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

• DOI: 10.1007/s10533-007-9119-x
• Publication Date: 2007-05-15T11:52:12Z
• Authors: J. Thad Scott; Robert D. Doyle; Jeffrey A. Back; S. I. Dworkin
• Keywords: Isotopes of nitrogen; Isotopic signature

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.