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.</p

Otolith microchemistry of modern versus well-dated ancient naked carpGymnocypris przewalskii: Implication for water evolution of Lake Qinghai

There is ongoing debate over how the water level and composition of the water in Lake Qinghai changed in the past and might change in future. This study of the microchemistry of otoliths from ancient naked carp explores the chemistry of a relict lake isolated from Lake Qinghai during the Little Ice Age (LIA). A close correlation between the ages measured on fish bone and otoliths by AMS-14C, and by optically stimulated luminescence on overlying sediments, confirms a high water level in Lake Qinghai before 680–300 years ago. The contrasting compositions of the ancient otoliths relative to modern otoliths and waters indicate that the relict lake became enriched in 18O, Mg, Li, B and to a lesser extent Ba, but depleted in 13C, owing to strong evaporation, authigenic carbonates precipitation, (micro-)organism activity, and less fresh water input after it was isolated. If there were long-term fresh water input, however, a reverse trend might occur. The most important observation is that, because the waters have been supersaturated with respect to carbonates, authigenic carbonate precipitation would result in low but consistent Sr/Ca ratios in the lakes, as recorded by both the ancient and modern otoliths. The geochemical records of ancient versus modern biogenic carbonates provide insights into the long-term hydroclimatic evolution processes of an inland water body

Stratigraphy and otolith microchemistry of the naked carp Gymnocypris przewalskii (Kessler) and their indication for water level of Lake Qinghai during the Ming Dynasty of China

Otoliths are biogenic carbonate minerals in the inner ear of teleost fish, whose compositions can record the physical and chemical conditions of the ambient water environment inhabited by individual fish. In this research, the fishbones and otoliths of naked carp sampled near the Bird Island, offshore Lake Qinghai, were dated and analyzed for mineralogy and microchemical compositions. Comparing the microchemical compositions of ancient otoliths with those of modern otoliths, we conclude that the ancient naked carps inhabited a relict lake formed when the lake shrank from a high lake level, by combining with the AMS-C-14 ages of fishbones and otoliths, the stratigraphy and surrounding topography of the sample site. AMS-C-14 dating results of ancient fishbones and otoliths show that these naked carps lived from 680 to 300 years ago, i.e. during the Ming Dynasty of China. The X-ray diffraction (XRD) patterns demonstrate that the ancient lapillus is composed of pure aragonite, identical to modern one, indicating that the mineral of lapillus didn&#39;t change after a long time burial and that the ancient lapillus is suitable for comparative analysis thereafter. Microchemical results show that both ratios of Mg/Ca ((70.12 +/- 18.50)x10(-5)) and delta O-18 ((1.76 +/- 1.03)parts per thousand) of ancient lapilli are significantly higher than those of modern lapilli (average Mg/Ca=(3.11 +/- 0.41)x10(-5) and delta O-18=(-4.82 +/- 0.96)parts per thousand). This reflects that the relict water body in which the ancient naked carp lived during the Ming Dynasty was characterized by higher Mg/Ca and delta O-18 ratios than modem Lake Qinghai, resulting from strong evaporation after being isolated from the main lake, similar to today&#39;s Lake Gahai. Based upon the stratigraphy and altitude of naked carp remains, it can be inferred that the altitude of lake level of Lake Qinghai reached at least 3202 m with a lake area of 4480 km(2) during the Ming Dynasty, approximately similar to 5% larger than it is today.</p