Effects of a two-year application of ammonium sulphate on growth,nutrient uptake,and rhizosphere microflora of juvenile Douglas-fir

Deposition of ammonium sulphate is often cited as a major factor causing forest dieback in The Netherlands. In this study, three-year-old Douglas-firs (Pseudotsuga menziesii [Mirb.] Franco) were potted and subsequently treated for two years with ammonium sulphate solutions, corresponding to total annual N applications of 5, 50 and 200 kg ha^-1 yr^-1. After 6, 18 and 23 months, five trees per treatment were harvested, and growth, nutrient concentrations, mycorrhizal frequency and bacterial population on the roots were determined, together with soil chemical factors. At the highest treatment level, ammonium accumulated in the soil causing increases in H⁺ and Al-ion concentrations. Dry plant weights and root/shoot ratio were not significantly affected but the specific root length, i.e. length per gram dry weight, decreased significantly with increasing ammonium application. Mycorrhizal frequency and total bacterial population on the roots were also reduced. Reduced uptake of nutrients, especially phosphorus, was associated with these changes in the soil chemical and biological status. Extrapolation of the results to natural ecosystems should be done with great care, but undoubtedly, a potential danger exists for natural, often poorly buffered-systems, since accumulation of ammonium occurs continuously over many years.