Influence of soybean residue management on nitrogen mineralisation and leaching and soil pH in a wet tropical environment

A study was undertaken at the Sugar Yield Decline Joint Venture sub-station at Feluga in north Queensland in 1998 to investigate the effect of soybean residue management on nitrogen mineralisation and leaching, and soil pH. Three residue management practices were involved: residue incorporated, residue left standing and residue slashed and left on the surface. Changes in soil mineral nitrogen were monitored over a 136 day period from the initiation of residue management treatments (70 days prior to sugarcane planting) to two months after sugarcane was planted. At planting of the sugarcane, those treatments where residue had been incorporated to 20 cm had accumulated 206 kglha of the nitrogen originally contributed by the soybean crop over a depth interval of 110 cm. By contrast, in those treatments where residue was either left standing or retained on the soil surface, only 81 and 58 kglha, respectively had accumulated over the same depth interval. By day 136, most of the mineralised nitrogen in the residue incorporated treatment had moved to below 30 cm and the net loss of nitrate (the amount that had moved below 110 em) was estimated to be 104 kg/ha. In those treatments where residue was not incorporated, the total amounts of nitrogen mineralised to nitrate appeared to be considerably less and most of this nitrogen was retained in the G-110 cm zone. In addition, with the mineralisation of nitrogen in the surface soil layers, significant shifts in soil pH were observed associated with the generation of protons from the mineralisation process. These findings have significant implications for the management of legume residue. In order to effect maximum utilisation by the plant crop of nitrogen from the legume break, establishment of the sugarcane crop should occur as soon as possible after maturity of the break crop. Further, the legume residue should not be incorporated but retained on the surface to slow down the rate of mineralisation and allow better utilisation of legume nitrogen by the sugarcane crop. It is suggested that maximisation of the benefits of such strategies will be achieved by adopting a controlled traffic strategy with minimal soil disturbance for planting of sugarcane.