Magnetic depletion and enhancement in the evolution of paddy and non‐paddy soil chronosequences

Abstract

SummaryThe objective of this study was to understand the rates and controlling factors of magnetic depletion and enhancement during anthropogenic soil evolution. To this end, the study compared the dynamic changes in magnetic properties as well as iron oxide species of paddy and non‐paddy soil chronosequences with the same parent materials. A two‐way analysis of variance (anova) showed that paddy management resulted in significant (P < 0.01) decreases in magnetic susceptibility (χ) and other magnetic properties. Paddy management‐induced χ losses increased gradually from 24 to 55% as the cultivation history increased from 50 to 700 years. The rates of χ decrease were most rapid within the first 50 years of paddy cultivation, after which the rate slowed. The rapid decline in χ is probably caused by accelerated depletion of fine‐grained maghemite and ultrafine magnetite by iron‐reducing bacteria during soil waterlogging and consequent reducing conditions. By contrast, a significant decrease in hard isothermal remanent magnetization (HIRM) occurred only after 700 years of paddy cultivation, which matches the time taken to leach CaCO3 from the profile. In contrast, although magnetic enhancement was observed in the non‐paddy surface horizon, there was no increasing trend at the millennium time‐scale, probably because of the large CaCO3 content of the soil. We show that magnetic properties of paddy and non‐paddy soil derived from calcareous sediments are mainly controlled by the changing soil moisture regime and soil carbonate status along different paths of soil development. Our study suggests that differences in soil moisture regime caused by land use are substantially more important than the period of cultivation.