Soil carbon changes upon secondary succession in Imperata grasslands (East Kalimantan, Indonesia)

Soil carbon changes upon secondary succession in Imperata grasslands are important both for their effect on potential production and for possible implications of forest degradation and regeneration on global climate change. We studied the effect of forest regeneration after fire in Imperata (speargrass) grasslands of East Kalimantan on soil properties, using 47 plots that last burned in 2004, 94 plots that last burned in 2003, 126 plots last burned before 2003, 43 plots of secondary forest, and 28 plots of primary forest. Although soil carbon stocks increase upon natural regeneration from grassland to secondary forest, highest carbon stocks are found in the later regeneration phases and lowest under primary forest. This is contrary to the situation in other forest systems. Low C stocks under primary forests may be due to extremely low fertility, combined with shallow soils and low root mass in the topsoil. Root density¿as observed in the field¿is much higher under the grass vegetation. The effects of regeneration on soil are strongest in the A-horizon, where soil carbon content increases with 14%, from 14.5 g kg¿ 1 in Imperata grassland to 16.5 g kg¿ 1 in secondary forest, while carbon stocks in the A-horizon increase from 16.51 ton C ha¿ 1 to 18.70 ton C ha¿ 1. This is accompanied by a decrease in pH and an increase in bulk density. The total soil carbon stocks in Kalimantan (fixed mass, approximate depth section 40 cm) are 36.19 ton ha¿ 1 in Imperata grassland, 38.98 ton ha¿ 1 in secondary forest and 33.19 ton ha¿ 1 in primary forest, which is considerably lower than in Sumatra. Above-ground C/below-ground C ratios are higher in Kalimantan primary forest but lower in Kalimantan secondary forest than in Sumatra. Soil carbon stocks in Imperata grassland could be lower than previously thought. This has important consequences for carbon sequestration projects in East Kalimantan, because carbon storage potentials could be highe