Opposite diel patterns of nitrogen fixation associated with salt marsh plant species (Spartina foliosa and Salicornia virginica) in southern California

Abstract

In marine wetlands, nitrogen fixation is a potentially important nutrient source for nitrogen-limited primary producers, but interactions between nitrogen fixers and different vascular plant species are not fully understood. Nitrogen fixation activity was compared in sediments vegetated by three plant species, Spartina foliosa, Salicornia virginica, and Salicornia bigelovii in the Kendall Frost Reserve salt marsh in Mission Bay (CA). This study addressed the effects of plant type, day and night conditions, and sediment depths on nitrogen fixation. Higher rates of nitrogen fixation were associated with S. foliosa than with either of the two Salicornia spp., which are known to compete more effectively than Spartina for exogenous nitrogen in the salt marsh environment. Rates of nitrogen fixation, determined by acetylene reduction, in sediments vegetated by S. virginica were low during the day (7.7 ± 1.2 lmol C 2 H 4 m )2 h )1 ) but averaged 13 ± 6.6 lmol C 2 H 4 m )2 h )1 at night, with particularly high rates in samples from locations with visible cyanobacterial mats. The opposite diel pattern was found for sediments containing S. foliosa plants, in which average daytime and nighttime rates of nitrogen fixation were 62 ± 23 and 21 ± 15 lmol C 2 H 4 m )2 h )1 , respectively. For S. foliosa, nitrogenase activity of rinsed roots and different sediment sections (0-1, or 4-5 cm depths) were measured. Although nitrogen fixation rates in vegetated sediment samples were substantial, all but one of rinsed S. foliosa root samples (n ¼ 12) and subsurface sediments at 4-5 cm depths failed to show nitrogen fixation activity after 2 h, suggesting that the most active nitrogen fixers in these systems likely reside in surface sediments. Further, nitrogenase activity in shaded and unshaded S. foliosa samples did not differ, suggesting that nitrogen fixers may not rapidly respond to changes in plant photosynthetic activity. Average nitrogen fixation rates in S. foliosa-vegetated samples from the Mission Bay salt marsh were on the same order as those of highly productive Atlantic coast marshes, and this microbially-mediated nitrogen source may be similarly substantial in other Mediterranean wetlands. Sediment abiotic variables seem to exert greater control upon nitrogen fixation activity than the effects of particular plant species. Nonetheless, dominant plant species may differ substantially in their reliance on nitrogen fixation as a nutrient source, with potentially important consequences for wetland conservation and restoration.

Heterotrophic nitrogen fixation in oligotrophic tropical marshes: changes after phosphorus addition

“…Research on biological N fixation and its role in maintaining soil N fertility in wetland ecosystems has focused on rice fields (Roger & Ladha, 1992;Roger, 1995) and coastal wetlands such as salt marshes and mangroves (Pelegri & Twilley, 1998;Bagwell & Lovell, 2000;Nielsen et al, 2001;Tyler et al, 2003;Moseman, 2007). However, such research is rather limited in freshwater marshes (Bristow, 1973;Tjepkema & Evans, 1976;Inglett et al, 2004;Scott et al, 2005).…”

“…Heterotrophic N fixation has been reported to be closely associated with plant roots (Boyle & Patriquin, 1981;Bergholz et al, 2001;Nielsen et al, 2001;Moseman, 2007), but the complex interaction between plant roots, root exudates, and microorganisms are poorly understood (Coleman, 2008). Most importantly, though, plants are known to provide a source of available carbon to sediment diazotrophs (Dakora & Drake, 2000;Bürgmann et al, 2005).…”

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

“…Therefore diazotroph incidence and activity depends on the availability of energy rich carbon (C) source. The continual input of easily available C into the plant rhizosphere sustains high activity of root associated microflora (Boyle and Patriquin 1981;Bergholz et al 2001;Nielsen et al 2001;Moseman 2007), and thus diazotrophs are closely associated with plant roots (Dakora and Drake 2000;Bürgmann et al 2005). In addition to C availability, the activity of diazotrophs in wetland sediments is repressed by high concentrations of ammonium (Howarth et al 1988;Č erná et al 2009) and low concentrations of available phosphorus (P) (Reed et al 2007).…”

“…Research on biological nitrogen (N) fixation and its role in maintaining soil N fertility in wetland ecosystems has focused on coastal wetlands such as salt marshes, mangroves (Pelegri and Twilley 1998;Bagwell and Lovell 2000;Nielsen et al 2001;Tyler et al 2003; Lee and Joye 2006;Moseman 2007), and on rice fields (Eskew et al 1981;Roger and Ladha 1992;Roger 1995). Data from freshwater wetlands, however, are rather limited (Bristow 1973;Tjepkema and Evans 1976;Scott et al 2008;Č erná et al 2009).…”

“…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).…”