Nitrogen and water flows under pasture–wheat and lupin–wheat rotations in deep sands in Western Australia. 1. Nitrogen fixation in legumes, net N mineralisation,and utilisation of soil-derived nitrogen

Abstract

Detailed studies on the eciency with which pastures and crops use soil-derived nitrogen (N) in southern Australia are limited. Inefficiencies in the N cycle are indicated by wide spread soilacidification and low N status in wheat grain. The aims of this study were to document rates of N₂ fixation by subterranean clover-based pastures and narrow-leaf lupin, plant uptake of soil-derived N, mineralisation of organic N during legume and cereal phases, and export of N from pastures, lupin,and wheat in relation to climate and soil water. These measurements were undertaken in a rotation experiment conducted on a deep sand located in the northern wheat belt of Western Australia at a site with a long-term average rainfall of 460 mm. The rotations examined over 3 years were 2 years pasture-wheat and lupin-wheat.

The ¹⁵N natural abundance technique was used to differentiate soil-derived N from atmospheric Nin legumes. Biomass production, grain yields, and N contents were standard plant measurements in all treatments. Net N mineralisation between growing seasons was as certained by measuring changes in soil inorganic N to 1·5 m. Growing season net N mineralisation was determined using an in situ method in which soil cores were isolated from plant roots. Anion exchange resin was used to trap NO^-₃ leached below the depth of the soil cores.

Nitrogen fixation by subterranean clover in a mixed pasture ranged from 29 to 162 kg N/ha whereas N₂ fixation by lupins was less variable, ranging from 90 to 151 kg N/ha. Pastures were large consumers of soil-derived N (range 58-154 kg N/ha), with capeweed being the most important sink (range 38-120 kg N/ha). In comparison, wheat and lupins were inefficient users of soil N, removing 29-51 kg N/ha within a season. Another 31-67 kg N/ha of inorganic N in soil was not utilised by wheat or lupin.

Annual net N mineralisation ranged from 80 to 130 kg N, confirming the high rate of decomposition of organic matter in the sandy soil. Mineralisation over summer and autumn, when crop and pastures were not grown, supplied ~25% of the inorganic N produced in soil profiles in 1995 and 20-40% in1996. The study indicated that legumes used in rotations with cereals on deep sands were able to add adequate organic N to soil to insure rates of net N mineralisation sufficient to support cereal yieldsin excess of current shire averages. However, in practice, the asynchrony in supply and demand for N resulted in the inefficient use of soil-derived N by wheat.