A legume rotation crop lessens the need for nitrogen fertiliser throughout the sugarcane cropping cycle

While the amount of nitrogen (N) contributed to agricultural soils by above ground legume biomass can be over 300 kg N/ha when grain is not harvested, its availability to subsequent sugarcane crops is uncertain. In Australia it is generally accepted that inorganic N fertiliser applied to the sugarcane (Saccharum officinarum L.) crop in the first year (termed the plant crop) following a ‘good’ legume fallow crop that is not harvested can be substantially reduced, or even eliminated. However, current recommendations do not consider the carry over of legume N to crops beyond the plant crop (termed ratoons), for which standard N fertiliser rates are considered necessary. Based on a simple field experiment extending the duration of a soybean (Glycine max L.) fallow and two sugarcane crops (plant and first ratoon), cropping system simulation was used to provide a first estimate of how long and how much soybean N remains available for uptake by sugarcane following a soybean break crop. The soybean and sugarcane rotation plot was carried out at Mossman, Queensland over 3 years during which crop yield, plant N, and total soil C and mineral N were measured. The soybean variety Leichhardt produced 9 t/ha (±0.7) above ground dry weight, containing 301 kg N/ha (±36) which contributed to the soil N stores. After independently developing parameter values for Leichhardt, model simulations were run. The model explained 91% of the variation observed in soil mineral N to a depth of 1.5 m. Additional legume and sugarcane rotation simulations were run for the Burdekin and Bundaberg sugarcane regions. Across all three study sites the simulations suggested that legume N was available to the sugarcane crop up to the fourth ratoon. Acknowledging the limitations of this initial exploratory study, it is hypothesized that potential reductions in fertiliser application rate could be up to approximately 100% in the first ratoon, and 60%, 25% and 10% in the subsequent ratoons. These findings require further validation but suggest a potential economic and environmental win–win outcome from refining N management practices in sugarcane-legume rotation cropping systems in Australia and other countries.