Trends in fertiliser use and efficiency in the Great Barrier Reef catchment area

Time series data of annual fertiliser application in river basins provide a foundational record of crop management practices and environmental change. Here we produce an annual time series of synthetic nitrogen (N) and phosphorus (P) fertiliser application for the river basins of the Great Barrier Reef (GBR) catchment area of north-eastern Australia spanning from 1910 to 2024. We draw on statistical data of basin-scale land use for various crop types coupled with known application rates to develop the time series, accounting for the uncertainties in both crop area and application rates. The compilation shows that N and P application has declined across four of the six natural resource management regions over the past one to two decades, however, there is considerable variability at the basin-scale, particularly in the timing of peak fertiliser application. This variability reflects changes in land use including shifts in crop types and/or cropped areas, as well as reductions in fertiliser application rates. We also show that mill mud/ash application to sugarcane crops provides extra plant-available N and P that should be incorporated into farm-scale nutrient budgets. Finally, we evaluate the long-term trends in N use efficiency in sugarcane, the dominant crop in the GBR catchment area, and reveal that the industry has improved markedly over the past one to two decades while maintaining consistent yields. These data contribute an important line of evidence for evaluating long-term trends in water quality and provide insights into the changing environmental footprint of croplands within the GBR catchment area.