Improving nitrogen fixation of crop legumes through breeding and agronomic management: analysis with simulation modelling

The nitrogen fixed by legumes is a valuable resource in agriculture, with crop legumes alone contributing as much as 20% of the nitrogen requirements of the world's grain and oilseed crops. Increasing legume nitrogen fixation through genetic improvement and more efficient management would have large economic benefits. Breeding for improved nitrogen fixation has, to a large extent, not been successful. Suggested reasons include the difficulty in combining single traits like nitrogen fixation with other traits, such as disease resistance, seed quality and yield, a lack of focus of programs and a lack of screening methodologies. Agronomic management of legume nitrogen fixation offers other opportunities. The challenge is to package those opportunities and provide legume growers with tools for understanding the factors determining nitrogen fixation, while at the same time providing them with site-specific management options. The potential of simulation modelling for assessing genetic and management options for enhancing nitrogen fixation of soybean grown at Warwick in south-eastern Queensland was investigated in a series of 30-year simulations using the APSIM modelling framework. The APSIM-soybean module was first adjusted to reflect observed responses of nitrogen fixation to soil nitrate. The subsequent simulations indicated that (genetically based) symbiotic nitrate tolerance would have only marginal benefits on residual soil nitrate (7 kg N/ha at sowing soil nitrate of 100 kg N/ha). Management of the crop for highest grain yield through optimising sowing dates, plant density and fallow length provided the best opportunities for increasing nitrogen fixation. The use of APSIM as a tool for managing legume nitrogen fixation appears to have merit.