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 understand ing the efficiency with which plants use soil-derived nitrogen (N). A deep sand located in the northern wheatbelt of Western Australia was m aintained under a lupin (Lupinus angustifolius)-wheat (Triticum aestiv um) and a subterranean clover (Trifolium subterraneum) based annual pa sture-wheat rotation from 1994 to 1996. Fluxes of water and NO3- throu gh, and beyond, the root-zone were examined. Drainage was calculated o n a daily basis from measurements of rainfall, evapotranspiration, and the change in soil water content to a depth of 1.5 m. Evapotranspirat ion was estimated from Bowen ratio measurements, and soil water conten t was determined by time domain reflectrometry. Soil was sampled in la yers to 1.5m at the onset of winter rains and analysed for NO3-. Ceram ic 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 throug h each layer by the estimated concentration of NO3- within the layer. The estimated concentration of NO3- in a layer was calculated by takin g into account NO3- either entering that layer through mineralisation 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 mineralisation, and daily N uptake was calculated from the measured ab ove-ground plant N derived from soil N. Root sampling was undertaken t o determine root length density under pastures, lupin, and wheat. Cumu lative drainage below 1.5 m was similar under wheat and lupin, and acc ounted 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 th ese periods was 169 mm from a wheat crop in 1995, and 178 mm from a lu pin crop in 1996. The amount of NO3- in soil at the start of the growi ng 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 l arge quantities of NO3- in the soil at the break of the season contrib uted substantially to NO leaching. Leaching of NO3- below 1.5 m in whe at crops accounted for 40-59 kg N/ha where these followed either lupin or pasture. In contrast, less NO3- was found to leach below 1.5m in p astures (17-28 kg N/ha). Greater N uptake by capeweed (Arctotheca cale ndula L.) than by either wheat or lupin was the main reason for the lo wer amount of NO3- leached in pastures.