HYPHAL UPTAKE AND TRANSPORT OF NITROGEN FROM TWO ¹⁵N‐LABELLED SOURCES BY <i>GLOMUS MOSSEAE</i>, A VESICULAR‐ARBUSCULAR MYCORRHIZAL FUNGUS ^*

SUMMARY Celery ( Apium graveolens L.) plants were grown in pots in which the root system was separated from the soil in a side chamber by a fine mesh screen. The side chamber was treated with either an organic (ground plant tissue) or inorganic [(NH 4 ) 2 SO 4 ] source of 15 N. Mycorrhizal ( Glomus mosseae ) and control (non‐mycorrhizal) plants were exposed to 15 N over a period of 30 days (inorganic‐ 15 N) or 88 days (organic‐ 16 N). Mycorrhizal and control plants did not differ in shoot dry weight or shoot P content. Dry weight of root was reduced in the mycorrhiza treatments. Mycorrhizal plants derived significantly ( P = 0.01) more 16 N, from both N sources, than did control plants. In the inorganic‐N treatment, 15 N in mycorrhizal plants was significantly ( P = 0.001) and positively correlated with percent mycorrhizal fungal colonization ( r = 0.58), number of hyphal crossings (±10 μ diameter) through the mesh into the area of 15 N placement ( r = 0.76), total length of hyphae per gram of soil ( r = 0.74), and length of hyphae of 5 μ diameter in the soil ( r = 0.77). No correlations were found between the 16 N content of mycorrhizal plants and any parameter in the organic‐N treatment. The 16 N content of control plants was not correlated with hyphal length in the outer chamber and there were no hyphal crossings of the size ( 10 μ diameter) which was counted for the mycorrhiza treatments. The presence of the organic matter (ground plant tissue) increased the total length of saprophytic hyphae per gram of soil but decreased the number of vesicular‐arbuscular mycorrhizal fungal hyphae crossing into the area of 16 N placement. The mean flux of N through the hyphae of G. mosseae was 7.42 × 10 −8 mol N cm −2 s −1 for the inorganic‐N treatment over a 30‐day period, and 1.74 × 10 −8 mol N cm −2 s −1 for the organic‐N treatment over an 88‐day period.