Nitrogen and water flows under pasture-wheat and lupin-wheat rotations in deep sands in Western Australia. 2. Drainage and nitrate leaching

Quantification of nitrate (NO₃/^-) 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 1994 to 1996. Fluxes of water and NO₃/^- 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 to 1.5 m at the onset of winter rains and analysed for NO₃/^-. 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 NO₃/^- leached from each layer was computed by multiplying the daily drainage through each layer by the estimated concentration of NO₃/^- within the layer. The estimated concentration of NO₃/^- in a layer was calculated by taking into account NO₃/^- either entering that layer through mineralization and leaching or leaving the layer through plant uptake. Mineral N was added to the surface 0.2 m in accordance with measured rates of net N mineralization, and daily N uptake was calculated from the measured above-ground plant N derived from soil N. Root sampling was undertaken to determine root length density under pastures, lupin, and wheat. Cumulative drainage below 1.5 m was similar under wheat and lupin, and accounted for 214 mm from 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 NO₃/^- in soil at the start of the growing season was affected by previous crop, with a lower range following wheat (31-68 kg N/ha) than following legumes (40-106 kg N/ha). These large quantities of NO₃/^- in the soil at the break of the season contributed substantially to NO₃/^- leaching. Leaching of NO₃/^- below 1.5 m in wheat crops accounted for 40-59 kg N/ha where these followed either lupin or pasture. In contrast, less NO₃/^- 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 NO₃/^- leached in pastures.