N2O emission dominates greenhouse-gas budgets of current and former peat swamp forests in Borneo and the Peruvian Amazon under disturbed water regimes

Biogeochemistry of tropical peatlands is vital for planet Earth, while little is known about their functioning under various water regimes and human disturbances. We ran measurement campaigns of soil respiration, methane (CH₄) and nitrous oxide (N₂O) fluxes in three pristine peat swamp forests around Iquitos (the Peruvian Amazon), in Maludam (Sarawak, Borneo, Malaysia) and Klias (Sabah, Borneo, Malaysia), manioc field in Iquitos, and oil palm plantations on peat soil in Sarawak and Sabah in dry seasons between 2017 and 2022. We extracted gross primary production values from the MODIS Terra satellite data for the field campaign days. Most of our sites were net carbon sinks. In a CO₂-equivalent greenhouse gas (GHG) budget, the carbon sinks were, however, offset by large N₂O emissions from the manioc field, oil palm plantations, as well as the Peruvian peat swamp forest, turning them into net GHG sources. Surprisingly for peat swamp forests, CH₄ comprised only a minor share of the GHG budgets. CH₄ may still be more important in wet seasons. Neither do our GHG budgets account for potential canopy effects, such as N₂O sinks in canopy air space (Mander et al., 2021) and respiration in trees.

To explain the high N₂O production, we collected peat samples from the Peruvian and Sabah sites, and used qPCR analysis to measure abundances of bacteria- and archaea-specific 16S rRNA, nitrification (AOA, AOB and COMAMMOX amoA), denitrification (nirK, nirS, nosZI and nosZII), nitrogen fixation (nifH) and DNRA (nrfA) marker genes in the peat samples. The N₂O emissions were positively correlated to archaeal amoA and nrfA gene abundances. This suggested that, contrary to expectation, nitrifiers produced much of N₂O emissions in the peatlands. The finding agrees with the global peatland microbiome study showing the importance of nitrifiers for N₂O emissions in disturbed soils (Bahram et al. 2022).

Further annual monitoring is needed to fully understand the GHG fluxes in the scarcely studied tropical peatlands.