COMPARISON OF LEGUME-BASED CROPPING SYSTEMS AT WARRA, QUEENSLAND .1. SOIL-NITROGEN AND ORGANIC-CARBON ACCRETION AND POTENTIALLY MINERALIZABLE NITROGEN

Effects on soil nitrogen accretion and potentially mineralisable nitro gen were studied as part of a long-term field experiment established i n 1986 to test alternative legume-based systems for restoring fertilit y in a Vertisol. Organic C accretion was also measured to ascertain th e changes in organic matter content. The systems, which were studied o nly during 1989 and 1990, were a grass+legume ley (purple pigeon grass , Rhodes grass, lucerne, annual medics) of 4 years duration followed b y wheat; a 2-year rotation of wheat (lucerne undersown) and lucerne; a 2-year rotation of wheat (medic undersown) and medic; a 2-year rotati on of chickpea and wheat; and continuous wheat as control. Soil total N and organic C significantly increased in the 0-10 cm soil layer only under the grass+legume ley. There was no significant change in the so il C/N ratio. Plant residues contained from 52 to 104 kg N/ha in 1990 st the end of the legume phase, with high values for root N in the gra ss+legume ley. A comparison of N accretion versus fixation at the end of the legume-based systems in 1990 showed that net accumulation of N exceeded fixation in soil under lucerne and grass+legume leys; in the latter, net accumulation of 779 kg N/ha over 3 . 75 years was measured compared with 384 kg N/ha for N-2 fixation. Part of the accumulation of N may have been due to uptake of NH4-N from the deep subsoil. Altho ugh values for soil mineral N (0-120 cm) were low at the end of all th e legume-based systems, a deep subsoil (120-300 cm) accumulation of NH 4-N was found in all treatments. The nitrogen mineralisation potential s (N-o) for 0-10 cm depth samples taken at the end of the legume phase in 1989 were higher in all the legume-based systems (105-182 mg N/kg) than the wheat control (57 mg N/kg). The rapid biological tests of N availability, both waterlogged and aerobic incubation, were more sensi tive to treatment differences than N, in the surface and subsoil (rang e 12-78 mg N/kg for 0-10 cm soil for the waterlogged procedure). The r apid chemical tests, hot KCl extraction and the autoclave index, showe d small treatment effects and did not appear to be useful availability indices. The pasture management (graced v. mown and removed) had no s ignificant effect on total N, organic C and N availability indices in this alkaline Vertisol during the study period.