Determination of the nitrogen source for arbuscular mycorrhizal fungi by N-15 application to soil and plants

The source of nitrogen in the spores of arbuscular mycorrhizal (AM) fungi w as quantified by a N-15-labeling technique. N was applied as coated urea to the soil and in solution to plant shoots. Soil-applied fertilizer had a si gnificant effect on spore % N-15 (P<0.01), with a 24-75% contribution to sp ore N. Fertilizer applied to either alfalfa shoots or bahia grass shoots ha d little effect on spore % N-15, accounting for 0-14% or 1-9% of spore N, r espectively. These results indicate that AM fungi obtain spore N mostly fro m the soil. The small amount of spore N originating from shoot-applied N ma y have been obtained via root exudation.