ABSTRACT
To determine to which extent root-derived carbon contributes to the effects of plants on nitrate reducers and denitrifiers, four solutions containing different proportions of sugar, organic acids and amino acids mimicking maize root exudates were added daily to soil microcosms at a concentration of 150 microg C g(-1) of soil. Water-amended soils were used as controls. After 1 month, the size and structure of the nitrate reducer and denitrifier communities were analysed using the narG and napA, and the nirK, nirS and nosZ genes as molecular markers respectively. Addition of artificial root exudates (ARE) did not strongly affect the structure or the density of nitrate reducer and denitrifier communities whereas potential nitrate reductase and denitrification activities were stimulated by the addition of root exudates. An effect of ARE composition was also observed on N(2)O production with an N(2)O:(N(2)O + N(2)) ratio of 0.3 in microcosms amended with ARE containing 80% of sugar and of 1 in microcosms amended with ARE containing 40% of sugar. Our study indicated that ARE stimulated nitrate reduction or denitrification activity with increases in the range of those observed with the whole plant. Furthermore, we demonstrated that the composition of the ARE affected the nature of the end-product of denitrification and could thus have a putative impact on greenhouse gas emissions.
Subject(s)
Bacteria/classification , Bacteria/isolation & purification , Biodiversity , Nitrates/metabolism , Nitrites/metabolism , Plant Roots/microbiology , Soil Microbiology , Bacteria/metabolism , Bacterial Proteins/genetics , Carbon/metabolism , DNA, Bacterial/chemistry , DNA, Bacterial/genetics , Molecular Sequence Data , Oxidation-Reduction , Oxidoreductases/genetics , Sequence Analysis, DNA , Zea maysABSTRACT
The aim of this study was to determine the effect of the rhizosphere of maize on the diversity of denitrifying bacteria. Community structure comparison was performed by constructing a collection of isolates recovered from bulk and maize planted soil. A total of 3240 nitrate-reducing isolates were obtained and 188 of these isolates were identified as denitrifiers based on their ability to reduce nitrate to N2O or N2. 16S rDNA fragments amplified from the denitrifying isolates were analysed by restriction fragment length polymorphism. Isolates were grouped according to their restriction patterns, and 16S rDNA of representatives from each group were sequenced. A plant dependent enrichment of Agrobacterium-related denitrifiers has been observed resulting in a modification of the structure of the denitrifying community between planted and bulk soil. In addition, the predominant isolates in the rhizosphere soil were not able to reduce N2O while dominant isolates in the bulk soil evolve N2 as a denitrification product.
