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.