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Abstract
N2-fixing bacteria convert N2 gas from the atmosphere into biologically available forms via the nitrogenase enzyme. N2-fixation is a natural flux of N to an ecosystem. The flux of N into some terrestrial and aquatic ecosystems via N2-fixation can be substantial and account for up to 97% of new N inputs. Yet, little is known about the importance of N2-fixation in streams. My objectives were to examine the controls on N2-fixation, evaluate current methods for measuring N2-fixation, and estimate the contribution of N 2-fixation to N cycling in Wyoming streams.
I examined how nutrients may partially control N2-fixation using 4-hour and 6-week nutrient incubations. During 4-hour incubations, the N2-fixation flux of intact stream biofilm was resistant to ammonium (NH4+), nitrate (NO3 -), and phosphate (PO43-) additions at varying concentrations and the N2-fixation fluxes remained unaltered. On the other hand, the 6-week additions of NO3- and PO43- altered chlorophyll a, algal assemblage structure, and N2-fixation fluxes. N2-fixation was inhibited by NO3- and stimulated by PO43- , and the inhibitory effect of NO3- was stronger than the stimulating effect of PO43-.
When measuring N2-fixation via the acetylene reduction method a conversion ratio of ethylene produced to N2-fixed (C2H 4:N2) must be assumed or calibrated. The C2H 4:N2 ratio for benthic biofilms in streams remained unclear. Using both the acetylene reduction method and 15N2 uptake methods, I examined the C2H4:N2 ratio, estimated controls on the ratio, and measured the immediate release of N 2-fixed. Our average estimate of the C2H4:N 2 ratio was 5.4 (n=85). Immediate release of N2-fixed as dissolved N was less than 1% of N2-fixation. Ethylene production increased 10 times faster than N2-fixation as stream temperature increased. Thus, stream temperature drove the variability observed in the ratios of ethylene produced to N2-fixed.
To estimate the contribution of N2-fixation to stream N cycling processes, I compared N2-fixation with dissolved inorganic N (DIN) uptake fluxes. N2-fixation exceeded NO3- uptake in 2 of 9 streams and NH4+ uptake in one stream. Throughout the summer season in one stream, Ditch Creek, N2 -fixation exceeded DIN uptake up to 8-fold. Ditch Creek N2-fixation fluxes surpassed denitrification rates for many streams. N2-fixation can be a dominant flux in stream N cycling, but as DIN concentrations increase in streams this flux may diminish.
