Predicting effects of N pollutant load on plant species based on a dynamic soil eutrophication indicator. Final report on Nitrogen Effects on Dune Species (NEDS) project
The effects of nitrogen (N) pollution on dune grassland were explored using a model chain that predicts how plant species are likely to respond to changes in soil chemistry. The model chain was calibrated to data from an N addition and grazing experiment at Newborough in Anglesey. The N14C soil model predicted increases in plant productivity and plant litter carbon (C) inputs with more N addition, resulting in an initial and persistent increase in soil C/N ratio. This contrasts with predictions of decreasing C/N ratio from the simpler N saturation model currently used to calculate nutrient-N critical load exceedance. All N addition rates also caused persistent increases in plant-available N. Using the MultiMOVE niche models for plant species typical of dune grassland, these soil changes were related to changes in the overall nutrient enrichment of the flora, as indicated by mean Ellenberg N score, and thereby to the habitat’s suitability for particular species. Declines in Habitat Suitability were interpreted as increasing risk to the species. At rates above 30 kg N ha-1y-1, the more sensitive species were placed at risk almost immediately, but at smaller rates species were placed at risk later on, with an increasing delay with less N addition. At rates lower than the critical N load for calcareous fixed dunes, more mesotrophic species were placed at risk. Species viewed as positive indicators of habitat condition were placed at risk under both high and low rates of N addition. Changes in Habitat Suitability due to changed grazing regime had greater simulated effects on Habitat Suitability. For more confidence in the model chain, differences between the spatial and temporal effects of N addition need to be addressed. More information on the effects of N on vegetation structure and litterfall would be very useful, and objective measurements of vegetation height should be included in monitoring schemes alongside floristic recording. Management was shown to be critical for mitigating the effects of N. Although N removal through grazing or mowing is unlikely to export sufficient N to prevent enrichment, reducing vegetation height can prevent competitive species shading out the more distinctive low-growing, light-demanding dune species
