Implications of natural ethylene cycling processes for forest soil acetylene reduction assays

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Abstract
Under aerobic conditions, ethylene production and oxidation occurred simultaneously in organic and mineral horizons of a northern hardwood forest. Ethylene oxidation rates in mineral soil were high (up to 25 nmol∙g−1∙day−1) relative to other forest soils, and exceeded production rates unless moisture contents were raised above saturation. Acetylene inhibits ethylene oxidation while allowing ethylene production to proceed; control samples for acetylene reduction assays should therefore contain acetylene plus a nitrogenase inhibitor such as carbon monoxide if forest soil nitrogenase activity is to be accurately quantified. A level of 7 kPa carbon monoxide gave complete nitrogenase inhibition in active forest floor samples in the presence of acetylene; carbon monoxide was applied 2 h in advance of acetylene to compensate for its lower solubility in water. Use of this methodology revealed a strong inhibitory effect of saturated water contents on forest floor acetylene reduction activity, suggesting that aerobic N2-fixing activity predominated in unamended forest soil samples.Key words: nitrogenase, acetylene reduction, ethylene formation, forest soil.

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