The processes regulating nitrification in soils are not entirely understood. Here we provide evidence that nitrification rates in soil may be affected by complexed nitrate molecules and microbial volatile organic compounds (mVOCs) produced during nitrification. Experiments were carried out to elucidate the overall nature of mVOCs and biogenic nitrates produced by nitrifiers, and their effects on nitrification and redox metabolism. Soils were incubated at three levels of biogenic nitrate. Soils containing biogenic nitrate were compared with soils containing inorganic fertilizer nitrate (KNO
3
) in terms of redox metabolism potential. Repeated NH
4
–N addition increased nitrification rates (mM NO
3
1-
produced g
-1
soil d
-1
) from 0.49 to 0.65. Soils with higher nitrification rates stimulated (
p
< 0.01) abundances of 16S rRNA genes by about eight times,
amoA
genes of nitrifying bacteria by about 25 times, and
amoA
genes of nitrifying archaea by about 15 times. Soils with biogenic nitrate and KNO
3
were incubated under anoxic conditions to undergo anaerobic respiration. The maximum rates of different redox metabolisms (mM electron acceptors reduced g
-1
soil d
-1
) in soil containing biogenic nitrate followed as: NO
3
1-
reduction 4.01 ± 0.22, Fe
3+
reduction 5.37 ± 0.12, SO
4
2-
reduction 9.56 ± 0.16, and CH
4
production (μg g
-1
soil) 0.46 ± 0.05. Biogenic nitrate inhibited denitrificaton 1.4 times more strongly compared to mineral KNO
3
. Raman spectra indicated that aliphatic hydrocarbons increased in soil during nitrification, and these compounds probably bind to NO
3
to form biogenic nitrate. The mVOCs produced by nitrifiers enhanced (
p
< 0.05) nitrification rates and abundances of nitrifying bacteria. Experiments suggest that biogenic nitrate and mVOCs affect nitrification and redox metabolism in soil.
The complex role of phylloplane microorganisms is less understood than that of rhizospheric microorganisms in lieu of their pivotal role in plant's sustainability. This experiment aims to study the diversity of the culturable phylloplane bacteria of Jatropha curcas and evaluate their growth-promoting activities towards maize seedling vigor. Heterotrophic bacteria were isolated from the phylloplane of J. curcas and their 16S rRNA genes were sequenced. Sequences of the 16S rRNA gene were very similar to those of species belonging to the classes Bacillales (50%), Gammaproteobacteria (21.8%), Betaproteobacteria (15.6%), and Alphaproteobacteria (12.5%). The phylloplane bacteria preferred to utilize alcohol rather than monosaccharides and polysaccharides as a carbon source. Isolates exhibited ACC (1-aminocyclopropane-1-carboxylic acid) deaminase, phosphatase, potassium solubilization, and indole acetic acid (IAA) production activities. The phosphate-solubilizing capacity (mg of PO solubilized by 10 cells) varied from 0.04 to 0.21. The IAA production potential (μg IAA produced by 10 cells in 48 h) of the isolates varied from 0.41 to 9.29. Inoculation of the isolates to maize seed significantly increased shoot and root lengths of maize seedlings. A linear regression model of the plant-growth-promoting activities significantly correlated (p < 0.01) with the growth parameters. Similarly, a correspondence analysis categorized ACC deaminase and IAA production as the major factors contributing 41% and 13.8% variation, respectively, to the growth of maize seedlings.
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