Soil organic carbon stabilization in dry tropical forests of Costa Rica

In contrast to tropical rain forests, mechanisms of soil organic carbon (SOC) stabilization in dry tropical forest (DTF) are less known. Therefore, the objective of this research was to compare mineral phase properties and chemically separated C fractions to 50-cm depth at three sites in the Sector Santa Rosa of Área de Conservación Guanacaste (ACG), Costa Rica: (i) > 400 yr. old-growth forest (Of), (ii) > 90 yr. old secondary deciduous forest (Df); and (iii) > 60 yr. old Quercus oleoides forest (Qu). The pool of chemically separated C fractions and their depth distribution varied depending on the separation technique. The amounts of C preferentially bound to soil minerals in 0–50 cm depth were higher in Of and Df compared to Qu (71.3 and 63.4 vs. 48.1 mg ha− 1, respectively), as indicated by treatment with HF to release mineral-associated SOC. In Of, however, the magnitude of functionally passive SOC pool was larger than in Df and Qu, (10.7 vs. 5.8 and 3.5 Mg ha− 1, respectively), as indicated by the H2O2 treatment. The amounts of stable SOC were not different among the sites, as was indicated by treatment with disodium peroxodisulfate (Na2S2O8). The regressions among selective dissolution data, mineral phase indicators and stabilized SOC indicated that poorly crystalline Fe oxides were the most important mineral phase components for SOC stabilization in DTF sub-soils. Larger sub-soil inputs of black C (BC) as charcoal through bioturbation over a long period at the oldest DTF site, in combination with high amounts of poorly crystalline Fe oxides which were evenly distributed in the soil profile through bioturbation contributed to the larger stabilized SOC pool and its depth distribution compared to the youngest forest site.