m in wheat crops accounted for 40-59 kg N/ha where these followed either lupin or pasture. In contrast, less NO^-₃ was found to leach below 1·5 m in pastures (17-28 kg N/ha). Greater N uptake by capeweed (Arctotheca calendula L.) than by either wheat or lupin was the main reason for the lower amount of NO^-₃ leached in pastures.