Scoping land conversion options for high DIN risk, low-lying sugarcane, to alternative use for water quality improvement in Dry Tropics catchments

Abstract

This project continues an assessment framework of scoping out land use transition options for land managers and government to consider in advancing towards the water quality improvements and long-term sustainability of coastal aquatic ecosystems and the Great Barrier Reef. • The framework here, applied in the Dry Tropics, builds on previous research in the Wet Tropics (NESP TWQ Project 2.1.2) where we combine both spatial and economic information to identify options for transitioning low-lying cane land, with a high risk of dissolved inorganic nitrogen (DIN) loss, to lower DIN-risk uses. • The cost-effectiveness of DIN reduction ($/kg DIN) was calculated for alternative land uses in the Lower Burdekin and Mackay regions. The levels of DIN credit payments required to deliver 5-, 10- and 15-year payback on landholders’ upfront investment in land use change were also calculated. Cane production that is consistent with the 2019 Reef Regulations and the Prescribed Methodology for Sugarcane Cultivation was used as a baseline land use throughout. • In terms of cost-effectiveness and the likely level of DIN credit payments required to incentivise land use change, conversion to cattle fattening, farm forestry and restored ecosystem wetlands perform well in all regions. Cattle fattening and farm forestry perform particularly well in Mackay region. Restored ecosystem wetlands perform well in the Burdekin Delta and particularly well in the Burdekin River Irrigation Area (BRIA). • There is a paucity of quantitative information on the DIN removal capacity and conversion costs for wetland restoration or engineered treatment wetlands in the Wet and Dry Tropics. Therefore, there is uncertainty around the cost-effectiveness values and end users should be cognisant of the assumptions used in the framework. In general, wetland restoration or engineered treatment wetlands are most cost-effective when conversion costs are low and DIN removal capacity is high – this is partly a function of the hydrology and maximising the residence time for processing nutrients in treatment wetlands. Hydrological regimes and nutrient loads are related to wetland size, position, wetland type, and connectivity with highDIN generating watersheds. Placing both forms of wetlands within an integrated treatment train might further improve water quality, though this needs to be weighed against the additional costs incurred. Further work is required to understand the timing of DIN movement through dry tropics catchments as this will heavily influence wetland treatment efficacy, and the size and positioning of wetlands necessary to achieve water quality objectives. • Land use transition could be considered as part of a mix of mechanisms to address DIN loss. It complements other mechanisms, if targeted at the relatively small areas of poorly performing sugarcane land, while best management practice adoption initiatives should focus on the remaining, more productive sugarcane land.