Influence of liquid manure on soil denitrifier abundance, denitrification, and nitrous oxide emissions
The influence of liquid dairy and swine manure, soil water-extractable C (WEC) and glucose on the amount of denitrification, the N2O molar ratio [N2O/(N2O + N2)], and the abundance of soil denitrifiers was investigated using repacked soil cores. Quantitative polymerase chain reaction was used to measure the abundance of the total bacteria (16S rRNA gene), two denitrifier guilds bearing the cnorB gene, and nosZ gene communities. The availability of both C and N oxides influenced the amount of denitrification and the N2O molar ratio for simple (glucose) and complex (liquid manure and WEC) C sources. A positive relationship (R2 = 0.77) was measured between respiration (cumulative CO2 emissions) and total denitrification. A negative relationship (R2 = 0.70) was found between respiration and the N2O molar ratio, demonstrating that C availability in soil promotes the reduction of N2O to N2. The abundance of the 16S rRNA and cnorBB (Bosea/Bradyrhizobium/Ensifer spp.) gene bearing bacteria was not significantly affected by the addition of C. The abundance of cnorBP (Pseudomonas mandelii and related species) gene bearing bacteria increased in response to glucose and manure addition, but only when also amended with NO3–. Significant positive correlations were found between the abundance of the cnorBP gene bearing bacteria and respiration. The nosZ gene bearing bacteria significantly increased only in soil amended with liquid manures. No significant relationships were found between the abundance of denitrifiers and total denitrification, N2O emissions, or the N2O molar ratio. Changes in denitrifier community abundance appeared to reflect changes in C substrate availability and were uncoupled to denitrification activity.