Ammonium deposition and the mycoflora in the rhizosphere of Douglas-fir

Abstract

Ammonium deposition seems to be a major factor in the observed forest dieback in The Netherlands. We studied the effects of ammonium sulphate (AS) on saprophytic fungi in the rhizosphere of Douglas-fir. In two Douglas-fir stands (Kootwijk and St Anthonis) with a different deposition rate of AS ( ca 22 and 30 kg N ha −1 yr −1, respectively), fungi were isolated from the roots and identified. Canonical correspondence analysis (CCA) revealed that the fungal populations in the two stands differed significantly at the community level. Regression analysis showed that Trichoderma viride and Pénicillium spinulosum occurred significantly more frequently in the Kootwijk stand, whereas Cylindrocarpon didymum and Sesquicillium candelabrum were dominant species in the St Anthonis forest. P. brevicompactum appeared to be common in both Douglas-fir stands. To estimate the possible role of AS in the observed differences between the two stands, a laboratory experiment was performed in which 5, 50 and 200 kg N ha −1 yr −1 was applied to 3yr old Douglas-firs for 23 months. Although the occurrence of Sesquicillium candelabrum, Trichoderma hamatum and T. viride increased and Sphaerodes retispora decreased with increasing ammonium deposition, CCA did not show that the overall fungal community was affected. The results of both CCA and regression analysis indicated that the differences between the two Douglas-fir stands were not primarily caused by different amounts of AS deposition. Separate analyses of the fungal communities of root tips and older roots showed that deposition of AS tended to alter the fungal community on the root tips; the occurrence of Sporothrix inflata was stimulated at increasing rates of AS, whereas the occurrence of ‘unidentified chlamydospores‘ decreased.