Hydrological conditions (i.e., high-flow versus low-flow) in peatland drainage streams influence both the quantity of dissolved organic carbon (DOC) exports and dissolved organic matter (DOM) composition. Yet, our knowledge on DOM fate after exports from the peatland remains limited while this highly reactive component sustains emissions and exports of carbon dioxide (CO2) from streams through degradation processes. The present study demonstrates the relationships between DOM composition evolution and catchment hydrological conditions along a 3 km long headwater stream running through a boreal peatland, from its source to the outlet. Our results show that hydrological conditions significantly influenced DOM composition evolution along the stream. DOM exported during high-flow conditions presented a composition similar to peat porewater in terms of DOC:DON ratio and aromaticity, but a lower average molecular weight, indicating preferential exports of low molecular weight DOM recently produced in the acrotelm. The DOM composition changed little along the stream during high-flow as it was rapidly flushed downstream. During low-flow conditions, DOM composition evolved along the stream in contrast to high-flow with a strong increase in DOM aromaticity and molecular weight along the stream. These changes were significantly correlated to the water residence time in the stream and to the estimated proportion of mineralized DOC to total DOC flux exported at the stream outlet. These results highlight the importance of hydrological conditions on DOM dynamics as DOM was locally mineralized during low-flow conditions, when DOC exports were low, while mineralization processes happened downstream under high-flow conditions which favored important DOC exports.

Tropical peatlands are one of the most effective long-term natural carbon stores on Earth. The drainage, conversion, and degradation of natural tropical peatlands for agricultural development shifts the magnitude and direction of their carbon balance, from net carbon sinks to sources. Yet, there are limited studies that globally synthesize information to constrain our general understanding of the characteristics and linkages between peat soil physicochemical properties and greenhouse gas (GHG) effluxes in tropical peatlands, as well as how their dynamics may be altered by land-use and land-cover change (LULCC). Here, we systematically reviewed more than 100 published field-based papers on soil physicochemical properties such as peat thickness, ages, bulk density, carbon and nitrogen contents, carbon to nitrogen ratio, water table, and CO2 and CH4 effluxes across three main tropical peatland regions, i.e., Latin America, Central Africa, and Southeast Asia. We report that Southeast Asian peatlands have the thickest layer with 537±230 cm compared to Latin America (150±104 cm) and Central Africa (250±160 cm). We also observed a strong natural variation of soil physicochemical properties within the region, which may imply variability of produced GHGs. Most managed peatlands have a higher bulk density compared to undisturbed ones (0.12±0.05 and 0.20±0.18 g cm-3) despite a slightly similar carbon content (44±9 and 47±10 %), which may suggest substantial peat subsidence and losses. Similarly, LULCC generates more than double increase in CO2 effluxes despite lowering CH4 effluxes compared to undisturbed peatlands. The global database constructed through this literature review will be valuable for future modelling improvement of peatland carbon estimates in the tropics, a significant carbon-rich system but often overlooked in the terrestrial climate model. Further, our synthesis and dataset will help provide science-based guidelines to set and monitor emissions reduction targets as part of the forestry and land-use sector.