『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 15N2 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.
Keywords: 15N; N budget; P limitation; Rhizosphere;
sulfur reducing diazotrophs』
Introduction
Materials and methods
Study site
Sediment and root sampling and analyses
Acetylene reduction assay (ARA)
15N reduction assay
Phospholipid fatty acid (PLFA) analysis
Estimation of N input via N fixation and budget construction
Data analysis
Result
Calibration of ARA through 15N2
reduction assay
N fixation of sediment, roots, litter and cyanobacterial mats
Contribution of sulfur reducing bacteria to total N fixation
Estimation of N input via N fixation and budget construction
Discussion
N fixation by sediment and root associated diazotrophs
Role of sulfate reducing diazotrophs
Budget
Conclusions
Acknowledgments
Open access
References