Organic carbon and nitrogen in soil particle-size aggregates under dry tropical forests from Guanacaste, Costa Rica - Implications for within-site soil organic carbon stabilization

In this study we report results on the soil organic carbon (SOC) pool (0-50 cm) from a chrono-sequence of dry tropical forest (dTf) of increasing age and a yearly burned ancient pasture in the "Sector Santa Rosa" at the "Area de Conservacion Guanacaste" (ACG) in northwestern Costa Rica, where intense human induced land-use modifications has occurred during the past century. The effects of land conversion on soil organic carbon (SOC) have mainly been conducted in the Atlantic humid forests while overlooking dTfs. We quantified the depth distribution of SOC concentration down to 50-cm and in physically separated mineral soil fractions, as these data are scanty from the dTf. Additional objectives were to identify the relationship with selected soil physical and chemical properties, including stabilized SOC fractions by means of multivariate ordination methods. Statistically significant differences were found for the main fixed factor ecosystem for all soil variables analyzed (ANOVA). SOC and N concentrations were significantly higher in the oldest dTf compared to the other dTfs. Soil physical properties like aggregate size distribution and bulk density changed with depth, and varied significantly among the three dTf stands sampled. The multivariate analysis, i.e. between-within class principal component analysis (PCA), revealed a significant ordination of dTfs (P<0.0001). The SOC concentration decreased in particle size fractions of <200 mu m aggregates with increasing soil depth. The lowest and highest C concentrations were obtained in the fine sand (105-200 mu m) and clay+silt (<20 mu m) fractions, respectively. Mineral-associated and stable SOC pool increased with depth, and poorly crystalline Fe oxides and ferrihydrite were the most important minerals for SOC stabilization at 40-50 cm depth. The highest SOC pool was found in the old-growth and >80 years-old dTfs, i.e., 228.9 and 150.3 Mg C ha(-1), respectively, values similar to those obtained in the Atlantic humid forests of Costa Rica. Comparatively to other studies, soils under dTf at Santa Rosa store a considerable amount of SOC with potentially large CO2 emissions if this ecosystem is not preserved. (C) 2011 Elsevier B.V. All rights reserved.