Diazotrophy in Alluvial Meadows of Subarctic River Systems

<div><p>There is currently limited understanding of the contribution of biological N₂ fixation (diazotrophy) to the N budget of large river systems. This natural source of N in boreal river systems may partially explain the sustained productivity of river floodplains in Northern Europe where winter fodder was harvested for centuries without fertilizer amendments. In much of the world, anthropogenic pollution and river regulation have nearly eliminated opportunities to study natural processes that shaped early nutrient dynamics of large river systems; however, pristine conditions in northern Fennoscandia allow for the retrospective evaluation of key biochemical processes of historical significance. We investigated biological N₂ fixation (diazotrophy) as a potential source of nitrogen fertility at 71 independent floodplain sites along 10 rivers and conducted seasonal and intensive analyses at a subset of these sites. Biological N₂ fixation occurred in all floodplains, averaged 24.5 kg N ha⁻¹ yr⁻¹ and was down regulated from over 60 kg N ha⁻¹ yr⁻¹ to 0 kg N ha⁻¹ yr⁻¹ by river N pollution. A diversity of N₂-fixing cyanobacteria was found to colonize surface detritus in the floodplains. The data provide evidence for N₂ fixation to be a fundamental source of new N that may have sustained fertility at alluvial sites along subarctic rivers. Such data may have implications for the interpretation of ancient agricultural development and the design of contemporary low-input agroecosystems.</p></div

Phylogenetic affiliation of sequences retrieved using cyanobacterial specific <i>nif</i>H genes as targets combined with DGGE analyses of sediment associated DNA samples from wetland meadows along three major rivers (Vindel, Pite, and Lais) in northern Sweden.

<p>The phylogram was constructed using the maximum likelihood distance method with TrNIG model. Sequences obtained here are given in bold. Each DGGE band sequence is designated by their accession number, a code and the name of the river/s were it was collected. The numbers given at the nodes represent bootstrap values.</p

Nitrogenase activity and resin sorbed inorganic N (NH₄⁺-N and NO₃⁻ −N) at fourteen different alluvial meadow sites at different river elevations along the Torne River in northern Sweden.

<p>Inorganic N was collected on ionic resin capsuel tethered and buried in alluvial sediments for nine months (September 2009 to June 2010). Resins buried in areas of greater human population density tended to have higher inorganic N concentrations (inorganic N was also related to river elevation following an exponential decline with increasing elevation).</p

Survey of nitrogenase activity in subarctic alluvial meadows at 71 total wet meadow sites along 10 individual river systems in northern Sweden (see SI).

<p>The horizontal solid line at 1150 µmol m⁻² d⁻¹ represents the average for all 71 sites. Bars represent one standard error of the mean acetylene reduction value at each site (n = 12).</p

Nitrogenase activity in four intensively sampled alluvial meadow sites along three major rivers (Vindel, Pite, and Lais) in Northern Sweden assayed from May 2008 to November 2010.

<p>Each data point represents average nitrogenase activity (acetylene reduction) across the four sampling areas and 12 subsamples at each site. Bars represent one standard error for the average at each site (n = 12).</p

Nitrogenase activity in thirteen individual alluvial meadow sites at different river elevations along the Torne River in Northern Sweden.

<p>Lower river elevations are associated with more dense populations, increasing agricultural development and increasing wastewater of effluent from communities. Each data point represents the mean of seven sampling dates with samples collected during the summer of 2009 with 12 subsamples collected at each site on each sample date, bars represent one standard error (n = 7).</p

Diversity measures.

<p>Non-parametric Shannon and Chao1 estimator calculated with sequences rarefied to the sample with minimum sequence number (<i>Clathria</i> sp. with 53,038 and <i>Myxilla (Burtonanchora)</i> sp. with 907 sequences for Bacteria/Archaea and Eukarya, respectively). SW: surrounding seawater sample.</p

Taxonomic composition.

<p>Taxonomic distribution of assigned tag sequences of Antarctic sponge-associated and surrounding seawater (SW) microbial communities. Bars represent relative abundance of sequences belonging to given classes superior to 0.5%. (A) Bacteria/Archaea assigned with the Silva database. (B) Eukaryotes assigned with the PR2 database. <i>MyxB</i>: <i>Myxilla (Burtonanchora)</i> sp.; <i>Clat</i>: <i>Clathria</i> sp.; uDem: undetermined Demospongiae; <i>Kvar</i>: <i>Kirkpatrickia variolosa</i>; <i>Htor</i>: <i>Hymeniacidon torquata</i>; <i>Lant</i>: <i>Leucetta Antarctica</i>; <i>HalG</i>: <i>Haliclona (Gellius)</i> sp.; <i>Mann</i>: <i>Megaciella annectens</i>; SW: surrounding seawater sample.</p

OTU distribution and clustering.

<p>Heatmaps representing the relative abundance (50 most abundant OTUs) of bacterial/archaeal (A) and eukaryote (B) taxa associated to Antarctic sponges and the corresponding surrounding seawater microbial communities. Their most resolved taxonomic assignation is included a side each OTU. Numbers represent taxonomic resolution level of the assignation, with (2) = Phylum, (3) = Class, (4) = Order, (5) = Family and (6) = Genus. Cluster above heatmap was generated using weighted Unifrac distance and group average clustering method. Color keys represent square root of relative abundance (in percentage). <i>MyxB</i>: <i>Myxilla (Burtonanchora)</i> sp.; <i>Clat</i>: <i>Clathria</i> sp.; uDem: undetermined Demospongiae; <i>Kvar</i>: <i>Kirkpatrickia variolosa</i>; <i>Htor</i>: <i>Hymeniacidon torquata</i>; <i>Lant</i>: <i>Leucetta Antarctica</i>; <i>HalG</i>: <i>Haliclona (Gellius)</i> sp.; <i>Mann</i>: <i>Megaciella annectens</i>; SW: surrounding seawater sample.</p

Summary of the sequencing data obtained.

<p>¹ Represent high quality sequences without chimera, undesired (metazoan, chloroplast, mitochondria) and rare sequences.</p><p>Summary of the sequencing data obtained.</p