The use of chronosequences in studies of paddy soil evolution: A review

Chronosequences and associated space-for-time substitutions are an important and fruitful means for investigating the rates and directions of soil and ecosystem evolution across multiple time-scales ranging from decades to millions of years. This paper reviews the use of chronosequences for studying biogeochemistry of paddy soil evolution to improve our understanding of the fundamental processes, the dynamic changes in soil properties and the associated environmental thresholds at different stages of paddy soil evolution under the intensive anthropogenic managements. Rice paddy cultivation results in accumulations of various nutrients (e.g. organic carbon, nitrogen, 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 dominated by rapid desalinization, loss of magnetic susceptibility, accumulation of topsoil organic matter and formation of a compacted plow pan due to extrinsic thresholds resulting from anthropogenic activities; the second phase lasts several centuries comprising Fe and clay enrichment in the illuvial horizon, and the loss of phosphorus and Mn coincident with the near complete removal of CaCO3 (recognized as the intrinsic threshold); in the third phase (N700 years), (trans-)formation and redistribution of metal oxides are accompanied by clearly visible hydromorphic patterns in paddy subsoils. We also note that after 2000-years, paddy soils still lack evidence of silicate weathering and neo-formation of pedogenic clay minerals. Paddy soil management is adjusted to match landscape positions (e.g. well-drained sloping uplands, alluvial plains with groundwater fluctuation, and poorly drained bog areas with near surface water table) and this influences the trajectory and magnitude of pedogenic changes with prolonged rice cultivation. However, the parent material effects on paddy soil evolution seem to diminish with the lapse of time and vary considerably among different soil properties or processes. Given our universal dependence on paddy soils for food production, their value as an excellent opportunity for investigating anthropedogenesis, and their critical roles in global biogeochemical cycling, we put forward several open questions that must be resolved to maintain the millennial-scale sustainability of these important wetlands.