Nitrogen fixation by Myrica gale root nodules Massachusetts wetland

Nitrogenase activity was measured by acetylene reduction in excised Myrica gale nodules collected throughout the growing season at two sites associated with a small lake in central Massachusetts. One site was in an open, weakly minerotrophic peatland dominated by M. gale and the other was on the lakeshore. Nitrogenase activity appeared in late May when the leaves unfolded, reached a peak in July when the maximum number of leaves was present, and declined until it was no longer measurable in late October several days after all leaves had fallen. Summer activities were substantially higher at the peatland site than the lakeshore. Maximum activities were 19.8±1.9 and 8.1±0.7 μmol/h x g dry weight (x±SE; N=20) at the peatland and lakeshore sites respectively.Nitrogenase activities were very low at 5° C and increased linearly from 10 to 30° C, the highest temperature examined. The maximum soil temperature measured was 20° C, and no significant diurnal fluctuation in activity was detected.Annual nitrogen fixation calculated from the seasonal nitrogenase activity curve was 34 kg N/ha x yr at the peatland site with mean dry weight nodule biomass of 104 kg/ha, and 24 kg N/ha x yr at the lakeshore with 111 kg/ha nodule biomass. These rates of nitrogen fixation are equivalent to 4-5x the amount of nitrogen contained in bulk precipitation and are major components in the nitrogen budgets of the M. gale plants and wetlands in which they grow.