Carbon amendment and soil depth affect the distribution and abundance of denitrifiers in agricultural soils

The nitrite reductase (nirS and nirK) and nitrous oxide reductase-encoding (nosZ) genes of denitrifying populations present in an agricultural grassland soil were quantified using real-time polymerase chain reaction (PCR) assays. Samples from three separate pedological depths at the chosen site were investigated: horizon A (0–10 cm), horizon B (45–55 cm), and horizon C (120–130 cm). The effect of carbon addition (treatment 1, control; treatment 2, glucose-C; treatment 3, dissolved organic carbon (DOC)) on denitrifier gene abundance and N₂O and N₂ fluxes was determined. In general, denitrifier abundance correlated well with flux measurements; nirS was positively correlated with N₂O, and nosZ was positively correlated with N₂ (P < 0.03). Denitrifier gene copy concentrations per gram of soil (GCC) varied in response to carbon type amendment (P < 0.01). Denitrifier GCCs were high (ca. 10⁷) and the bac:nirK, bac:nirS, bac:nir^T, and bac:nosZ ratios were low (ca. 10⁻¹/10) in horizon A in all three respective treatments. Glucose-C amendment favored partial denitrification, resulting in higher nir abundance and higher N₂O fluxes compared to the control. DOC amendment, by contrast, resulted in relatively higher nosZ abundance and N₂ emissions, thus favoring complete denitrification. We also noted soil depth directly affected bacterial, archaeal, and denitrifier abundance, possibly due to changes in soil carbon availability with depth.