Background, results and conclusions of the Dutch Priority Programme on Acidification

The second phase of the Dutch Priority Programme on Acidification was carried out from 1988-1991. The main research areas were ammonia emissions, the deposition of SOx, NOy and NHx exposure-effect relationships for forest ecosystems (on dry sandy soils) and heathlands, and effectiveness of control measures. The last topic included the evaluation of emission-deposition scenarios and the assessment of critical loads. In 1989, 54% of the acid deposition in the Netherlands originated from the country itself. NHx is the main component in the acid deposition (46% in 1989, 80% from NL). Agriculture contributes the largest portion to the NL fraction of the acid deposition (62% in 1989). Nevertheless, sulphur is the most important factor in soil acidification (contribution about 65% versus nitrogen 35%). Soil acidification is the main effect of acid deposition in the Netherlands, resulting in depletion of the Al buffer and deterioration of the groundwater quality. Disturbance of nutrient balances occurs (also caused by excess nitrogen), resulting in an increasing risk of damage caused by traditional stress factors like drought and pests. In the Netherlands, no mono-causal relationship exists between acid load and forest vitality in terms of needle loss and discolouration; acidification plays a role but it is not the only factor. In general, its impact is a weakening of the tree's resistance to attack by other stress factors. From calculations with the Dutch Acidification Systems model an average deposition of 1400 mol H+ per ha for the Netherlands as a whole in the year 2010 and an average deposition of 1400 mol H+ per ha for forests in the year 2050, seems sufficient to minimise the impact of acidification. 3 refs., 10 figs., 3 tabs