Organic carbon stock in forest soils in Japan

Abstract

An accurate estimate of the organic carbon stock in the soil in a country allows the global estimate to be refined. This study estimated the organic carbon stock in forest soil in Japan using available data. Forest soil was classified into 15 soil units, and the extent of each soil unit was determined using digital soil and land use data. A representative value of the organic carbon density for each unit was determined from 3391 soil profiles obtained in a nationwide soil survey. The mean organic carbon density of the forest soils differed in each soil unit; the density to a depth of 1 m ranged from 33.0 kg m −2 for Black soils (Andisols) to 8.2 kg m −2 for Immature soils (Regosols). The variation within soil units was also large, and the coefficient of variation was almost 40%. The area-weighted average organic carbon density was 9.0 kg m −2 in the first 30 cm of soil, and 18.8 kg m −2 in the upper 1 m. The estimated total organic carbon stock in the forest soils of Japan was 4570±500 Tg (1 Tg=10 12 g) in the first 1 m (±95% confidence limit), excluding carbon held in the surface organic horizons and buried horizons. The Brown forest soils group (mostly Cambisols) made the largest contribution, with over 3000 Tg C. Forty-eight percent of the total organic carbon stock (2180 Tg C) was contained in the top 30 cm. The contribution of the surface layers to the organic carbon was clear. The wide variation in organic carbon density by soil type and the high density of organic carbon on a national scale in Japan is related to the distribution of volcanic ejecta. The concentration of organic carbon in the surface layers implies that soil conservation is important both to maintain land productivity and to counter global warming. The analysis of variability in the organic carbon density with soil type demonstrated that estimation methods should be improved.