Rhizosphere N2 Fixation in a Forest Ecosystem in situ Assays and Evaluation of the Acetylene Reduction Technique on the Utah State University Forest


Inger Borjesson

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In situ assays of N2 fixation activity, using the acetylene reduction technique, were performed in four successional stages of a Northern Wasatch Mountain subalpine forest ecosystem, elevation 2,800 m. Emphasis was made on rhizosphere fixation in association with Antennaria microphylla and Achillea millefolium. The vegetation period was approximately 100 days. Assays were performed in Saran bags. A defined amount of propane was injected at initiation of the assay and acetylene was generated from CaC2. Samples were analyzed for ethylene and propane. Data were evaluated assuming that the ethylene production was directly proportional to the increase in the ratio of ethylene to propane in the samples. Input of N by soil free-living N2 fixers in the meadow, the aspen, the fir and the spruce was 0.5, 0.3, 0.2 and 0.3 kg N ha-1 y-1, respectively. A higher activity in the presence of plant, as compared to the soil activity, was measured in 10 of 16 assays, however, the increase was significant at three testings only. This might indicate a contribution by rhizosphere N2 fixation, but to an extent lower than was expected and therefore not detectable with the method used. Leakage of gases from the test device not corrected for in the method used for evaluation of data. This introduces an overestimation of the obtained activities that increases exponentially with a more rapid effusion rate. Correction for effusion from a closed device can be made provided quantitative analyses of the tracer gas. To determine a small difference between enclosures with and without plants the accuracies in the effusion rates must be high. Quantitative analyses were not required for evaluation according to the method used and therefore, the obtained effusion rates have too wide standard deviations for correction of the effusion rate. It was shown that the determined effusion rates with corresponding standard deviations might obscure a low rhizosphere N2 fixation activity. Acetylene reduction assays performed in open devices can not easily be corrected for diffusion of gases. The initial very rapid diffusion from an open device leads to a vastly overestimated acetylene reduction activity, when the diffusion is not corrected for.


This item is a dissertation published by a student who attended Utah State University. Abstract can be accessed through the remote link. Fulltext not available online.