Dr Audrey Leopold1, Eng. Julien Drouin1, Elia Drohnu1, Jacques Wamejonengo1
1Institut Agronomique Néo-Calédonien – Equipe SolVeg, Noumea, New Caledonia
The Loyalty Islands, part of the French Archipelago of New Caledonia, are old raised coral atolls on which soils develop from pumice and volcanic ashes. These soils have a very high content of organic matter (‘humic’) but they are very thin, i.e., less than 50 cm-depth in average. Because of the absence of clay minerals in these soils, the organic matter plays a very important role on the soil fertility and on the quality of the freshwater lens, which is the main source of freshwater on these islands. Historically, local people have practiced slash-and-burn agriculture; several years of cultivation were followed by a fallow for which the duration could be as long as thirty years. Because of the evolution of economic and social models, the length of the fallow tends to decrease. Moreover, the development of perennial cropping systems is funded to generate revenues from the agricultural sector. We assessed the impact of land use changes on soil carbon storage by evaluating the amount of soil organic carbon (SOC) stored in the first 30 cm of Gibbsic Ferralsols occupied by forest, of a fallow of traditional farming or of Avocado orchard. SOC stocks are high, ranging between 69.5 tC.ha-1 and 170.2 tC.ha-1. However, mean SOC stocks in Avocado orchard were significantly lower, i.e., 106.0±29.0 tC.ha-1 than those of fallow and forest, i.e., 127.5±23.7 tC.ha-1 and 121.9±23.0 tC.ha-1, respectively. Even if the perennial agricultural systems are not intensive on these islands, they impact more soil organic carbon content than traditional farming. Consequently, they could endanger soil health, food security, as well as the freshwater lens quality. Moreover, even if The Loyalty Islands are small territories, the degradation of their soils could release high quantities of CO2 in the atmosphere, whereas New Caledonia emits already as much CO2 per capita as USA.