Soll organic matter and nitrogen contents of saga polder rice fields: Changes with depth and time

Abstract

Thirteen rice soil profiles developed at intervals from 0 to some 700 years on the soil derived from shallow sea sediments along the Ariake Bay were examined to discover what changes in organic C, total N, and related constituents occurred with time. On an organic C and total N basis, organic matter content of the surface soils, with the exception of the early years of rice cultivation, had a general tendency to increase as a linear function of logarithm of time. By contrast, the organic matter content in the subsoil B horizons decreased significantly. Thus, the organic matter profiles indicated the development of A and B horizons, causing distinct horizontal gradients with time. Using total N content, C/N ratio varied from 16.08 to 7.81 with the lowest values found in the B horizons as time elapsed. Inorganic N was present mainly as fixed NH4+-N accounting for 5 to 31% of the total N, whereas exchangeable NH4+-N was the dominant extractable inorganic N. Fixed NH4+-N occurred in overall equal abundance in...