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

Abstract

The role of nitrogen (N 2 ) fixation in balancing N supply to wetland metaphyton was assessed by comparing primary production with enzymatic, isotopic, and elemental correlates. Primary production, N 2 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. N 2 fixation and phosphatase activity in metaphyton were negatively correlated with inorganic N and P concentrations, respectively. Although metaphyton N 2 fixation demonstrated a clear spatial pattern along the nutrient gradient, N 2 fixation rates varied monthly and decreased sharply in September. However, the percent contribution of N 2 fixation to N uptake by metaphyton consistently decreased throughout the summer. Furthermore, the decreased contribution of N 2 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 N 2 source through September at the most downstream (nutrient poor) site even though the percent contribution of N 2 fixition to N uptake was lowest in that month. This suggests that metaphyton were efficiently accumulating and recycling fixed N 2 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 N 2 fixation in N poor areas may adequately supplement community N requirements in metaphyton, thereby causing limitation by other elemental resources such as P.

The role of macrophytes in wetland ecosystems

“…An important contribution to N supply to wetland macrophytes can be from N-fixation (Scott et al 2007), either by symbiotic N fixers (Elliott et al 2009) …”

Nutrient enrichment in tropical wetlands: shifts from autotrophic to heterotrophic nitrogen fixation

“…Two types of organisms are often responsible for N fixation in wetlands: autotrophic cyanobacteria, which occur in benthic, floating, or periphytic cyanobacterial mats (Rejmánková and Komárková 2000;Rejmánková et al 2004;Inglett et al 2004Inglett et al , 2009Scott et al 2005Scott et al , 2007Lee and Joye 2006), and heterotophic N fixers in the rhizosphere or in bulk sediments (Moseman 2007;Č erná et al 2009). Autotrophic N fixation prevails in oligotrophic and mesotrophic wetlands where cyanobacterial mats are often the main primary producers because macrophytes are nutrient limited (Rejmánková and Komár-ková 2000;Scott et al 2007).…”

“…Autotrophic N fixation prevails in oligotrophic and mesotrophic wetlands where cyanobacterial mats are often the main primary producers because macrophytes are nutrient limited (Rejmánková and Komár-ková 2000;Scott et al 2007). After eutrophication, however, the growth of macrophytes often increases, replacing cyanobacteria, and thus reducing autotrophic N fixation.…”

Denitrification, dissimilatory nitrate reduction to ammonium, and nitrogen fixation along a nitrate concentration gradient in a created freshwater wetland

“…The NO 3 -concentration in waters entering LWW is reduced by more than 90% in the first 500 m downstream of the inflow (Scott et al 2005). As a result, microbial communities in the downstream areas of LWW are severely N limited (Scott et al 2005(Scott et al , 2007Scott and Doyle 2006). Therefore, LWW is an ideal system in which to study NO 3 -removal and N transformations at the sediment-water interface of freshwater wetlands.…”