The influence of pH and nitrate on Mycorrhizal associations of Pinus radiata D. Don

Abstract

Growth of 5-month-old Pinus radiata seedlings was 60-80% lower in soil at pH 8.0 than at pH 6.2. At pH 8.0 mycorrhiza formation was lower by c. 60% and ectotrophic mycorrhizas were compIetely replaced by ectendotrophic mycorrhizas.

In a laboratory medium at pH 5.0, 575 p.p.m. nitrate did not depress growth of the mycorrhizal fungi Rhizopogon luteolus, Suillus granulatus, and Cenococcum graniforme, but at pH 8.0 growth was slight regardless of the nitrate concentration in the medium. Rhizosphere colonization by Rhizopogon luteolus was studied by comparing its growth along roots of P. radiata seedlings with that along glass fibres in sterile soil. At pH 5 .0 appreciable colonization of the root occurred but this was less with 115 p.p.m. than with 12 p.p.m. soil nitrate. Growth along fibres was significantly less than along roots. Alkalinity produced the most marked effect on colonization, there being no growth of R. luteolus along the root at pH 8.0 at both nitrate levels. It is concluded that decreased mycorrhiza formation in alkaline soils is caused by inhibition of growth of some mycorrhizal fungi in the rhizosphere in addition to possible nitrate inhibition of infection but nitrate inhibition of mycorrhiza formation under acid conditions is mainly due to inhibition of infection.