Analysis of nifH transcripts in Argentinean soils: An approach to evaluate biological nitrogen fixation

CALDEROLI PRISCILA; RAMOS, E; COLLAVINO M.M; AGUILAR O.M

Córdoba

Congreso; XI Congreso Argentino de Microbiología General (SAMIGE).; 2015

Asociación Argentina de Microbiología General

Biologically available N is often a limiting nutrient in agricultural soil and other environments. About 4.32 x 105 tons of nitrogen fertilizers are used annually in the productive agricultural region of Argentina. In this context, we were interested in analyzing the active diazotrophic community in the Argentinean soils by assessing nifH-mRNAs transcripts using pyrosequencing technology. We sampled production fields located in Pergamino (Buenos Aires) subjected to the following no-till treatments: intensive crop rotation, nutrient replacement and minimal use of agrochemicals (good agricultural practices, GAP); soybean monoculture, low nutrient replacement and high agrochemical use (poor agricultural practices, PAP), and a grassland soil used as a reference natural 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-quality reads (low-quality base-calling, frameshift and chimeras errors) were removed and the final set of 28171 reads were 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 using reference nifH datasets (Zher et al., 2004) with the ARB software. The following nifH subclusters were represented in our datasets: 45% of 1A (Anaeromyxobacter y Desulfuromonadales), 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 depths analyzed. 1A OTUs (facultative anaerobic deltaproteobacteria) increased their relative proportion in the 20 cm whereas subcluster 1K (aerobic alphaproteobacteria) displays the opposite trend. On the other hand, subclusters 1B (cyanobacteria) and 3B (anaerobic bacteria) were represented at a significant proportion (>1%) only in the first 10 cm (1B 33%) and 20 cm (3B 15%), respectively. Most abundant OTUs from subclusters 1A, 1K, 1B and 3B where related to Geobacter, Bradyrhizobium, Nostoc and Anabaena, and uncultured bacteria, respectively. Soil management practices also seem to affect the distribution of the active diazotrophs in both depths differentially. Subcluster 1K predominated (98%) in the first cm and 1A (99%) at 20 cm in the soil under crop rotation (GAP). Under monoculture (PAP) treatment cyanobacteria (98%) predominated at 10 cm decreasing at 20 cm where the subcluster 1K (52%), 3B (45%) and 1A (3%) were found in greater proportion. By contrast, in the grassland soil (NE) the communities of both depths were similar, with a high proportion of subclusters 1A (79-87%) and 1K (11%).We concluded that the structure of the active diazotrophic community displays features associated with the use, management and soil depth examined.