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 1.5m 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 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 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 NO 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 1.5m 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.