Soil acidification effects on fine root growth of Douglas-fir on sandy soils.

The ammonium sulphate deposited in forest ecosystems in the Netherlands as a result of air pollution currently exceeds 80 kg N ha <sup>-1</SUP>yr <sup>-1</SUP>locally. To study the influence of this air pollution on fine root density and its dynamics, fine root growth was monitored for three years in two young mature stands of Douglas-fir on sandy soils in the central part of the Netherlands in the ACIFORN project (ACIdification of FORests in the Netherlands). In the drier site the fine root density in early summer was higher and more strongly reduced after a dry spring than at the more favourable site. The fine root density appeared to depend on the proximity of the tree stems in the stand. In normal years, the main peak of fine root growth occurred in spring, with less growth during summer. After a dry spring, fine root growth on the drier site reacted strongly to periods with rain in summer. In a greenhouse trial with one-year-old seedlings of Douglas-fir in sand, ammonium sulphate was added to simulate the present acidic deposition. Part of the ammonium was nitrified into nitrate. This resulted in different ammonium, nitrate, aluminium levels and pH in the different treatments. In the treatments with the highest applications, fine root length was reduced by 50 % compared to the control, and the specific root length was reduced by 40 %.A critical review of the results reported in the literature, to ascertain the total effect of the soil chemical changes since the start of the industrial revolution on the fine root density, revealed that aluminium is the main factor reducing fine root growth, even at sub-lethal concentrations in the soil solution. The fine root length is probably reduced by 50 % in the topsoil, and by 75 % in the subsoil. Excessive nitrogen availability reduces the total fine root biomass by 30 %. The net result is that the fine root system is less dense and more shallow than it would be without acidic deposition. This reduces the potential for water uptake on coarse sandy soils, and increases fine root mortality in long dry periods, especially on poor sites. An elementary model to describe fluctuations in the fine root density during and after a drought is used to illustrate the effects of soil acidification and nitrogen enrichment on drought susceptibility. Accumulated deposition has enhanced the risks of tree mortality or forest dieback induced by severe drought