Soil moisture and the mycorrhizal association of Pinus radiata D. don

Abstract

Five ectomycorrhizal fungi were found to have an optimum growth in Mt. Burr sand at total soil water potentials of −300 to −600 kPa but could grow or survive at a total water potential of −4000 kPa when glucose was supplied exogenously. Different fungi responded differently to reduced soil water potential induced by the addition of KCl solutions to soil. Growth characteristics of mycorrhizal fungi in the rhizosphere were poorly related to growth in soil with added glucose and yeast extract. Colonisation of Pinus radiata roots by Rhizopogon luteolus was greatest at soil water contents of 13% (−120 kPa osmotic potential) and was significantly depressed at 6.5%, (−510 kPa) and at 27% (−40 kPa). Cencoccum graniforme colonised roots most at 20%, (−60 kPa). Colonisation by this fungus was depressed at soil water contents of 13 to 6.5% and at 27%. Mycorrhizal infection was greatest at soil water contents of 13% (−120 kPa osmotic potential) to 20% (−60 kPa) and was significantly depressed at 27%, (−40 kPa). The depression of fungal growth in the rhizosphere and infection at soil water contents approaching saturation is discussed in terms of soil void space and aeration problems.