Carbon and nitrogen fixation in rhizomatous species of Coriaria

The rhizomatous species Coriaria sarmentosa and C. angustissima bear coralloid clusters of root nodules which actively fix atmospheric nitrogen. Using rooted cuttings, comparative measurements were made of growth, net photosynthesis, dark respiration and nitrogen fixation in the two species. In both, the optimum temperatures for growth and net photosynthesis are 16-18¡C. Over a range of temperature and light intensity C. sarmentosa possesses a higher rate of net photosynthesis than its more diminuitive sister species C. angustissima. When grown in soil collected beneath rhizomatous Coriaria in the field, nodulated C. sarmentosa respended significantly to sulphur and phosphorus when added together. The acetylene reduction method was used to measure rates of nitrogen fixation in excised modules of both species in various environmental conditions. Nitrogenase activity appears most vigorous in C. sarmentosa nodules at temperatures approaching 22¡C. Rates of acetylene reduction appear to be similar in both species but vary according to clone and pre-treatment temperature. Nodule efficiency is higher in young nodules than in old and compares favourably with those reported for other nodulated nitrogen fixers. These results tend to confirm some of our earlier assumptions about the ecology of these successional species. Though considered as weeds on grazing land they may have a place in planting programmes for erosion control in wet scree country