The soil organic carbon in particle-size separates under different regrowth forest stands of north eastern Costa Rica

Despite the importance of the secondary forest (SF) in tropical areas, few studies have quantified the soil organic carbon (SOC) pool in Costa Rica. Most of the studies conducted to date in this country have focused mainly on changes in the soil C pool following conversion of forests to pastures, which is the predominant land use in the tropics. The aim of this study was to measure SOC concentration and pool in particle-size fractions down to 50 cm depth in four SF stands regenerating from different intensities of prior land use in loamy sand and sandy loam soils of northeast Costa Rica: (i) a gallery forest (GF), (ii) a 15-year-old SF enriched with commercially planted native trees (15SF), (iii) a 25-year-old SF (25SF), and (iv) an abandoned Theobromma cacao plantation >60 years old (60SF). Additional objectives were (1) to determine the relationship of SOC concentration with selected physical and chemical soil properties, and (2) to establish the key determinants of the depth distribution of SOC in order to identify meaningful trends in the SOC pool. The SOC pool was highest under the 60SF (221.4 Mg C ha⁻¹) followed by the 15SF (212.1 Mg C ha⁻¹), the 25SF (195.9 Mg C ha⁻¹) and the lowest in the GF (183.5 Mg C ha⁻¹). The SOC concentration decreased significantly from 59.7 to 94.1 g kg⁻¹ in the 0–10 cm layer down to 31.0 to 45.5 g kg⁻¹ in the 40–50 cm layer in all forest stands. The fine silt + clay fraction contained the highest values of SOC concentration in all forest stands. Soil texture and the age of the SF were identified as the main factors that explained the variability in SOC. The age of SF stand influenced the distribution of size class aggregates and SOC.