Soil organic matter dynamics in density and particle-size fractions following destruction of tropical rainforest and the subsequent establishment of Imperata grassland in Indonesian Borneo using stable carbon isotopes

Large portions of the deforested areas in Southeast Asia have been ultimately replaced by the invasive grass Imperata cylindrica, but the dynamics of soil organic matter (SOM) during such land transitions are poorly understood. This study presents SOM dynamics in density and particle-size fractions following rainforest destruction and the subsequent establishment and persistence of Imperata grassland. We examined soil C stock and natural C-13 abundance in these fractions to depths of 100 cm. We predicted future soil C storage and evaluated C turnover rates in these fractions using a simple exponential model. Because soil texture strongly affects soil C storage, two chronosequences of soils differing in soil texture were compared (n = 1 in each chronosequence). The clay-associated SOM increased in all soil layers (0-100 cm) along the forest-to-grassland chronosequence, whereas light-fraction SOM in the surface soil layer (0-5 cm) decreased. In the surface layer, all SOM fractions exhibited rapid replacement of forest-derived C to grassland-derived C, indicating fast turnover. Meanwhile, delta C-13 values of the light fraction in the surface layer indicated that forest-derived charcoal and/or occluded low-density organic matter constituted unexpectedly large proportions of the light fraction. Mathematical modelling (0-50 cm) showed that grassland-derived C in the clay and silt fractions in all soil layers increased almost linearly for at least 50 years after grassland establishment. In the meantime, the forest-derived C stock in the clay fraction constituted 82 % of the total stable C pool at 0-50-cm depths even under steady-state conditions (t = a), indicating that residue of forest-derived SOM associated with clay largely contributed to preserving the soil C pool. Comparing soils with different soil textures, clay and silt particles in coarse-textured soil exhibited a substantially higher degree of organo-mineral interactions per unit volume of clay or silt compared to fine-textured soils.