Dissolved inorganic nitrogen (DIN) are typically the main focus of nutrient management strategies; however, some studies have found that dissolved organic nitrogen (DON) can be the dominant form of total nitrogen (TN) in several Australian estuaries and catchments. To better understand nitrogen cycling and explore the relationships between measured groundwater DON and environmental factors, thirteen machine learning (ML) techniques were compared in this study. DON was simulated under two scenarios using a range of input variables: 1) detailed nutrient data with landscape and sampling factors, and 2) limited nutrient data with landscape and sampling factors. Most of the tested ML algorithms more accurately predicted DON than when it was estimated from the difference between TN and DIN. Some models show greater adaptability to different modelling conditions, with only a few approaches able to predict with high accuracy using limited input variables (scenario 2). From the models tested, bagged mars, cubist and random forest were selected as optimal. Sample depth, sampling date and specific surface water area were the important non-nutrient input variables for DON prediction, which reveals the significant effect of surface environmental factors and seasonality on groundwater DON.
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