Abundance and activity of N2-fixing bacteria in decaying wood

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Abstract
Populations of N2-fixing bacteria exceeded 106/g in moderately decayed trembling aspen (Populustremuloides Michx.) and white birch (Betula papyrifera Marsh.) logs. Lower numbers were found in red maple (Acerrubrum L.) and conifer logs (Abiesbalsamea (L.) Mill., Pinusbanksiana Lamb.) at similar decay states. Populations were assayed using a combined-carbon medium in semisolid agar under aerobic conditions. Single carbon sources generally yielded lower and more variable numbers. With water and carbon supplements, aspen and birch wood showed high potential nitrogenase activity (>400 nmol C2H4•g−1•day−1) at 30 °C under aerobic conditions, and even greater activity (>1000 nmol C2H4•g−1•day−1) during a 3-day incubation under waterlogged conditions. In the absence of supplemental carbon, aspen wood remained active but birch wood did not. Wood from other species supported much lower nitrogenase activity under aerobic conditions, and waterlogging was strongly inhibitory. A highly decayed white pine (Pinusstrobus L.) log had low populations (4/g) of N2-fixing bacteria and did not reduce C2H2 under any treatments. Results support the hypothesis that a relationship exists between (i) populations and activity of N2-fixing bacteria and (ii) the decay susceptibility of wood of different species.

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