An Empirical Approach for Assessing the Relationship Between Nitrogen Deposition and Nitrate Leaching from Upland Catchments in the United Kingdom Using Runoff Chemistry

Samples were collected from 13 upland sites (main inflow and loch outflow) in the UK along an N deposition gradient of 12-50 kg ha-1 yr-1 to determine the relationship between N deposition and NO3- concentrations in surface waters. There was no direct correlation between NO3- leaching and soluble inorganic N deposition at these sites, but a significant relationship with NO3- was found using a deposition function incorporating dissolved organic carbon (DOC) flux from each catchment. A similar but less significant relationship was found between NO3- concentration and DOC:DON ratio in runoff water. Catchments showed evidence of N saturation, i.e., when mean NO3- concentration exceeded 5 µeq L-1, when the mean DOC:DON ratio fell below an approximate value of 25. Five other large loch sites (LLS) with multiple subcatchments were used to test these relationships and for four of these mostly heathland sites the predicted NO3- concentrations closely matched measured values. At the fifth site, where most subcatchments were forested, the deposition function and DOC:DON ratios gave conflicting predictions and both methods generally underestimated measured NO3- concentrations. If the capacity of these catchments to retain deposited N is determined by C supply then many upland catchments in the UK may experience increasing NO3- concentrations in runoff in the future at current or increased levels of N deposition.