Soil organic carbon in relation to cultivation and topsoil removal on sloping lands of Kolombangara, Solomon Islands

Soil organic carbon (SOC) concentration was evaluated under three land use practices on sloping land of Kolombangara in Solomon Islands (5–12°S, 155–170°W). Three soil management treatments were: (1) natural forest, (2) traditional farming practice, and (3) topsoil removal. The SOC concentration was high in the surface layer of soil under natural forest and decreased sharply with depth. The SOC concentration of the 0–15 cm depth was significantly greater under natural forest (67 g kg−1) compared to the traditional farming (24 g kg−1) and topsoil removal (7 g kg−1) treatments. The depletion of SOC concentration in traditional farming and topsoil removal treatments decreased the mean weight diameter (MWD) of water stable aggregates. In this strongly structured soil, the traditional farming treatment had no detrimental effect on water stable macroaggregates (WSA) for either of the two depths. WSAs in natural forest had significantly higher SOC concentration compared to the microaggregates. There was no difference in SOC concentration among different aggregate size fractions in the topsoil removal treatment. The traditional farming treatment based on slash-and-burn agriculture depleted the SOC pool in the top 30 cm layer by 52.2 Mg C ha−1 compared with natural forest. The loss of SOC pool in the topsoil removal treatment was 94.8 Mg C ha−1 compared with natural forest. Severe erosion, leading to a complete loss of topsoil as in the case of the topsoil removal treatment, has drastic adverse impacts on productivity and quality of the exposed sub-soil. Thus, there is a strong need to identify viable alternatives to slash-and-burn agriculture now widely practiced in the Solomon Islands. In this regard, a judicious use of chemical fertilizers and biosolids may be crucial to achieving sustainable production.