Doctoral Dissertations

Orcid ID

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Plant, Soil and Environmental Sciences

Major Professor

Frank E. Löffler

Committee Members

Mark Radosevich, Karen G. Lloyd, Jie Zhuang


N2O is a long-recognized greenhouse gas (GHG) with potential in global warming and ozone depletion. Terrestrial ecosystems are a major source of N2O due to imbalanced N2O production and consumption. Soil pH is a chief modulating factor controlling net N2O emissions, and N2O consumption has been considered negligible under acidic conditions (pH <6). In this dissertation, we obtained solids-free cultures reducing N2O at pH 4.5. Furthermore, a co-culture (designated culture EV) comprising two interacting bacterial population was acquired via consecutive transfer in mineral salt medium. Integrated phenotypic, metagenomic and metabolomic analysis dictated that the Serratia population excreted certain amino acid to support Desulfosporosinus population growth. Characterization of co-culture EV demonstrated that organisms synthesizing functional NosZ exist in acidic soils. To further close the knowledge gap of low pH N2O reduction, we conducted enrichment experiments on two contrasting soils representing natural and agricultural soils, respectively. With varying combination of carbon sources and H2, N2O reduction activity was observed in a total of six solids-free cultures at pH 4.5. Comparative growth experiments documented that N2O was essential for N2O-reducing organisms’ growth. Reconstruction of the communities suggested the cultures were highly enriched following consecutive transfer efforts. Surprisingly, N2O-reducing organisms recovered from the cultures were all identified as clade II lineage or a novel clade lineage. These results together suggested that clade II and the putative novel clade N2O-reducing organisms are closely associated with low pH N2O reduction. The observation of low pH N2O reduction in current research was inconsistent to field observation, where the abiotic-biotic interaction occurs frequently. To simulate potential abiotic-biotic interaction of low pH N2O reduction, we tested the impact of geochemistry factors on N2O reduction by culture EV. The impeded N2O reduction of co-culture EV in the presence of trace amount (as low as 0.01 mM for nitrate and 0.001 mM for nitrite) of nitrogen oxyanions was unexpected, which may explain the weak N2O reduction activity under field conditions. These observations indicated N2O reduction in acidic soils may be limited by interacting abiotic factors, instead of pH itself.

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