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.Denitrification is the microbial reduction of NO 3 Ϫ via NO 2 Ϫ to gaseous NO, N 2 O, and N 2 , which are then lost into the atmosphere (36). It therefore results in considerable loss of nitrogen, one of the most limiting nutrients for crop production in agriculture (20). Denitrification is also of environmental concern since, together with nitrification, it is the main biological process responsible for N 2 O emissions (7). N 2 O is a potent greenhouse gas which has a global warming potential about 320 times greater than that of CO 2 and has a lifetime of approximately 120 years (32). In the stratosphere, N 2 O can also react with O 2 to produce NO, which induces the destruction of stratospheric ozone (8). N 2 O can be released into the atmosphere by incomplete denitrification due to the effect of environmental conditions on the regulation of the different denitrification reductases (14, 41, 51), but it has recently been suggested that it could also be due to lack of nitrous oxide reductase in some denitrifiers (19,41). Since N 2 O is an intermediate in the denitrification pathway, both the amount of N 2 O produced and the N 2 O/(N 2 O ϩ N 2 ) ratio are important in understanding and predicting N 2 O fluxes from soils. The main environmental factors known to influence the N 2 O/(N 2 O...

Responses of soil nitrous oxide production and abundances and composition of associated microbial communities to nitrogen and water amendment

“…However, in the red soil, the Principal coordinate analysis (PCoA) of AOA amoA, AOB amoA, nirK, and nosZ gene community profiles in response to NH 4 + and NO 3 − and without N addition in the alluvial soil (a-d) and the red soil (e-h) abundances of nirK and nosZ genes decreased under the ammonium treatment, implying the response of denitrifying genes to ammonium addition varied depending on soil properties. Soil pH was obviously reduced in the ammoniumtreated soils in comparison with the control after incubation which could significantly inhibit the growth of denitrifiers (Čuhel et al 2010;Bakken et al 2012;Huang et al 2014b). A notable discovery was that the community composition of nirK and nosZ responded differently to the water and N amendment in the two soils.…”

Annual emissions of CH<sub>4</sub> and N<sub>2</sub>O, and ecosystem respiration, from eight organic soils in Western Denmark managed by agriculture

“…Following a rise in water is present (Madsen and Jensen, 1988;Jørgensen et al, 2009). Also, the relative importance of N 2 O as product of denitrification is higher at low pH (Cuhel et al, 2010). Groundwater SO 2− 4 was high at the sites W-AR, N-AR and N-RG with significant N 2 O emissions, but also at site W-PG with no significant N 2 O emission (Fig.…”

Contrasting denitrifier communities relate to contrasting N2O emission patterns from acidic peat soils in arctic tundra

“…Denitrification occurs at acidic soil pH in other systems as well, although denitrification capacities of neutral soils are often higher (Parkin et al, 1985). However, denitrification capacities of cryoturbated peat soil were much higher than those of many more neutral habitats, indicating an acid-tolerant denitrifier community in cryoturbated peat that can cope remarkably well with low pH (Cuhel et al, 2010). Apparent K M values for both nitrate and nitrite Figure 6 Phylogenetic tree of representative nirS sequences retrieved from unturbated and cryoturbated peat soils.…”

“…Denitrification rates and the product ratio of N 2 O to N 2 are regulated by the denitrifying community and in situ conditions (for example, pH, temperature, C-to-N-ratio, as well as the availability of substrates and electron acceptors; van Cleemput, 1998). Acidic pHo5 impairs denitrification and increases the product ratio of N 2 O to N 2 (Simek and Cooper, 2002;Cuhel et al, 2010). The increased product ratio of N 2 O to N 2 is likely caused by posttranscriptional effects of low pH on N 2 O reductase formation .…”