Statistical analyses based on the community composition of sequences received from pyrosequencing of narG, nirK, nirS, and nosZ from different peatland soils were conducted to assess the effect of different environmental parameters on community composition of nitrate reducers and denitrifiers. Environmental parameters were classified as primary environmental parameters (i.e., pH, temperature, precipitation, nitrate and ammonium content, total carbon, total nitrogen, C/N ratio) or derived parameters (i.e., N2O emission, ratio of N2O to total N-gases, relative abundance of narG or nosZ). Canonical correspondence analyses (CCA) were conducted for each gene marker on the basis of rarified species-level OTU tables to cluster the communities according to their habitat and to elucidate the primary or derived environmental parameters that influence community composition.
CCA of narG revealed a clear separation of Puukkosuo and Schl¨oppnerbrunnen fen soil communities from each other and from the permafrost affected communities (Figure 23 A), as was already observed for direct comparison of OTU relative abun-dances (Figure 18 A). Community composition was affected by pH, total soil carbon and mean annual precipitation (Figure 23 A). The relative abundance of OTU 1 was negatively correlated with pH and soil water content (Spearman rank correlations with R = −0.8, P = 0.08 and R ≈ −1.0, P = 0.02, respectively), and positively correlated with total soil carbon (R = 0.9, P = 0.08), while OTU 2 was positively and negatively correlated with soil water content and total soil carbon, respectively (R≈1.0,P < 0.001 and R=−0.9, P = 0.08, respectively).
CCA ofnirK showed a clear separation of all studied peatland communities (Fig-ure 23 B). ThenirK community of Puukkosuo fen was most dissimilar to the com-munities in the other peatland soil, as it was also indicated earlier (Figure 18 B, Ta-ble 9). pH and total soil carbon determined the community composition of detected
3.3 Processes involved in turnover of N2O
Figure 23: Canonical correspondence analyses (CCA) based on relative species-level OTU abundances of narG (A), nirK (B), nirS (C), and nosZ (D). Blue circles represent the different sampling sites (Table 6).
Primary and derived environmental parameters affecting the community com-position of denitrification-associated genes are displayed in red. OTU abun-dances from rarified datasets(100 iterations at sampling depths of 500, 1000, 100, and 100 for narG,nirK,nirS, and nosZ, respectively) were used for the calculation of CCAs.
nirK (Figure 23 B). Correlations between environmental parameters and individ-ual OTUs were observed for OTU 1, which was predominant in frost-affected soils and was positively, negatively and negatively correlated with total soil carbon, soil water content and soil pH, respectively (Spearman rank correlations with R = 0.9, P = 0.08, R ≈ −1.0, P = 0.02, and R = −0.8, P = 0.08, respectively). On the other hand, the relative abundance of OTU 5 which was abundant in Puukkosuo and
3 Greenhouse gas production in pristine peatlands
Schl¨oppnerbrunnen fen soil was positively and negatively correlated with soil water content and total soil carbon, respectively (R = 0.98, P = 0.02 and R = −0.87, P = 0.08, respectively), indicating that total soil carbon does not only affect the nirK community composition in total but also more directly the relative abundance of individual OTUs. OTU 2 was the only OTU that showed a correlation with in situ N2O emission, the relative abundance of OTU 2 was negatively correlated with N2O emission (R =−0.9, P = 0.08), indicating that denitrifiers carrying this type of nirK might be important for N2O reduction in situ.
CCA based on relative abundances of detected nirS revealed 3 distinct clusters of nirScommunities, thus supporting the visual comparison (Figure 23 C, Figure 18 C).
ThenirS community from Puukkosuo fen was clearly distinct from the community in the other peatland soils, and Skalluvaara palsa peat and peatland tundra as well as Schl¨oppnerbrunnen fen and cryoturbated peat circle communities clustered together (Figure 23 C). Community composition of detected nirS was influenced by pH as a primary environmental factor and by the relative abundance of narG and the ratio of N2O to total N-gases (when 10µM nitrite were supplied) as derived factors (Figure 23 C). The relative abundance of several OTUs was correlated to observed in situ N2O emissions: OTU 1 was positively correlated within situ N2O emissions (Spearman rank correlation withR = 0.98, P = 0.02), while OTUs 2, 4 and 6 were negatively correlated within situ N2O emissions (R =−0.98, P = 0.02,R =−0.87, P = 0.08, and R = −0.98, P = 0.02, respectively), indicating that denitrifiers harboring those types of nirS might be involved in production and consumption of N2O in peatland soils, respectively.
CCA analysis based on relative abundances ofnosZ in peatland soils likewise sup-ported the observed grouping of peatland soils (Figure 23 D, Figure 18 D), as the nosZ community of Puukkosuo fen was clearly separated from the other soils, while
3.3 Processes involved in turnover of N2O
nosZ communities of Schl¨oppnerbrunnen fen and Skalluvaara palsa peat as well as nosZ communities of permafrost tundra and cryoturbated peat circles grouped together (Figure 23 D). Community composition of nosZ was influenced by the primary environmental parameters pH and temperature as well as by the relative abundance ofnosZ in peatland soils as a secondary parameter (Figure 23 D). Unlike OTUs of nirK and nirS, OTUs of nosZ were not correlated with observed in situ N2O emissions. However, OTUs 1 and 2 were negatively and positively correlated with the ratio of N2O to total N-gases in unsupplemented anoxic microcosms, re-spectively (Spearman rank correlations with R ≈ −1.0, P = 0.02 and R = 0.9, P = 0.08, respectively), indicating that denitrifiers harboringnosZ of OTU 1 might be highly efficient in N2O consumption and cause the observed differences in N2O emissions.
The collective data indicate that (i) nitrate reducer and denitrifier communities in different peatland soils are clearly distinct, (ii) the primary and derived environ-mental factors affecting community composition of denitrification associated genes differ between the genes, and (iii) several OTUs of nirK, nirS, and nosZ are likely determining N2O emissions from peatland soils.
3 Greenhouse gas production in pristine peatlands