Microbial nitrogen cycle in sub-tropical peatland cloud forest and wetland ecosystems of Réunion Island

Peatlands play an essential role in the regulation of carbon and nitrogen cycles. Nitrogen-rich peatlands under warm and dry conditions can be a source of N₂O, a potent greenhouse gas. Research on microbial activity, particularly in relation to N₂O emissions in sub-tropical peatlands and wetlands, is very limited. In the current study, we investigated two peatland cloud forest sites in Reunion Island, namely Plaine des Cafres (characterized by dominant species Erica reunionensis and Alsophila glaucifolia) and Forêt de Bébour (featuring Erica reunionensis exclusively), alongside one RAMSAR wetland site located in Saint Paul. Both cloud forest sites were located at an altitude of 1500-1600 m, while the wetland was at an altitude of 4 m. to clarify the microbial dynamics of the nitrogen cycle in sub-tropical peatland cloud forests and wetlands.

DNA extraction was performed on these samples, followed by quantification of genes associated with the nitrogen cycle using quantitative polymerase chain reaction (qPCR). Analyses are ongoing for plant samples. In addition, soil samples underwent analyses to assess levels of ammonium (NH₄⁺-N) and nitrate (NO₃⁻-N). Soil N₂O fluxes were determined by collecting gas samples from the chamber headspace of static soil chamber systems at 20-minute intervals during one-hour sessions. The concentration of N₂O was determined from gas samples using a gas chromatographer (Shimadzu, 2014).

All sites emitted negligible soil N₂O fluxes (mean: 0.9 µg N m⁻² h⁻¹). However, a substantial amount of soil NH₄⁺-N was found across all sites (mean: 77.2 mg/kg). The forests dominated by Erica reunionensis showed the highest values (mean: 106 mg/kg). Soil NH₄⁺-N significantly correlated with the abundance of the nifH gene (R² = 0.7, p<0.05). This indicates a high potential for microbial nitrogen fixation in all sites. Soil NO₃⁻-N varied significantly among different ecosystems. The cloud forests dominated by Erica reunionensis showed the highest values (mean: 139 mg/kg) as compared to the mixed forest (mean: 53.7 mg/kg) and the wetland site (mean: 2.53 mg/kg). Archaeal amoA gene abundance and proportion in soil were higher (p<0.05) in cloud forest sites than in wetland sites, positively correlating with NO₃⁻-N (R² = 0.7, p<0.05). This reveals an archaeal nitrification potential in cloud forests. The nir:amoA ratio, as well as the nirS gene proportion, was significantly higher in the wetland (p<0.05), indicating the anaerobic denitrification potential, which explains the low NO₃⁻-N values there. Meanwhile, low soil N₂O fluxes in the cloud forest soils can be attributed to the high abundance and proportions of nosZI-type denitrifiers.

The canopy soil from Erica reunionensis had a higher abundance of nirK and nosZI genes than Alsophila glaucifolia's canopy soil (P<0.05). However, it was the opposite in the case of fungal nirK abundance. The presence of denitrification genes in the canopy indicates an aboveground potential denitrification pathway in the cloud forests of the Réunion island.