Abstract
The microbial ecology of the nitrogen cycle in agricultural soils is an issue of major interest. We hypothesized a major effect by farm management systems (mineral versus organic fertilizers) and a minor influence of soil texture and plant variety on the composition and abundance of microbial nitrifiers. We explored changes in composition (16S rRNA gene) of ammonia-oxidizing archaea (AOA), bacteria (AOB), and nitrite-oxidizing bacteria (NOB), and in abundance of AOA and AOB (qPCR of amoA genes) in the rhizosphere of 96 olive orchards differing in climatic conditions, agricultural practices, soil properties, and olive variety. Majority of archaea were 1.1b thaumarchaeota (soil crenarchaeotic group, SCG) closely related to the AOA genus Nitrososphaera. Most AOB (97%) were identical to Nitrosospira tenuis and most NOB (76%) were closely related to Nitrospira sp. Common factors shaping nitrifiers assemblage composition were pH, soil texture, and olive variety. AOB abundance was positively correlated with altitude, pH, and clay content, whereas AOA abundances showed significant relationships with organic nitrogen content and exchangeable K. The abundances of AOA differed significantly among soil textures and olive varieties, and those of AOB among soil management systems and olive varieties. Overall, we observed minor effects by orchard management system, soil cover crop practices, plantation age, or soil organic matter content, and major influence of soil texture, pH, and olive tree variety.
Highlights
Reactive nitrogen (Nr), the forms of N that have a reduced oxidation state relative to N2, is an essential nutrient for plant growth
The ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) communities in the rhizosphere were dominated by Nitrosospira and Nitrospira species, respectively
AOB and NOB accounted for 0.4% and 0.2% of the total bacterial pyrotags sequences, respectively, and were detected in >95% of the rhizospheres sampled
Summary
Reactive nitrogen (Nr), the forms of N that have a reduced oxidation state relative to N2 (mostly NO3- and NH4+), is an essential nutrient for plant growth. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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