The availability of nitrogen from sugarcane trash on contrasting soils in the wet tropics of North Queensland

Sugarcane crop residues ('trash') have the potential to supply nitrogen (N) to crops when they are retained on the soil surface after harvest. Farmers should account for the contribution of this N to crop requirements in order to avoid over-fertilisation. In very wet tropical locations, the climate may increase the rate of trash decomposition as well as the amount of N lost from the soil-plant system due to leaching or denitrification. A field experiment was conducted on Hydrosol and Ferrosol soils in the wet tropics of northern Australia using N-15-labelled trash either applied to the soil surface or incorporated. Labelled urea fertiliser was also applied with unlabelled surface trash. The objective of the experiment was to investigate the contribution of trash to crop N nutrition in wet tropical climates, the timing of N mineralisation from trash, and the retention of trash N in contrasting soils. Less than 6% of the N in trash was recovered in the first crop and the recovery was not affected by trash incorporation. Around 6% of the N in fertiliser was also recovered in the first crop, which was less than previously measured in temperate areas (20-40%). Leaf samples taken at the end of the second crop contined 2-3% of N from trash and fertilizer applied at the beginning of the experiment. Although most N was recovered in the 0-1.5 m soil layer there was some evidence of movement of N below this depth. The results showed that trash supplies N slowly and in small amounts to the succeeding crop in wet tropics sugarcane growing areas regardless of trash placement (on the soil surface or incorporated) or soil type, and so N mineralisation from a single trash blanket is not important for sugarcane production in the wet tropics

Nitrogen management guidelines for sugarcane production in Australia: can these be modified for wet tropical conditions using seasonal climate forecasting?

Sugarcane is a highly valuable crop grown in tropical and subtropical climates worldwide primarily for the production of sucrose-based products. The Australian sugarcane industry is located in close proximity to sensitive environments and the apparent declining health of the Great Barrier Reef has been linked to damaging levels of land-based pollutants entering reef waters as a result of sugarcane cultivation undertaken in adjacent catchments. Unprecedented environmental scrutiny of N fertiliser application rates is necessitating improved N fertiliser management strategies in sugarcane.Over time the focus of N fertiliser management has shifted from maximising production to optimising profitability and most recently to improved environmental sustainability. However, current N calculations are limited in their ability to match N fertiliser inputs to forthcoming crop requirements. Seasonal climate forecasts are being used to improve decision-making capabilities across different sectors of the sugarcane value chain. Climate is a key driver of crop growth, N demand and N loss processes, but climate forecasts are not being used to guide N management strategies. Seasonal climate forecasts could be used to develop N management strategies for 'wet' and 'dry' years by guiding application rate, timing and/or frequency of N inputs and the benefit of using alternative forms of N fertiliser. The use of seasonal climate forecasts may allow more environmentally sensitive yet profitable N management strategies to be developed for the Australian sugarcane industry