NITROGEN SOURCES AND ‘A’ VALUES FOR VESICULAR–ARBUSCULAR AND NON‐MYCORRHIZAL SORGHUM GROWN AT THREE RATES OF 15N‐AMMONIUM SULPHATE*†

Abstract

SummaryMycorrhizal and non‐mycorrhizal sorghum [Sorghum bicolor (L.) Moench] plants were grown in soil treated with one of three rates of (15NH4)2SO4, fertilizer. Mycorrhizal plants did not significantly differ from non‐mycorrhizal plants in total plant dry weight, total plant N, or percent utilization of the applied N by the whole plant at any fertilizer rate. However, the atom percent 15N excess in shoots, roots and the whole plant, and percent N derived from fertilizer were significantly higher for non‐mycorrhizal plants at the intermediate level of fertilizer N. The percent N derived from native soil sources, and ‘A’ values were significantly higher for mycorrhizal plants, compared to non‐mycorrhizal plants, at the intermediate but not low or high fertilizer rates. The results suggest that, at the intermediate fertilizer rate, where plants produced the most growth, mycorrhizal sorghum plants derived N from a source that was less available to non‐mycorrhizal plants.