Whether nutrients increased or decreased in host tissues were
contingent on soil nutrient levels. Though Allium was more responsive
overall, AM fungi reduced nutrients in Allium under low
fertilizer, but increased shoot nutrients when soil nutrients were
abundant. It may be that at low nutrient levels there is competition
between fungus and plant for limiting nutrients. Such competition
between roots and fungi has been suggested as a mechanism to
explain nitrogen budgets in AM plants (Hodge et al., 2010) and carbon
sequestration in the common mycelial network (Robinson and
Fitter, 1999). However, evidence on the pervasiveness or the effect
size of such competition is almost entirely lacking, and deserves
further study.