The organic carbon content in topsoil is high and declined with depth, except for Anak Krakatau and profiles at a higher elevation (Figure 6). The highest is in Rakata, followed by Sebesi, Panjang, and Anak Krakatau. Organic C contents in soils of Rakata and Sebesi (mean 1.97 and 2.34%) were significantly different from those found in Anak Krakatau (1.05%). Higher values of organic carbon content had been reported by A Shinagawa et al. (1992) in topsoils; 9.05% in Rakata, 1.34% in Panjang, and 0.6% in Anak Krakatau. While Hardjowigeno (1992) reported a range of 3.42 to 8.81% OC in Rakata soils. Our results are in line with those findings and suggest that the accumulation of organic matter did not change the SOC content significantly over the past 20 years.
Higher contents of OC in Rakata and Sebesi soils were related to the more extended period of accumulation of organic matter on these two islands compared to Panjang and Anak Krakatau. Repeated volcanic activities of the Anak Krakatau damaged the vegetation cover both in Panjang and Anak Krakatau. Vegetation succession and plant colonization in Panjang and Anak Krakatau were destroyed after the eruption of Anak Krakatau in 1932-35, 1952-53, 1972-1973, and 1980-81 (R. Whittaker et al., 1992).
The initial carbon storage on the Krakatau islands started three years after the eruption in 1886 with twenty-six species of ferns and algae, grasses after eleven years, and then the first trees, Casuarina equisetifolia,  started to grow afterward (Ernst, 1908). Plants developed more advanced in Rakata and Sebesi and accumulated more organic matter in soils as they were less affected by the past eruptions of Anak Krakatau. A study by (D Fiantis, Nelson, Shamshuddin, Goh, & Van Ranst, 2010) (Dian Fiantis, Nelson, Shamshuddin, Goh, & Ranst, 2016) on tephra of Mt. Talang in West Sumatra reported that blue-green algae started to colonize the reconstructed ash layer after four months followed by lichens within 16 months and vascular plants after 24 months. This type of pioneering plants was responsible for building an initial soil organic matter and facilitated the development of higher-order plants.