Soil organic matter composition under primary forest, pasture, and secondary forest succession, Region Huetar Norte, Costa Rica

Secondary forests are increasingly wide-spread on neotropical soils. In thi s study, we investigated if, and how, the establishment of a secondary fore st on abandoned pasture affect the quality of soil organic matter (SOM). We approached this by a combination of physical fractionation of soil, where particulate SOM (light fraction and sand-associated SOM) is separated from mineral-bound SOM (silt- and clay-associated SOM), and structural chemical analyses, including measurements of well-decomposable carbohydrates and the more refractory lignin. Particle-size separation revealed that agricultura l use of a soil being formerly under primary forest resulted in a depletion of the particulate SOM pool, whereas clay- and silt-bound SOM was less aff ected. Abandonment of the pasture and growth of a secondary forest raised t he C content in all separates to a pre-cultivation level within 18 years, a nd sand-associated C was even higher as compared to the primary forest. The lignin and carbohydrate signature showed that the land use rarely affected the chemical composition of SOM within the different fractions. This was c orroborated by solution C-13 NMR spectroscopy of the NaOH-soluble SOM. The results suggested that land use primarily influences the C balance across t he light fraction and the size separates, with the particulate SOM pool bei ng the most significant SOM component in the context of management impacts on these soils. While the gross chemical composition of SOM within the frac tions remained unaffected, some molecular differences indicated a shift in the microbial community and/or activity at transformation of primary forest into pasture and after abandonment of the pasture with growth of secondary forest. (C) 1999 Elsevier Science B.V. All rights reserved.