Response of mycorrhizal western red cedar to organic phosphorus sources and benomyl

Abstract

In some British Columbia forests, western red cedar (Thuja plicata Donn ex D. Don) grows well in soils with phosphorus (P) mainly in organic form and shows less response to P fertilization than other conifers. It was hypothesized that cedar or their associate arbuscular mycorrhizae could mineralize organic P. In a year-long greenhouse study, 2-year-old cedar trees in Turface were fed every 9 days with one of 11 P treatments (in Long Ashton solution): no P; high and low phytic acid (for all treatments, high is 50 mg P∙L−1, and low is 10 mg P∙L−1); high and low ATP; high and low glycerophosphate; high and low pyrophosphate; high and low orthophosphate. Ten mycorrhizal trees were used per treatment; of these, five trees per treatment were treated with the fungicide benomyl, which significantly reduced colonization in only two treatments. Growth was good with high rates of glycerophosphate, ATP, and pyrophosphate; low rates of these compounds resulted in P deficiencies. The trees grew poorly with phytic acid, which may have been adsorbed to the Turface, and which may have complexed Ca, Zn, and Cu from the nutrient solution, inducing deficiencies. The P source influenced the activities of rhizosphere acid and alkaline phosphatase, but had no significant effect on phosphodiesterase or pyrophosphatase activity. Use of the fungicide benomyl caused a significant increase in foliar N, either directly from the fungicide or indirectly from effects on N cycling. Key words: Thuja plicata, arbuscular mycorrhizae, phosphatase.