Temporal dynamics of soil microbial C and N cycles with GHG fluxes in the transition from tropical peatland forest to oil palm plantation.

Tropical peatlands significantly influence local and global carbon and nitrogen cycles, yet they face growing pressure from anthropogenic activities. Land use changes, such as peatland forests conversion to oil palm plantations, affect the soil microbiome and greenhouse gas (GHG) emissions. However, the temporal dynamics of microbial community changes and their role as GHG indicators are not well understood. This study examines the dynamics of peat chemistry, soil microbial communities, and GHG emissions from 2016 to 2020 in a logged-over secondary peat swamp forest in Sarawak, Malaysia, which transitioned to an oil palm plantation. This study focuses on changes in genetic composition governing plant litter degradation, methane (CH₄), and nitrous oxide (N₂O) fluxes. Soil CO₂ emission increased (doubling from approximately 200 mg C m^-2 h^-1), while CH₄ emissions decreased (from 200 µg C m^-2 h^-1 to slightly negative) following land use changes. The N₂O emissions in the oil palm plantation reached approximately 1,510 µg N m^-2 h^-1, significantly higher than previous land uses. The CH₄ fluxes were driven by groundwater table, humification levels, and C:N ratio, with Methanomicrobia populations dominating methanogenesis and Methylocystis as the main CH₄ oxidizer. The N₂O fluxes correlated with groundwater table, total nitrogen, and C:N ratio with dominant nirK-type denitrifiers (13-fold nir to nosZ) and a minor role by nitrification (a threefold increase in amoA) in the plantation. Proteobacteria and Acidobacteria encoding incomplete denitrification genes potentially impact N₂O emissions. These findings highlighted complex interactions between microbial communities and environmental factors influencing GHG fluxes in altered tropical peatland ecosystems.IMPORTANCETropical peatlands are carbon-rich environments that release significant amounts of greenhouse gases when drained or disturbed. This study assesses the impact of land use change on a secondary tropical peat swamp forest site converted into an oil palm plantation. The transformation lowered groundwater levels and changed soil properties. Consequently, the oil palm plantation site released higher carbon dioxide and nitrous oxide compared to previous land uses. As microbial communities play crucial roles in carbon and nitrogen cycles, this study identified environmental factors associated with microbial diversity, including genes and specific microbial groups related to nitrous oxide and methane emissions. Understanding the factors driving microbial composition shifts and greenhouse gas emissions in tropical peatlands provides baseline information to potentially mitigate environmental consequences of land use change, leading to a broader impact on climate change mitigation efforts and proper land management practices.