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

Abstract

We used established long-term experimental P-amended plots in freshwater marshes of northern Belize to determine the impact of P input on nitrogen (N) fixation. Marshes with different conductivities and sulfate concentrations were selected to elucidate the effect of salinity and the contribution of sulfur reducing bacteria to the overall N fixation. Rates of N fixation in sediment, roots, and cyanobacterial mats was measured in laboratory incubation experiments (acetylene reduction assay calibrated by 15 N 2 reduction assay) with and without the addition of sodium molybdate (sulfur reducing bacteria inhibitor). P has increased macrophyte primary production significantly, which led to the rapid elimination of cyanobacterial mats and the elimination of autotrophic N fixation. P addition enhanced heterotrophic N fixation in both the sediments and rhizosphere due primarily to increased C supply to the sediment. When expressed on a dry weight basis, root associated N fixation was higher than sediment N fixation, but the contribution of the root associated fixation to the total N fixation was small when expressed per square meter. Sulfur reducing bacteria were an important component of N fixation, contributing from 20 to 53% to the overall N fixation. A simple N budget was created to determine if N demands are met following P addition. The heterotrophic N fixation substituted in part for autotrophic cyanobacterial N fixation when P limitation was alleviated.

Variation in N2Fixation in Subarctic Tundra in Relation to Landscape Position and Nitrogen Pools and Fluxes

“…The regulating role of nutrients was confirmed in laboratory experiments (Table 4) and was significant within a short time period. These trends were consistent with other studies evaluating long-term fertilization experiments (Liengen, 1999b;Weiss et al, 2005;DeLuca et al, 2007;Šantrůčková et al, 2010). High soil concentration of ammonium as an end product of the enzymatic reaction process suppresses nitrogenase activity (Paul and Clark, 1996).…”

A plant–microbe interaction framework explaining nutrient effects on primary production

“…2c). This mechanism is documented for symbiotic N 2 fixation 38,39,40,41 and mycorrhizal colonization of plant roots 42,43 , which increases with P fertilization.…”

Heterotrophic N2-fixation contributes to nitrogen economy of a common wetland sedge, Schoenoplectus californicus

“…Controls run as samples without acetylene addition as well as blanks (tubes without plant tissue incubated with acetylene) showed no endogenous ethylene production. Samples were kept for dry weight determination after terminating the exposure (see [ 11 ] for more detailed description).…”

“…Bacteria capable of performing N 2 -fixation can colonize both root surfaces (and are referred to as “epiphytes” here) as well as the internal tissues (“endophytes”) (for review see [ 7 ]. The reduction of N 2 to ammonia during its biological fixation is an energetically expensive process and the input of easily available C from roots into the plant rhizosphere can sustain high activity of root associated diazotrophs [ 8 , 9 , 10 , 11 ]. Epiphytic diazotrophs establishing loosely organized associative relationships in the rhizosphere have been documented frequently for tropical grasses [ 8 , 12 , 13 ] and among wetland plants for littoral macrophytes [ 14 , 15 , 16 ].…”

“…Nonsymbiotic nitrogen fixation includes fixation by the true free-living diazotrophs (e.g.,heterotrophic N 2 -fixation in leaf litter and soil or water) [ 31 , 32 ], as well as by autotrophic and heterotrophic organisms of intercellular and epiphytic growth associated with certain species of bryophytes [ 33 ]. In the past, the majority of studies dealt with the symbiotic N 2 -fixation in legumes, however more recently, the important role of free-living and epiphytic N 2 -fixation is being acknowledged and current evidence suggests that free-living N 2 -fixation represents a critical N input to many terrestrial and aquatic ecosystems, particularly those lacking large numbers of symbiotic N 2 -fixing plants [ 11 , 21 ]. Most of the studies on endophytic N 2 -fixation have been focused on crop plants such as sugar cane and rice, while only a few studies on natural plant populations are available and none, to our best knowledge, reports endophytic diazotrophs from any Cyperaceae family.…”