Factors influencing nitrogen saturation in Sitka spruce stands in Wales,UK

Preliminary empirical nitrogen critical load exceedance maps for the UK have identified large areas of Wales where nitrogen deposition exceeds the nitrogen critical load, indicating that some ecosystems are at risk from eutrophication. This paper synthesises the monitoring and experimental work which have been carried out to collect evidence for exceedance in spruce plantations in the uplands and to investigate the implications for acidity and eutrophication in these areas. The results have conclusively demonstrated that current nitrogen deposition to mature Sitka spruce stands, planted on freely draining acid soils, is in excess of ecosystem requirements and results in elevated nitrate leaching losses. In contrast, stands with large biological sinks such as aggrading stands or stands with high denitrification potential have low ntirate leaching losses. The controls on the magnitude of leaching losses in the mature stands on this soil type are discussed within the context of two categories: (i) “nitrate saturated” stands which exhibit no retention of incoming nitrate-N but retain incoming ammonium-N as demonstrated in the Welsh NITREX experimental site and (ii) more nitrogen-rich stands which are saturated for both nitrate-N and ammonium-N and respond directly to incoming ammonium-N with immediate increases in nitrate production and thus nitrate leaching losses. There is little evidence for any adverse effects on tree growth or health in response to excess nitrogen deposition, however, tree growth in the most mature stands is now limited by phosphorus and potassium deficiency. The risk of a reduction in soil and stream water quality in acid sensitive areas of Wales due to the link between nitrate leaching and aluminium concentrations has also been confirmed. The results are discussed within the framework proposed by Aber et al. 1989 for the sequence of changes in ecosystem function which occur following long-term chronic nitrogen deposition.