The role of N₂ fixation in alleviating N limitation in wetland metaphyton: enzymatic, isotopic, and elemental evidence

The role of nitrogen (N₂) fixation in balancing N supply to wetland metaphyton was assessed by comparing primary production with enzymatic, isotopic, and elemental correlates. Primary production, N₂ fixation (acetylene reduction, AR), phosphatase activity, C:N:P ratio, and N isotopic composition of metaphyton were measured along a nutrient gradient in a freshwater marsh during May through September 2004. N₂ fixation and phosphatase activity in metaphyton were negatively correlated with inorganic N and P concentrations, respectively. Although metaphyton N₂ fixation demonstrated a clear spatial pattern along the nutrient gradient, N₂ fixation rates varied monthly and decreased sharply in September. However, the percent contribution of N₂ fixation to N uptake by metaphyton consistently decreased throughout the summer. Furthermore, the decreased contribution of N₂ fixation to N uptake corresponded with an increase in metaphyton N content during the growing season. Nitrogen isotopic data suggested the sustained importance of an atmospheric N₂ source through September at the most downstream (nutrient poor) site even though the percent contribution of N₂ fixition to N uptake was lowest in that month. This suggests that metaphyton were efficiently accumulating and recycling fixed N₂ in support of primary production. Over the course of the summer, metaphyton primary production showed a weak inverse correlation with metaphyton phosphatase activity (r ² = 0.58). The largest residuals in this regression corresponded to the largest vartiation in metaphyton N content. When metaphyton primary production was normalized to metaphyton N content, production rates for the entire growing season were more strongly inversely correlated with metaphyton phosphatase activity (r ² = 0.78). Results of the study suggest that N₂ fixation in N poor areas may adequately supplement community N requirements in metaphyton, thereby causing limitation by other elemental resources such as P.