FACTORS INFLUENCING NITROGEN SATURATION IN SITKA SPRUCE STANDS IN WALES, UK

Preliminary empirical nitrogen critical load exceedance maps for the U K have identified large areas of Wales where nitrogen deposition excee ds the nitrogen critical load, indicating that some ecosystems are at risk from eutrophication. This paper synthesises the monitoring and ex perimental work which have been carried out to collect evidence for ex ceedance in spruce plantations in the uplands and to investigate the i mplications for acidity and eutrophication in these areas. The results have conclusively demonstrated that current nitrogen deposition to ma ture Sitka spruce stands, planted on freely draining acid soils, is in excess of ecosystem requirements and results in elevated nitrate leac hing losses. In contrast, stands with large biological sinks such as a ggrading stands or stands with high denitrification potential have low nitrate leaching losses. The controls on the magnitude of leaching lo sses in the mature stands on this soil type are discussed within the c ontext of two categories: (i) ''nitrate saturated'' stands which exhib it no retention of incoming nitrate-N but retain incoming ammonium-N a s demonstrated in the Welsh NITREX experimental site and (ii) more nit rogen-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 litt le evidence for any adverse effects on tree growth or health in respon se to excess nitrogen deposition, however, tree growth in the most mat ure 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 wit hin the framework proposed by Aber et al. 1989 for the sequence of cha nges in ecosystem function which occur following long-term chronic nit rogen deposition.