Evaluation of biological nitrogen fixation in coarse woody debris from Eastern Canadian boreal forests

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The contribution of Coarse Woody Debris (CWD) to N inputs in boreal ecosystems has been reported in intensively managed Scandinavian forests in Europe and in Pacific coastal rainforest in North America. These estimates range from 0.085 to 2 kg N ha−1 y−1. Data from Eastern Canadian forests that receive fewer sylvicultural treatments than do Scandinavian forests and are exposed to a drier climate than the Canadian rainforest are scare. We evaluated contributions of CWD to total BNF in two forest sites dominated by balsam fir and black spruce. We also measured the conversion ratio (R ratio = ARA/N2) for the acetylene reduction assay, which is the method most often used to estimate N2-fixation, using parallel 15N2 incubations to account for potential methodological biases. We further tested two factors that can affect the R ratio, hance quality of BNF estimates in deadwood; inhibition of N2-fixing methanotrophs by acetylene and molybdenum limitation of N2-fixation. We report that N2-fixation by CWD in balsam fir and black spruce from eastern boreal forest is low (0.05–0.12 kg N ha−1y−1) compared to other estimates for coniferous forests. We also show that moss-covered CWD can significantly contribute to CWD N2-fixation and that improper validation of the ARA can lead to significant underestimation of N inputs.