ANALYSIS OF nifH TRANSCRIPTS IN ARGENTINEAN SOILS: AN APPROACH TO EVALUATE BIOLOGICAL NITROGEN FIXATION

PRISCILA A. CALDEROLI; EFRÉN RAMOS CABRERA; MÓNICA MARIANA COLLAVINO; ORLANDO MARIO AGUILAR

Córdoba Capital

Congreso; XI Congreso Argentino de Microbiología General; 2015

Asociación Argentina de Microbiología General

Biologically available N is often a limiting nutrient in agricultural soil and other environments. About4.32 x 105 tons of nitrogen fertilizers are used annually in the productive agricultural region ofArgentina. In this context, we were interested in analyzing the active diazotrophic community in theArgentinean soils by assessing nifH-mRNAs transcripts using pyrosequencing technology. Wesampled cropping fields located in Pergamino (Buenos Aires) subjected to the following no-tilltreatments: intensive crop rotation, nutrient replacement and minimal use of agrochemicals (goodagricultural practices, GAP); soybean monoculture, low nutrient replacement and high agrochemicaluse (poor agricultural practices, PAP), and a grassland soil non disturbed by human activity used as anatural reference environment (NE). Each treatment was sampled at depths of 0-10 cm and 10-20 cm.A total of 112.493 reads were retrieved from the six pyrosequencing-derived datasets. Poor-qualityreads (low-quality base-calling, frameshifts and chimeras errors) were removed and the final set of28171 reads was clustered into 1661 OTUs defined at 98% amino acid sequence similarity. Finally,437 OTUs with more than 3 sequences were selected and phylogenetic analysis was performed usingreference nifH datasets (Zher et al., 2004) with the ARB software.The following nifH subclusters were represented in our datasets: 45% of 1A (Anaeromyxobacter yDesulfuromonadales), 29% of 1K (Rhizobiales), 17% of 1B (Cyanobacteria) and 8% of 3B(Desulfovibrionales y Verrucomicrobiales), while the remaining subclusters represented less than 1%of the total number of OTUS. The proportions of subclusters were clearly different in the soil depthsanalyzed. 1A OTUs (facultative anaerobic deltaproteobacteria) increased their relative proportion inthe 20 cm whereas subcluster 1K (aerobic alphaproteobacteria) displays the opposite trend. On theother hand, subclusters 1B (cyanobacteria) and 3B (anaerobic bacteria) were represented at asignificant proportion (>1%) only in the first 10 cm (1B 33%) and 20 cm (3B 15%), respectively. Mostabundant OTUs from subclusters 1A, 1K, 1B and 3B were related to Geobacter, Bradyrhizobium,Nostoc and Anabaena, and uncultured bacteria, respectively. Soil management practices also seem toaffect the distribution of the active diazotrophs in both depths differentially. Subcluster 1Kpredominated (98%) in the upper layer and 1A (99%) at 20 cm depth in the soil under crop rotation(GAP). Under monoculture (PAP) treatment cyanobacteria (98%) predominated at 10 cm decreasingat 20 cm where the subcluster 1K (52%), 3B (45%) and 1A (3%) were found in greater proportion. Bycontrast, in the grassland soil (NE) the communities were found similar at both depths, with a highproportion of subclusters 1A (79-87%) and 1K (11%).We concluded that the structure of the active diazotrophic community displays features associatedwith the use, management and soil depth examined.