Long-term effects of nitrogen fertilization on methane emissions in drained tropical peatland

Nitrogen (N) fertilization improves crop productivity. However, the long-term effects of N application on methane (CH4) emissions in drained peat soils, particularly under different hydrological conditions, remain poorly understood. Accurate quantification of CH4 emissions from peatlands is essential for assessing carbon losses and formulating effective climate change mitigation strategies. This study was conducted to investigate the impact of N fertilization on CH4 emissions and identify the main factors influencing CH4 emissions from drained tropical peatlands. This study was conducted on an oil palm plantation in Sarawak, Malaysia, a randomized block design included four N fertilizer treatments: Control (0 kg N ha− 1 yr− 1) (T1); low (31.1 kg N ha⁻¹ yr⁻¹) (T2), moderate (62.2 kg N ha⁻¹ yr⁻¹) (T3), and high (124.3 kg N ha⁻¹ yr⁻¹) (T4). Soil CH4 fluxes showed no statistically significant differences between treatments or across years, with emissions ranging from − 163.6 to 320.7 µg C m− 2 hr− 1 at T1, -86.7 to 285.8 µg C m− 2 hr− 1 at T2, -131.6 to 274.1 µg C m− 2 hr− 1 at T3 and − 125.7 to 185.9 µg C m− 2 hr− 1 at T4 (p > 0.05). Although ammonium sulfate fertilization did not significantly alter CH4 emissions, its pronounced acidifying effect on soil pH, particularly at application rates above 62.2 kg N ha⁻¹ yr⁻¹ along with elevated sulfate (SO42−) inputs and nitrogen pools exceeding the critical threshold (> 400 ppm), likely suppressed methanogenic activity and constrained soil organic matter decomposition. Water-filled pore space (WFPS) influenced CH4 emissions more than groundwater level (GWL), with the low GWL at the site limiting its impact. Increased WFPS (60–80%) reduced nitrate (NO3−) through enhanced denitrification, lowering its inhibition on CH4 production and thus increasing emissions. This study highlights the key role of soil moisture and nitrogen cycling in regulating CH4 emissions in peatland.