Abstract
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 (KNO3) in terms of redox metabolism potential. Repeated NH4–N addition increased nitrification rates (mM NO31- 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 KNO3 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: NO31- reduction 4.01 ± 0.22, Fe3+ reduction 5.37 ± 0.12, SO42- reduction 9.56 ± 0.16, and CH4 production (μg g-1 soil) 0.46 ± 0.05. Biogenic nitrate inhibited denitrificaton 1.4 times more strongly compared to mineral KNO3. Raman spectra indicated that aliphatic hydrocarbons increased in soil during nitrification, and these compounds probably bind to NO3 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.
Highlights
Nitrification is a key biogeochemical process for the global nitrogen cycle (Nelson et al, 2016)
How does nitrification progress under repeated N amendment? Second, how does nitrification influence redox metabolism? Third, how does the nitrate produced from nitrification differ from inorganic nitrate? Fourth, do the nitrifiers communicate by means of volatile organic compounds (VOCs)? Nitrification activity was observed under three repeated N amendments
An initial nitrification phase inhibited redox rates compared to the addition of an equivalent amount of inorganic NO31− (KNO3)
Summary
Nitrification is a key biogeochemical process for the global nitrogen cycle (Nelson et al, 2016). Nitrification of ammonia to nitrate is a two-step process usually performed by two distinct groups of chemolitho-autotrophic microbes (Alfreider et al, 2017), one step oxidizes NH4+ to NO21−, while the other oxidizes NO21− to NO31− (Li Y. et al, 2018). Most of the NH4+ is converted to NO21−, but a small portion of the N is emitted as N2O (Liimatainen et al, 2018). This is produced as a byproduct when the intermediate HNO is produced during the oxidation of NH2OH to NO21−. HNO is further oxidized to NO21− and to Biogenic Nitrate and mVOCs Affect Nitrification
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