Natural abundance of 15N 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 legumes. A range of actinorhizal plants was surveyed to determine whether the nodules of any of them are also substantially enriched in N-15. The nonlegume Parasponia, nodulated by a rhizobium, was also included. Four of the actinorhizal genera and Parasponia were grown in N-free culture, and three actinorhizal genera were collected from the field. Nodules of Parasponia, Casuarina and Alnus were N-15 enriched relative to other plant parts, but only Parasponia approached the degree of enrichment found in some legume nodules. The nodules of Datisca, Myrica, Elaeagnus, Shepherdia, and Coriaria were depleted in N-15. Thus many actinorhizal nodules are depleted in N-15 compared to other plant parts and enrichment is modest when it does occur. Whole plant N-15 content (delta(15)N) in four actinorhizal plants and Parasponia 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 appears to vary little in plants grown in N-free culture.