MANAGEMENT OF LEGUME BIOMASS TO MAXIMISE BENEFITS TO THE FOLLOWING SUGARCANE CROP

PREVIOUS studies on the wet tropical coast have shown that the management of soybean biomass can have important implications for residue decomposition and mineralisation of nitrogen. The traditional incorporation of soybean as a green manure resulted in rapid mineralisation and, at least on permeable soils under wet seasonal conditions, rapid movement of nitrate nitrogen below the root zone of the subsequent cane crop. However, it was shown that the rate of mineralisation could be slowed and more mineral nitrogen made available to the subsequent cane crop if the legume was managed as a standing crop or slashed and left on the surface. In these previous studies, yields of the following cane crop were not recorded due to uneven cane establishment, which was not related to legume management treatments. A subsequent experiment was conducted between 2001–2003 at Tully Experiment Station where a range of different soybean management treatments were imposed and then planted to sugarcane in June 2002 without any applied nitrogen. The mineralisation of nitrogen from the legume residue followed the patterns previously established, even though the 2002–2003 growing season was one of the driest on record and the soil type was a heavy clay. The cane yield following standing soybean or slashed soybean biomass left on the surface was not significantly different to that following green manure soybean, and in fact there was a strong trend for higher cane yields following standing or surface managed soybean biomass. Where soybean biomass was removed and no nitrogen fertiliser applied, the cane yield was reduced by up to 42%. The good yields following the surface and standing management of soybean biomass were shown to be associated with larger stalks that could be related to more nitrogen being available later in the growing season. Regardless of how soybean biomass was managed, CCS was unaffected. Managing legume biomass on the surface or as a standing crop ideally suits minimum/zero tillage, controlled traffic, legume-based farming systems that the Sugar Yield Decline Joint Venture is developing.