Investigating relationships between nitrogen inputs and in-stream nitrogen concentrations and exports across catchments in Victoria, Australia

Abstract. Accurate information on nitrogen (N) inputs to landscapes is crucial for understanding and predicting in-stream N concentrations and downstream N export. However, obtaining detailed catchment-scale data remains challenging due to spatial and temporal variability. We developed a statistical model based on mean annual rainfall to estimate fertiliser N inputs for four agricultural land uses in Victoria, Australia. These estimates, together with contributions from purchased feed, human food N, and biological fixation, were used to (a) examine how N inputs relate to stream total nitrogen (TN) concentrations and export, and (b) assess the influence of rainfall, hydrology, and other catchment characteristics on TN export across 59 diverse catchments. The model revealed a strong positive correlation between average rainfall and fertiliser N input for each land use at the Catchment Management Authority (CMA) (i.e., regional) level, with R2 values ranging from 0.55 to 0.72. Stream TN concentrations were strongly correlated with total N inputs (R2 = 0.72) and fertiliser N inputs (R2 = 0.68). Stream TN export also showed significant relationships with total N inputs (R2 = 0.50) and fertiliser N inputs (R2 = 0.53). The proportion of total N inputs exported varied widely, ranging from 1.4 % to 50 %, with an average of 11 %. This variation was strongly influenced by agricultural activity and hydroclimatic factors. Moreover, the average export proportion was notably lower than values reported for other regions globally, which may reflect Australia's generally lower N input levels. These findings provide a useful tool for water quality assessment and can guide targeted strategies to reduce nitrogen pollution in streams.