Nitrogen mineralisation of legume residues

LEGUME BREAK CROPPING is an important part of the improved sugarcane cropping system, with the potential to supply fixed nitrogen (N) to the succeeding cane crop. However, the effectiveness of a legume crop in achieving this outcome is directly related to the rate of mineralisation of the legume residues. Therefore practices that can potentially manipulate the rate of mineralisation of legume residues are required. A 200 day incubation study was undertaken to: 1) compare the carbon (C) and N mineralisation dynamics of three different legume residues, namely a soybean at maturity, a soybean sprayed out early (green manure) and a peanut at maturity; 2) compare the C and N mineralisation dynamics of three legume management practices, namely surface applied legume residue, incorporated legume residues and legume residues incorporated with a nitrification inhibitor and; 3) examine the effects of legume type and management on nitrous oxide (N2O) fluxes. The incubation was undertaken on three soils from Bundaberg, Ayr and Ingham, at 50% water holding capacity and 25 ?C. Mature soybean exhibited net N mineralisation throughout the 200 days, while mature peanut and green manure soybean had initial net N immobilisation followed by net mineralisation. Mature soybean mineralised the largest (80.9 mg N/kg soil) amount of N, followed by green manure soybean (47.6 mg N/kg soil) and mature peanut (30.1 mg N/kg soil). This was due to the lower C:N ratio of the mature soybean, compared with other legumes. On average, incorporated residues mineralised 192% more N than unincorporated residues, while incorporated residues released 324% more N2O than unincorporated residues. The application of a nitrification inhibitor to the legume residues, was effective at reducing nitrification for 10 days in Ayr and Ingham soils, and up to 30 days in the Bundaberg soil, due to its higher sand content. However, the nitrification inhibitor was only effective at significantly reducing cumulative N2O flux from mature soybean. This study has shown that the management of residues can manipulate their N mineralisation dynamics, and if used in the appropriate situation could help to maximise legume N supplied to the succeeding plant cane crop.