delta C-13 values of organic carbon in cropland and forest soils in Japan

The carbon content and natural C-13 abundance (delta C-13) of soil organic carbon were determined for Japanese cropland and nearby forest soils by con tinuous dow CN analysis with mass spectrometry. The delta C-13 values of or ganic carbon from cropland soils in the Hokkaido District ranged between -2 7 and -24 parts per thousand, suggesting that soil organic carbon originate d mainly from C-3 plants, while the delta C-13 values from those in Main Is land, Shikoku, and Kyushu Districts ranged between -25 and -17 parts per th ousand, indicating that up to 70% of soil organic carbon originated from C- 4 plants (presumably Miscanthus sinensis), The delta C-13 values of organic carbon from Imaichi and Tanegashima forest soils showed that carbon was la rgely of C-3 plant origin in the surface soils but in deeper horizons the v alues were strongly affected by the organic carbon of C-4 plant origin. Sug arcane cultivation for 36 y in Tanegashima cropland soils had resulted in t he increase of the soil delta C-13 values to around -15 parts per thousand. In Shizuoka Prefecture, the delta C-13 values of soil organic carbon of no n-volcanic ash Andosols under forests at Iwata and Makinohara were within t he ranges of C-3 plant origin. The values in volcanic ash Andosols at Fuji indicated that the soil carbon in the surface horizons mainly originated fr om C-3 plants while that in deep soil horizons largely from C-4 plants. Tea cultivation for more than 40 y had led to a decrease of the delta C-13 val ues at Fuji by 2-4 parts per thousand. In the Miyako Island sugarcane field s, the delta C-13 values of the plow layers were apparently affected by sug arcane, a C-4 plant species, compared to the subsoils where a large fractio n of carbon originated from C-3 forest plants. The shift from forest to sug arcane cultivation markedly decreased the carbon content and increased the delta C-13 values of soil organic carbon from -27--25 parts per thousand to -17--14 parts per thousand within 25 y.