Removal of previously planted tree and shrub species and the impacts on the shallow groundwater chemistry of coastal dune systems in northern Ireland

Nitrogen (N) input to naturally oligotrophic dune systems is a global issue. This study assessed the groundwater nitrogen impact of growing sea buckthorn, sea buckthorn and tree removal, a golf course and a small stream on two coastal sand dune systems over several years. Groundwater nitrogen concentrations were monitored at both sites, during and after the removal of previously planted species. Sea buckthorn and Corsican Pine were removed in a single season from site one, however a series of sea buckthorn removal events occurred at site two over a 4 year period. Sea buckthorn was pulled up with the roots and burnt at site, but the tree roots were left in the ground. Concentrations of NO3-N and total N from shallow piezometers and eluted sand samples were assessed against pre-defined criteria which treat N as a groundwater contaminant in coastal dune systems. It was found that neither the management of the golf course at the first site, or the removal of the Corsican pines at the second increased groundwater nitrogen above the 'level of concern', set at <0.2 mg/l N. Growing sea buckthorn was seen to cause 'possible contamination' in groundwater (between 0.2 and 1 mg/l N). Sea buckthorn removal increased the groundwater NO3-N concentrations above the Threshold Value of 3 mg/l NO3-N and subsequent attenuation of groundwater nitrogen concentrations to 'below concern' took <5 years. Dual nitrate isotopes provided strong evidence for denitrification as the primary reason for this decrease in concentration.