Effect of conversion of sugarcane plantation to forest and pasture on soil carbon in Hawaii

It is well known that land use change can affect soil C storage of terrestrial ecosystems either by altering the biotic processes involved in carbon cycling or by altering abiotic processes such as carbon adsorption on soil minerals. Relatively few studies, however, have examined the dynamics of soil C pools after conversion of farmland to forest or pasture. We selected three pairs of secondary forests and pastures that originated from the same abandoned sugarcane (interspecific hybrids of Saccharum spp.) land in the wet tropics of Hawaii to examine whether forest or pasture converted from farmland is more effective in sequestering C in soils. We compared the soil C pool, soil chemistry, and stable C isotope ratios between the forests and pastures. We found that total soil C was greater (P < 0.01) in forests than in the pastures 22 years after land conversion. The percentages of SOC4 in the pastures were significantly higher than in the secondary forests in both soil layers. The percentages of SOC3 in the pastures were lower than in the secondary forests in both soil layers. The net SOC3 increase in the forest soils at 0–10 and 10–25 cm was 28.6 ± 5.6 and 43.9 ± 3.2 Mg ha−1 while net SOC4 increase in pasture soils at these respective depths was 18.8 ± 2.2 and 26.1 ± 2.7 Mg ha−1. We found that the net increases of SOC3 in both soil layers in the forest were greater (P < 0.01) than the net increases of SOC4 in the respective soil layers in the pasture. Aluminum saturation was greater (P < 0.01) in the forests than the pastures in both soil layers. There was no difference in oxalate extractable Fe concentration between the forests and the pastures but oxalate extractable Al concentration in both soil layers was greater (P < 0.05) in forests than the pastures. Our findings indicated that reforestation of abandoned sugarcane farmland in Hawaii is more effective in soil C increase and stabilization than conversion to pasture.