Insights into the effect of soil pH on N(2)O and N(2) emissions and denitrifier community size and activity

Abstract

The objective of this study was to investigate how changes in soil pH affect the N(2)O and N(2) emissions, denitrification activity, and size of a denitrifier community. We established a field experiment, situated in a grassland area, which consisted of three treatments which were repeatedly amended with a KOH solution (alkaline soil), an H(2)SO(4) solution (acidic soil), or water (natural pH soil) over 10 months. At the site, we determined field N(2)O and N(2) emissions using the (15)N gas flux method and collected soil samples for the measurement of potential denitrification activity and quantification of the size of the denitrifying community by quantitative PCR of the narG, napA, nirS, nirK, and nosZ denitrification genes. Overall, our results indicate that soil pH is of importance in determining the nature of denitrification end products. Thus, we found that the N(2)O/(N(2)O + N(2)) ratio increased with decreasing pH due to changes in the total denitrification activity, while no changes in N(2)O production were observed. Denitrification activity and N(2)O emissions measured under laboratory conditions were correlated with N fluxes in situ and therefore reflected treatment differences in the field. The size of the denitrifying community was uncoupled from in situ N fluxes, but potential denitrification was correlated with the count of NirS denitrifiers. Significant relationships were observed between nirS, napA, and narG gene copy numbers and the N(2)O/(N(2)O + N(2)) ratio, which are difficult to explain. However, this highlights the need for further studies combining analysis of denitrifier ecology and quantification of denitrification end products for a comprehensive understanding of the regulation of N fluxes by denitrification.

FIG. 1.

Dynamics of total N (N₂O + N₂) and N₂O in situ fluxes after the addition of ¹⁵N-labeled NO₃⁻ to acidic, natural pH, and alkaline soils. Mean values ± standard errors of the means are shown (n = 12).

FIG. 2.

In situ cumulative losses of N (separately as N₂O and N₂) (A) and relative N₂O production expressed as the N₂O/(N₂O + N₂) molar ratio (B) over the 74 h after the addition of ¹⁵N-labeled KNO₃ to acidic, natural pH, and alkaline soils. Mean values and ± standard deviations are shown (n = 12). The different letters next to the bars indicate significant differences between the specific pH treatments (P < 0.05).

FIG. 3.

DEA (separately as N₂O and N₂ production) of soils with different pH treatments (A) and relative N₂O production during DEA determination expressed as the N₂O/(N₂O + N₂) molar ratio (B). DEA (A) was also estimated and is expressed as g N m⁻² 74 h⁻¹ (right vertical axis). Mean values ± standard deviations are shown (n = 12). The different letters next to the bars indicate significant differences between the specific pH treatments (P < 0.05).

FIG. 4.

Abundances of the 16S rRNA, narG, napA, nirS, nirK, and nosZ genes in soils with different pH treatment expressed as the number of gene copies ng⁻¹ of DNA (A), the specific denitrification gene/16S rRNA gene ratio (B), and nosZ/nirS, nosZ/nirK, and nosZ/(nirS + nirK) ratios (C). Mean values and standard deviations are shown (n = 12). The different letters above the bars indicate significant differences between the specific pH treatments (P < 0.05).