Differences of nitrogen cycle microbes and nitrous oxide emissions from natural and degraded tropical peatland sites in Malaysia

Tropical peatlands in Southeast Asia are severe threats due to rapid changes in land-cover. Estimates suggest that approximately 25% of natural peatlands in Malaysia have been converted for large-scale agro-industrial development in the last two decades, primarily driven by the rapid expansion of oil palm cultivation. These intense and rapid land-use changes contribute to adverse ecological impacts, including increased greenhouse gas (GHG) emissions, soil degradation, deforestation and the loss of biodiversity. The aim of the study was to assess the genetic potential of nitrogen (N) transformation processes by quantifying functional marker genes of N cycle and measuring nitrous oxide (N₂O) emissions from natural tropical peatland and oil palm plantation. This was conducted to understand the effect of land-cover changes on microbial N transformation processes and gaseous N emissions, using the closed chamber method and quantitative polymerase chain reaction (qPCR) analysis. N₂O emissions were measured in the natural tropical peatland forest in Maludam (Sarawak, Malaysia) and in the oil palm plantation on peat soil in Betong (Sarawak, Malaysia) in September 2022. qPCR was used to measure the abundance of bacterial and archaeal specific 16S rRNA, nitrification (AOB, AOA and COMAMMOX amoA genes), denitrification (nirK, nirS, nosZ clade I and nosZ clade II genes) and dissimilatory nitrate reduction to ammonium (DNRA; nrfA gene) marker genes in the collected soil, litter and leaf samples.

The average soil N₂O emissions were relatively higher from the oil palm plantation, ranging from 2.04 to 131.9 µg N m^-2 h^-1. Soil N₂O emissions from the natural peatland forest were negligible. Quantification of N cycle genes revealed variations in the microbiome between natural peatland and deep-drained oil palm plantation. The microbial analysis showed that the archaeal abundance in leaves did not vary significantly between the two sites, but the abundance of bacteria in leaves was higher in the oil palm plantation. The abundance of denitrifying microorganisms was significantly higher in the natural peatland soil compared to the peat soil in the oil palm plantation. However, the abundances of bacterial amoA and archaeal amoA were found to be lower in the soil of natural site compared to the soil in oil palm plantation, suggesting a higher genetic potential of nitrification in the oil palm plantation. In addition, microbes possessing the archaeal amoA gene seemed to be the primary nitrifiers in the soil of oil palm plantation. The study’s findings indicate that hydrological interventions cause significant changes in the microbial N cycle and N₂O emissions of tropical peatlands.