Factors limiting organic matter decomposition and the nutrient supply of soils on the Daito Islands

抄録

One hundred soil samples were collected from the Okinawa Islands, and soil properties relating to the nutrient supply capacity were evaluated. Among the 4 major soil types (Kunigami mahji, Shimajiri mahji, Jahgaru, and Daito mahji), Daito mahji showed significantly (p<0.05) higher labile organic carbon content than the others. However, Daito mahji showed the lowest value (15.3 ± 6.2 mg kg^<-1>) of available nitrogen. No significant relationships were observed in Daito mahji between the labile organic carbon and available nitrogen contents, in contrast to previous studies. The ratio of labile to total organic carbon of Daito mahji was significantly (p<0.05) higher than that of the other soil types, and a significant negative correlation was shown between the ratio and soil pH, suggesting that the lower pH in Daito mahji suppressed the decomposition of the labile organic matter and resulted in higher contents of the labile organic matter. An incubation study using Daito mahji showed a highly (p<0.001) significant positive correlation between the decomposition rate of added sunn hemp and soil pH, and liming significantly increased the decomposition rate (p<0.05). These results revealed that Daito mahji with higher contents of labile organic carbon could have potentially higher available nitrogen, although this would be impeded by soil acidity. Therefore, liming is a reasonable way to enhance soil productivity in sugarcane cultivation through the improvement of the ability of Daito mahji to decompose organic matter.