Iron loss of paddy soil in China and its environmental implications

Iron(Fe)is an important element for the terrestrial and marine ecosystems through its biogeochemical cycling on the Earth’s surface.China has a long rice cultivation history,with extensive rice distribution across many types of paddy soils.Paddy soils are the largest anthropogenic wetlands on earth with critical roles in ecosystem functions.The periodic artificial submergence and drainage during paddy soil evolution result in significant changes in soil moisture regime and redox conditions from the natural soils,which facilitate the increase of Fe solubility and mobilization.However,there is a lack of systematic assessment on the magnitude of the migration and loss amount of Fe from paddy soils.In order to quantify the Fe loss and assess the dynamic evolution of Fe in the soils after rice cultivation,seven paddy soil chronosequences derived from different landscapes(bog,plain,terrace)and parent materials(acidic,neutral,calcareous)with cultivation history from 0 to 2,000 yr were studied.Results showed that the rates and trajectories of Fe evolution showed distinct patterns among the studied seven paddy soil chronosequences.However,net losses of Fe from 1 m soil depth occurred at all studied paddy soil chronosequences regardless of the original landscapes and parent materials.Fe in the paddy soils derived from the calcareous lacustrine sediments in the bog area showed a slight accumulation during the initial stage(50 yr)of paddy cultivation,with a loss rate of0.026 kg m^(-2)yr^(-1) during the 50-to 500-yr time period.For the paddy soils developed on the calcareous marine sediments in the plain area,Fe evolution was dominated by the internal movement in soil profiles through coupled reducing-eluviation reactions in the surface horizons and oxidation-illuviation in the subsurface horizons within 1,000 yr of paddy cultivation,with an averaged net loss rate of 0.029 kg m^(-2)yr^(-1) during the 1,000-to 2,000-yr time period of rice cultivation.In contrast,Fe in the paddy soils derived from the acidic and neutral parent materials in the plain and terraced upland areas was rapidly lost during the initial stage of paddy cultivation,with a maximum loss rate of 1.106 kg m^(-2)yr^(-1),while the Fe loss rate decreased gradually with increasing paddy cultivation age.Soil pH,CaCO_(3),and organic matter contents of the original soils,the length of time of paddy cultivation,landscape types and positions,and changes in soil moisture regime and redox condition induced by artificial submergence and drainage were the main factors controlling the rates and trajectories of Fe loss during paddy soils evolution.The amount of Fe loss caused by rice cultivation at the national scale was estimated based on the data collected from this study and the literature.The Fe loss fluxes of paddy soils in China were about 46.4–195.7 Tg yr^(-1),and the amounts of Fe losses from paddy fields nationwide were about 5,121.5–9,412.2 Tg.Quantifying Fe loss from paddy fields is important to scientifically assess the impact of paddy cultivation on the Fe biogeochemical cycle.