Nitrogen fixation and methane metabolism in a stream sediment-water system amended with leaf material.

The reduction of acetylene and the production and oxidation of methane in a stream sediment--water system amended with either fresh leaves or autumn-shed leaves in the presence and absence of air were studied. Net methane production by the sediment--water system occurred only when leaf material was added, with fresh leaves giving 2.2 times the methane accumulation as autumn-shed leaves. Static incubation in the presence of air had little effect on net methane production, with such production being about the same as (with fresh leaves) or 79% of (with autumn-shed leaves) the anaerobic rates. Acetylene reduction was more affected by the presence of air, with anaerobic rates being at very low levels. The presence of fresh leaves was again stimulatory, with the acetylene reduction rate being 5.5-fold greater than that occurring with autumn-shed leaves.