Development of bulk density, total C distribution and OC saturation during paddy soil evolution.

Considerable amounts of organic matter (OM) are stabilized in paddy soils, and thus a large proportion of the terrestrial carbon is conserved in wetland rice soils. Nonetheless the mechanism for stabilization of total organic carbon (TOC) in paddy soils as driven by redox cycling is largely unknown. The aim of the project is to identify the role of organo(mineral complexes for the stabilization of organic carbon in a chronosequence of paddy soil use and thus duration of management(induced paddy soil formation. Soil profiles were sampled, including paddy sites and non(irrigated control sites. First analyses include bulk density, the TOC and total inorganic carbon (TIC) concentrations of bulk soil and the concentration of organic carbon as well as the organic carbon stocks of physical soil fractions. First results indicate distinctly different depth distributions between paddy and non(paddy (control) sites. The paddy soils are characterized by relatively low bulk densities in the puddled layer (between 0.9 and 1.3 g/cm 3 ) and high values in the plough pan (1.4 to 1.6 g/cm 3 ) and the non(paddy soils by relatively homogeneous values throughout the profiles (1.3 to 1.4 g/cm 3 ). In contrast to the carbonate(rich non(paddy sites, we found a significant loss of carbonates during paddy soil formation, resulting in decalcification of the upper 20 cm in 100 y old paddy soils, and decalcification of the total soil profile in 700, 1000 and 2000 y old paddy soils. The calculation of the organic carbon stocks of each horizon indicate that paddy sites have always higher values in top(soils compared to non(paddy sites, and show increasing values with increasing soil age. The capacity of fine soil fractions to preserve OC was calculated by using the formula of Hassink (1997). With increasing duration of paddy soil use, the fine fractions indicate an increasing saturation level of the potential capacity to preserve OC.