Variation of main terrestrial carbon stocks at the landscape-scale are shaped by soil in a tropical rainforest

Economic incentives to offset carbon dioxide (CO2) emissions associated with deforestation and other human activities affecting forest ecosystems depend on robust estimates of changes in forest carbon (C) stocks. Such stocks are difficult to assess in heterogeneous landscapes where the soil properties and the forest structure and functionality vary in space and time. Here we show that geopedological mapping is useful to quantify the above and belowground C stocks in the different land units of the Lacandon tropical rainforest, southeast Mexico. We used an ordination method to recognize major gradients in the soil and we applied regression analyses to identify relationships between soil properties and AGB. Total forest C stocks differed among land units (287 to 478 Mg C ha(-1) in limestone mountains and fluvial terraces, respectively). Soil constrains like rooting depth (ranging from 0.13 to 1.34 m), available water storage capacity (ranging from 32.3 to 161.4 L m(-2)) and Al saturation in the ion exchange complex (0 to 22.6% Al-sat) were correlated with the aboveground biomass (AGE) C stock by affecting the stem size and density of trees. Soil organic carbon (SOC) in the solum represented 22 to 46% of the total forest C stock in the different landscape units, of which 28 to 45% was stored below 30 cm depth. Therefore, an accurate assessment of forest C stocks must consider not only the variation between land units with contrasting soil properties, but also the solum depth. Our results indicate that stratified sampling based on geopedologic mapping is useful to allocate incentives assessment of C storage at relatively low costs and with reasonable effort.