Natural abundance of N-15 in actinorhizal plants and nodules

Substantial enrichment of some plant parts in N-15 relative to the rest of the plant is unusual, but is found in the nitrogen-fixing nodules of many l egumes. A range of actinorhizal plants was surveyed to determine whether th e nodules of any of them are also substantially enriched in N-15. The nonle gume Parasponia, nodulated by a rhizobium, was also included. Four of the a ctinorhizal genera and Parasponia were grown in N-free culture, and three a ctinorhizal genera were collected from the field. Nodules of Parasponia, Ca suarina and Alnus were N-15 enriched relative to other plant parts, but onl y Parasponia approached the degree of enrichment found in some legume nodul es. The nodules of Datisca, Myrica, Elaeagnus, Shepherdia, and Coriaria wer e depleted in N-15. Thus many actinorhizal nodules are depleted in N-15 com pared to other plant parts and enrichment is modest when it does occur. Who le plant N-15 content (delta(15)N) in four actinorhizal plants and Paraspon ia showed a relatively narrow range of -1.41 to -1.90. Hence regardless of the degree of nodule enrichment or depletion, whole plant N-15 content appe ars to vary little in plants grown in N-free culture.