Large CO2 and CH4 emissions from drainage ditches in oil palm plantations on peat soil

Tropical peatlands contain approximately 17% of the total global peat carbon and are under pressure for deforestation and the formation of oil palm plantations. The conversion of large peatland forests in Malaysia and Indonesia has resulted in these plantations becoming substantial sources of greenhouse gases. While previous research has focused on estimating the C loss from the soil, the impact of drainage ditches on the overall C budget remains largely unexplored. However, on average, drainage ditches with free surface water cover roughly one-third of the total drained land. Hence, these ditches could be significant CO₂ and CH₄ sources and while not considered for C budget calculation it could lead to significant underestimation of total C loss from these ecosystems. Here we represent the CO₂ and CH₄ emissions from drainage ditches in an oil palm plantation located in Sarawak, Malaysia. CO₂ and CH₄ samples (n=107) were collected from a recently created plantation (~5 y.o.) and from the plantation, which is under second rotation using a floating chamber and LI7810 analyzer (LICOR Biosciences). Additional parameters such as water pH, electrical conductivity, dissolved oxygen concentration, temperature, turbidity, salinity, water level, and dissolved gas concentration (dCO₂ and dCH₄; analyzed in the lab with GC-2014, Shimadzu) were measured from each sampling spot. After measurements, we collected sediment samples for soil TN, TOC, TIC, DOC, DIC, and DN analyses. The results revealed that the average net CH₄ flux (combining both diffusive and ebullitive emissions) from drainage ditches in the first and second rotations was 0.31 ± 0.65 g m^-2 d^-1 and 0.29 ± 0.54 g CH₄-C m^-2 d^-1, respectively. The average CO₂ flux from the first and second rotations was 4.27 ± 2.1 g CO₂-C m^-2 d^-1 and 4.4 ± 2.5 g CO₂-C m^-2 d^-1, respectively. To estimate surface water coverage at the whole site, green vegetation, open water, and bare soil were mapped from the site drone imagery collected in Spring 2023 using object-based supervised classification and spectral indicators computed from red, green, and blue image bands. The total surface water coverage will give us an understanding of the total CO₂ and CH₄ flux in the entire region that originates from drainage ditches. Our results strongly underscore the significant role of drainage ditches in contributing to the overall carbon loss from oil palm plantations on organic soils. Proper consideration of these emissions is essential for accurate carbon budget calculations and for devising effective strategies to mitigate greenhouse gas emissions in these ecosystems.