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article i nfo Chronosequencesand associated space-for-time substitutionsare animportant and fruitful means for investigat- ing the rates and directions of soil and ecosystem evolution across multiple time-scales ranging from decades to millions of years. Thispaperreviews the useof chronosequencesfor studying biogeochemistry of paddy soil evo- lution to improve our understanding of the fundamental processes, the dynamic changes in soil properties and the associatedenvironmentalthresholdsatdifferentstagesofpaddysoilevolutionundertheintensiveanthropo- genic managements. Rice paddy cultivation results in accumulations of various nutrients (e.g. organic carbon, ni- trogen, and phosphorus) over a much longer time period than predicted by typical long-term (b50 years) field experiments, although it is not clear how long it takes paddy soils with different origins to reach a steady-state of these important nutrients. Extensive investigations of a 2000-year paddy soil chronosequence derived from calcareous marine sediments in the coastal region of Zhejiang Province (P.R. China) illustrate three phases of paddy soil evolution and the associated pedogenic thresholds: an initial phase during the first few decades dom- inated by rapid desalinization, loss of magnetic susceptibility, accumulation of topsoil organic matter and forma- tion of a compacted plow pan due to extrinsic thresholds resulting from anthropogenic activities; the second phaselastsseveralcenturiescomprisingFeandclayenrichmentintheilluvialhorizon,andthelossofphosphorus |
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