Influence of ammonium on fine root development and rhizosphere pH of Douglas-fir seedlings in sand

Ammonium sulphate is a major component of the air pollutants deposited on forests in the Netherlands. Different amounts of NH₄ ⁺ were added to Douglas-fir seedlings grown in tall containers of sand, to study the influence of high concentrations of NH₄ ⁺ in the soil on the development of fine roots and the effects of nitrogen uptake on rhizosphere pH. At the end of this eight-month experiment part of the ammonium appeared to have nitrified into nitrate. High doses of ammonium negatively affected root length and root length per unit of dry matter (specific root length). Although Douglas fir shows a preferential ammonium uptake in nutrient solutions the increases in the pH of the rhizosphere in this experiment indicate that nitrogen was mostly taken up as nitrate. When the ammonium concentration in the soil is low, it cannot be taken up readily because of its low mobility in soil. Shoot growth was stimulated by high availability of nitrogen. The possible effects of high doses of ammonium on long-term forest vitality are discussed.