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Diversity and function from the ground up : microbial mediation of wetland plant structure and ecosystem function via nitrogen fixation


Plant-dependent functions of coastal wetlands are strongly influenced by nitrogen availability. Diazotrophs, microbes that fix nitrogen, in surface sediments and rhizospheres (roots and surrounding sediments) of plants may fundamentally affect wetland ecosystems. In testing roles of nitrogen fixing microbes in niche differentiation between two key plants, Spartina foliosa and Salicornia virginica, a mensurative experiment reveals plant-specific diel patterns of nitrogen fixation (acetylene reduction). Functional disparities in nitrogen fixation rates between late- and early-successional salt marshes in Tijuana Estuary (1 pair) and Venice lagoon, Italy (2 pairs) also show roles of diazotrophs in facilitating marsh development. Nitrogen fixation rates are consistently greater in marshes with less plant growth, which is not always a function of marsh age. Fates of fixed nitrogen are tested in isotopic enrichment experiments within an early successional marsh (Tijuana Estuary). Newly fixed nitrogen reaches S. foliosa roots and several animal consumers within 3-8 days. Thus, nitrogen fixation has broad significance for wetland ecosystem function. The role of diazotroph diversity in enhancing or conferring stability to the nitrogen fixation (acetylene reduction) rates was tested in wetlands experiencing biological invasion, restoration, and sediment and nutrient stresses, via genetic fingerprinting (T-RFLP) of the nifH gene (coding dinitrogenase reductase). The invasive mussel, Musculista senhousia, salt cedar, Tamarix spp., and mangrove, Avicennia marina each produced different effects on nitrogen fixation rates, despite maintenance of diazotroph diversity. In the early successional marsh at Tijuana Estuary, positive relationships among diazotroph diversity, nitrogen fixation rates, and S. foliosa height during one season of plant growth (Fall) demonstrate context-dependent complementarity. Effects of anthropogenic nutrient and sediment loading on nitrogen- fixing microbes are tested by field manipulations. Ammonium nitrate additions decrease nitrogen fixation rates but increase diversity of surface diazotrophs within 17 days, while sediment inputs enhanced and prolonged ammonium concentrations. As nitrogen fixation is highly responsive to the range of explored environmental changes, concepts of functional redundancy may not easily extend to microbial realms. Wetland management should more fully consider the role of plant-microbe interactions in mediating ecosystem functional responses to future global changes

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