Abstract
Water holding capacity, soil physical properties and organic matter properties in abandoned terrace paddy fields were investigated in order to evaluate the effect of afforestation on water conservation function. We studied 12 abandoned terrace paddy fields where sugi (Cryptomeria japonica ) or hinoki {Chamaecyparis obtsusa) were planted. The stand ages ranged from 12 to 33 years. The water holding capacity and permeability of surface soil in abandoned terrace paddy fields increased with increasing stand age. They became almost equal to those in artificial forests of sugi or hinoki in mountainous area about 30 years after afforestation. The water holding capacity correlated with soil carbon content and can be predicted from the stand age and carbon content at the organic layer by multiple regression analysis. Increase of the stand age indicates increases of litterfall, carbon content in mineral soils and activity of soil fauna. As a result, soil aggregate develops well so that the water holding capacity increases. Comparing similarly aged
stands, the water holding capacity and carbon content in mineral soils were greater in lower carbon content at organic layer. This suggests that decomposition of litterfall is important for the water holding capacity. These results suggest that water conservation function in soil of abandoned terrace paddy fields would be elevated following afforestation.
