FACILITATION OF PLANT PHOSPHATE ACQUISITION BY ARBUSCULAR MYCORRHIZASFROM ENRICHED SOIL PATCHES .1. ROOTS AND HYPHAE EXPLOITING THE SAME SOIL VOLUME

Mycorrhizal contribution to nutrient acquisition from spatially hetero geneous soil was investigated using two-way comparisons of non-mycorrh izal and arbuscular mycorrhizal Agropyron desertorum (Fisch. ex Link) Schult. in soil with either a patchy or a uniform nutrient distributio n. Double-labelling allowed a comparison between acquisition of an ion of lower abundance and mobility, phosphate (P), and an ion of higher abundance and mobility, nitrate. As expected, root density increased i n enriched-nutrient patches relative to either unenriched soil or to s oil where the same quantity of nutrients was applied to the entire soi l volume (uniform-nutrient treatment). However, the increase of local root density in patches was less for mycorrhizal plants than for non-m ycorrhizal plants. Mycorrhizal infection enhanced plant acquisition of labelled P relatively more in the enriched patches (patchy-nutrient t reatment) than in soil with uniform nutrient distribution, both when e xpressed on a shoot mass and on a root-length basis. As expected, plan t nitrate acquisition and shoot nitrogen concentration were not signif icantly affected by either mycorrhizal infection or patterns of soil n utrient distribution. Hence, arbuscular mycorrhizas can enhance root P acquisition and, thus, contribute to plant exploitation of soil P het erogeneity.