The prediction of nitrate leaching with the first-order acidity balance (FAB) model for upland catchment in Great Britain

The relative contribution of N deposition to the acidification of freshwaters in Great Britain has increased over the last few years as S deposition has fallen in line with reduced emissions. In certain high deposition areas of Great Britain, NO3 --based acidity can equal or exceed the contribution of SO4 2--based acidity in some upland waters. Here we apply the first-order acidity balance model (FAB) to predict the maximum N leaching from 13 study catchments at future steady state. Using mean water chemistry and catchment soils data, along with long-term default values for N sink processes, we predict NO3-N leaching at much higher rates than currently are being measured in surface waters, with a mean increase of 10.5 kg ha-1 yr-1. As a result, mean acid neutralizing capacity would decline to less than 0 meq L-1 at 4 sites. While there are uncertainties associated with model parameterization relating to the short-term storage of N within catchment soils and vegetation, model outputs do indicate much greater leaching of N at some time in the future as steady-state is achieved.