Chronosequential changes of selected pedogenic properties in paddy soils as compared with non-paddy soils

A series of paddy and non-paddy soil profiles that are developed in chronosequences from approximately 50 to 1000 years of exposure time on fairly uniform marine deposits under nearly identical landscape and climate conditions were sampled from Cixi, Zhejiang Province, China, where rice has been traditionally grown under water submergence. Master horizons were identified and sampled. Selected pedogenic properties such as soil organic carbon (SOC) content, clay content, fractions of iron, total manganese and pH were used to characterize the horizons. To compare the differences between paddy and non-paddy profiles, all paddy profiles are taken as one group and all non-paddy profiles as another. The measured properties were averaged over 0–100 cm soil depth. Results showed that paddy group contained significantly (p < 0.05) higher amount of clay and total Fe than non-paddy group, but significantly lower amount of total Mn. The increased clay content was mainly found in horizons bellow about 40 cm of paddy profiles. SOC compared as such is insignificantly different between paddy and non-paddy groups. However, SOC in surface horizons (roughly 0–20 cm) is significantly higher in paddy profiles than that in non-paddy profiles. The chronosequence of soil development is obvious in paddy profiles in term of distribution/redistribution patterns of total Fe and free Fe oxyhydrates in the profiles, but is not seen in non-paddy profiles. Longer history of rice cultivation tended to result in more profound horizon differentiations, but longer history of non-rice cultivation did not have this effect. Fifty years of rice cultivation seem to be long enough to differentiate paddy profiles from non-paddy ones in terms of SOC contents in surface horizons. However, 100 years of rice cultivation do not seem to be long enough to differentiate them in terms of clay contents and distribution/redistribution of total Fe and free Fe oxyhydrates. In conclusion, our data demonstrate that paddy management has profound impact on soil formations. SOC accumulation in surface horizon, the increase of clay content in horizons bellow about 40 cm depth, horizon differentiation characterized by distribution/redistribution patterns of total Fe and free Fe oxyhydrates, and net losses of total Mn from the profiles are the characteristic soil-forming processes of paddy soils.