Uncoupled surface and below-ground soil respiration in mangroves: implications for estimates of dissolved inorganic carbon export

Potential disparities between rates of surface and below-ground respiration were examined in seven mangrove forests of different topographic height in Timor Leste. Differences in surface respiration between air-exposed and inundated soils were inconsistent, but surface respiration rates increased, with tidal elevation. Net primary production (NPP) on air-exposed soils declined with increasing forest cover indicating light limitation beneath the canopy. NPP and respiration were linearly related under both air-exposed and inundated conditions. Rates of DIC release from the soil surface varied among forests, correlating only with soil carbon (TOC) and nitrogen (TN) and their stoichiometric ratios. Sulfate reduction was detected to maximum depth of unconsolidated soil, correlating only with TOC and TN content at discrete depth intervals. DIC concentrations in drainage channels were equivalent to porewater concentrations. The rate of carbon mineralized by sulfate reducers (SRC) was equivalent to rates of total carbon oxidation (TCO) measured at the soil surface in forests at tidal heights?≤0.5?m above mean sea-level (MSL). However, SRC was increasingly greater than TCO in forests residing from 1.0 up to 2.5?m above MSL. Most carbon mineralized in subsurface deposits appears to seep out of the forest via groundwater. Rates of surface respiration therefore underestimate rates of total benthic carbon mineralization in forests at topographic heights?≥0.5?m above MSL, suggesting that the amount of respiratory carbon exported from many mangrove forests has also been underestimated.