Little is known about the effect of tillage erosion on the distribution of CaCO3, phosphorus and changes in the ratio of CaCO3/available phosphorus (AP) in the hillslope landscape. The aims of the present study were to elucidate the mechanisms underlying changes in CaCO3 and AP concentrations induced by tillage erosion along slope transects and to reconstruct the historical changes in CaCO3 in soil layers at different landscape positions. Two adjacent slopes were selected from the Sichuan Basin, China, one with downslope tillage (Slope 1) and the other with upslope tillage (Slope 2) for 29 years. Then, consecutive downslope tillage by hoeing was applied five and 20 times on Slope 1. Under normal tillage (both downslope and upslope) conditions, CaCO3 concentrations increased exponentially with soil depth. However, the mixing effect of consecutive tillage (five and 20 tills) changed the vertical CaCO3 distribution patterns. For downslope tillage, the topsoil layer CaCO3 concentration was significantly lower at the toeslope than at other slope positions, but there were no significant differences between toeslope and other slope positions for upslope tillage. Consecutive tillage with five and 20 tills increased CaCO3 concentrations in the topsoil layer by 27.7% and 30.8% respectively compared with downslope tillage, but AP concentrations decreased by 26.1% and 29.0% respectively. Under normal tillage, AP concentrations decreased with increasing CaCO3 concentrations due to the adsorption and precipitation of AP by CaCO3, but this relationship disappeared after consecutive tillage. After consecutive tillage with five and 20 tills, the mean CaCO3/AP ratios of the topsoil layer were 93.5% and 88.4% greater than those for downslope tillage respectively, whereas there were no significant differences between downslope and upslope tillage. In conclusion, tillage is a process of CaCO3 replenishment and AP dilution in the surface layer of soil derived from carbonate-rich bedrocks.
Distribution and Geochemical Significance of Rearranged Hopanes in Jurassic Source Rocks and Related Oils in the Center of the Sichuan Basin, China
