Linking nitrogen transformations and soil δ15N to assess drivers of nitrogen losses across a tropical peat swamp forest and an oil palm plantation

Conversion of tropical peatland forests to oil palm plantations has been associated with elevated nitrous oxide (N2O) emissions and nitrate (NO3−) leaching, yet the biogeochemical mechanisms driving these nitrogen (N) losses remain poorly understood. To address this gap, soil N2O fluxes, gross N transformation rates, microbial gene abundances, annual NO3− leaching rates, and bulk soil δ15N signatures were quantified along transects in Malaysia that included intact tropical peat swamp forest and drained oil palm plantation. Oil palm plantation sites had higher topsoil oxygen (O2) concentrations than forest sites, indicating that drainage shifts soils from predominantly anaerobic to aerobic conditions. These redox shifts, together with fertilizer inputs, increased nitrifier gene abundances and enhanced gross nitrification rates, which exceeded ammonification rates by up to 186%, promoting soil NO3− accumulation. The elevated soil NO3− concentrations stimulated N2O production, probably through coupled nitrification-denitrification, and increased NO3− leaching. Overall, these results showed that oil palm plantation sites had higher gaseous and hydrological N losses than peat forest sites. Additionally, nitrification rates, hydrological N export, and gaseous N losses were positively correlated with more enriched bulk soil δ15N values along the land use gradient. These findings demonstrated that bulk soil δ15N patterns may have the potential to serve as indicators of long-term N cycling and losses in tropical peatlands associated with human-induced land use changes.