Acetylene reduction (nitrogen fixation) in a Delaware,USA salt marsh

Acetylene reduction (nitrogen fixation) was measured in several vegetational areas in a Delaware, USA salt marsh. Samples were collected for 1 yr and the results showed a seasonally variable pattern of acetylene reduction at all stations. Peak rates were generally recorded during the later summer and early fall (September–October). The seasonality was influenced mainly, although not exclusively, by the soil temperature. In addition, samples collected in short Spartina alterniflora stands exhibited rates which were up to 20-fold higher than those found in samples from tall S. alterniflora stands. Over 50% of the total yearly ethylene production occurred from mid-August until the beginning of December at the tall and short S. alterniflora stations. Maximum activity occurred at 5 cm depth for all stations. Surface activity accounted for only 3–4% of the total measured in the top 20 cm. Addition of glucose or mannitol resulted in considerable increases in activity, thus suggesting that heterotrophic acetylene reduction is carbon and/or energy limited. The results obtained in this study indicate that the measured rates are only potential rates and that considerable caution must be used in extrapolating from acetylene reduction rates to nitrogen fixation rates in situ.