Soil Organic Matter Form and Fluxes in Tropical Wetlands.

Soils contain the largest carbon stock on Earth, and tropical soils account for up to one-third of the soil carbon pool. An important control on soil organic matter (SOM) quantities is the mineralization (oxidation) time, which ranges from days to millennia. SOM oxidation rates are affected by climactic factors that control microbial metabolic rates, and thus the distribution and fluxes of SOM.

The major factors contributing to SOM accumulation (e.g., precipitation, clay content, temperature, biological activity) are often at extremes in the tropics. We therefore used the accelerated (potential) rate of carbon processing to examine the factors controlling carbon turnover within the mega deltas of Asia, specifically focusing on the Mekong Delta. We compared the abundance and chemical state of SOM at three wetland sites with varying degrees of saturation by measuring bulk carbon content and chemistry, the latter deduced with C near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. Permanganate oxidizable carbon (POXC) measurements suggest that microbially oxidizable C is concentrated in the top 50-100 cm of the deltaic wetland soils, even when normalized by bulk C content, and that deeper SOM is minimally available for microbial oxidation.

To assess the impact of soil saturation on the chemical composition and bulk abundance of SOM, CO₂ flux and soil moisture contents were monitored at the wetlands over the dry season. Periods of sub-saturation allow more O₂ diffusion into sediment, stimulating SOM oxidation. Over time, soils exposed to seasonal oxygen incursions accumulate less SOM than permanently flooded soils and sediments. Short pulses of aeration may provide pathways to degrade carbon normally stabilized under sustained anaerobic conditions. As soils in the Mekong Delta undergo rapid changes, our results will provide estimates of carbon budgets for wetland soils and sediments under various land-use conditions.