Humic acid-oxidizing, nitrate-reducing bacteria in agricultural soils

Area of Science:

• Environmental microbiology
• Soil science
• Biogeochemistry

Background:

• Nitrogen loss from soil systems is a significant environmental concern.
• Humic acids (HA) are abundant organic compounds in soil, but their role in microbial metabolism is not fully understood.
• Nitrate reduction is a key process in soil nitrogen cycling.

Purpose of the Study:

• To investigate the prevalence and diversity of microorganisms capable of oxidizing reduced humic acids (HA) coupled with nitrate reduction in agricultural soils.
• To determine the impact of reduced HA as an electron donor on denitrification processes and bacterial community composition in soil.

Main Methods:

• Most probable number (MPN) enumeration to quantify HA-oxidizing, nitrate-reducing microorganisms.
• Isolation and phylogenetic characterization (16S rRNA gene sequencing) of these microorganisms.
• Advective up-flow column experiments to assess HA oxidation and nitrate reduction rates.
• High-density oligonucleotide microarray (PhyloChip) analysis to profile bacterial community composition.

Main Results:

• Large populations (10(4) to 10(6) cells g(-1) soil) of nitrate-reducing microorganisms utilizing reduced HA were found.
• Isolated organisms belonged to diverse phylogenetic groups, including Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria.
• Reduced HA significantly enhanced nitrate reduction and HA oxidation in soil columns, supporting bacterial communities enriched in Acidobacteria, Firmicutes, and Betaproteobacteria.

Conclusions:

• This study identifies a previously unrecognized role for reduced humic acids in stimulating denitrification in saturated soil systems.
• Reduced humic acids serve as electron donors for a diverse group of soil microorganisms, impacting nitrogen loss.
• These findings highlight the influence of reduced humic acids on soil geochemistry and indigenous bacterial community structure.