Acetylene reduction and 15N2 uptake rates for Alnus tenuifolia and Alnus crispa in six different successional habitats

This study sought to determine the conversion factor between acetylene reduction and N-fixation rates by root nodules excised from naturally-occurring Alnus tenuifolia and Alnus crispa plants in 18 sites in the BNZ-LTER. N-fixation rate was measured via uptake of 15N2 by excised nodule tissue. Sites were chosen to represent three replicates of early, mid, and late successional habitats on the Tanana River floodplain and in surrounding upland areas. A. tenuifolia was sampled in floodplain sites, and A. crispa in uplands. Parameters measured: acetylene reduction rate, N2 fixation rate, ratio of acetylene reduced to N2 fixed, soil temperature, leaf del 15N, and specific leaf weight. Most of the fixed nitrogen (N) that enters the N cycle in Alaskan forests is fixed biologically by alder (Alnus spp.) in symbiosis with the N-fixing bacterial genus Frankia. Estimates of gross N inputs resulting from this process ultimately rely on accurate measurement of N-fixation by nodule tissue. These rates are typically estimated by measuring rates of acetylene reduction by nodule tissue, then using an empirically determined conversion factor to obtain the corresponding N fixation rate. The purposes of this study were to determine: 1) the conversion factor(s) for Alnus tenuifolia and Alnus crispa in the Bonanza Creek Experimental Forest, and 2) whether the conversion factor varied between plants occurring in different successional habitats.