Effects of soil microorganisms on mycorrhizal contribution to growth of big bluestem grass in non-sterile soil

Abstract

Plant dry weight and mycorrhizal root colonization of big bluestem ( Andropogon gerardii Vitman) inoculated with Glomus etunicatum Becker and Gerd. were suppressed if non-sterile prairie soil sievings or filtrate were added to pasteurized soil. Addition of prairie soil microorganisms, isolated onto peptone yeast extract, King's B, or starch casein agar media, to pasteurized soil also reduced dry weight and mycorrhizal root colonization of big bluestem inoculated with G. etunicatum. In contrast, addition of non-sterile soil sievings or filtrate or organisms isolated onto potato dextrose or pseudomonas isolation agar to pasteurized soil improved growth of non-mycorrhizal big bluestem. These effects of soil micro-organisms on plant growth were further quantified by comparing 32P uptake by fungicide treated and untreated mycorrhizal plants and by non-mycorrhizal plants in pasteurized and non-sterile soil. Mycorrhizal plants grown in pasteurized soil absorbed ≅10 times more 32P than mycorrhizal plants grown in non-sterile soil. Application of propiconazole (Tilt) or fenarimol (Rubigan), fungicides which inhibit 32P uptake by mycorrhizas, resulted in approx. 40- and 5-fold reductions in 32P uptake in pasteurized and non-sterile soil, respectively. Thus, more 32P is absorbed in pasteurized than in non-sterile soil, probably because soil microorganisms limit mycorrhizal activity in non-sterile soil. Assessments of mycorrhizal contribution to plant growth conducted in sterilized soils may significantly overestimate the effects of VAM fungi because other soil microorganisms are not considered.