Developing a nitrogen load apportionment tool: Theory and application

•A new tool estimating nitrogen load apportionment was developed.•The tool is an alternative approach to complex conceptual models.•It integrates TN Soil System Budget, riverine export, export coefficient model.•Setting export coefficients without any calibration leads to a bias in TN loads. In this work, a new nitrogen load apportionment tool was developed to quantify the anthropogenic pressures from point sources and diffuse sources (DSs) on the water in a basin, and to identify the areas contributing most of the total nitrogen (TN). The model, which is an alternative approach to complex conceptual models, was tested in the Canale d’Aiedda Basin (SE Italy). It integrates the TN soil system budget (SSB), TN riverine export (NRE) and TN export coefficient modelling. The results of the SSB showed a TN surplus for productive land (∼60 kg ha−1 yr−1). Major TN inputs were derived from fertilisers (∼89 kg ha−1 yr−1, 77% of the total input) and animal manure (∼13 kg ha−1 yr−1, 11% of the total input). Crop uptake was the main output (∼39 kg ha−1 yr−1, 70% of the total output). NRE was estimated through the measurement of streamflow and TN concentrations in two stream sections. The average NRE per unit area of productive land was 5.22 kg ha−1 yr−1. Groundwater was the major receptor of the TN from DSs. The TN runoff export coefficients, estimated on the basis of environmental factors and calibrated with riverine load measurements, were lower than the TN leaching fractions. The results show that setting export coefficients based only on environmental factors, without any calibration, leads to an overestimation of TN load in runoff, and to an underestimation of TN load in leaching.