Determination of Soil Loss Tolerance of an Entisol in Southwest China

Soil loss tolerance (T value) serves as an ultimate criterion for determining if erosion control measures are necessary to preserve long-term soil productivity; therefore, it must be determined scientifically and rationally. In this study, soil formation rates (SR) of the purple soils (Entisols in the U.S. Soil Taxonomy and Regosols in the FAO soil classification) in the hilly area of Sichuan, China, were determined in field plots (measured SR) for three treatments: (i) soil type U.S, J(3)s, and J(3)p Soils) and parent materials or bedrock, (ii) vegetation (wheat [ Triticum aestivum L.]-maize [Zea mays L.], loquat tree [Eriobotrya japonica (Thunb.) Lindl.], and perennial ryegrass [Lolium perenne L.), and (iii) soil depth (10, 20, 40, and 60 cm). The measured values were further used to test the applicability of the Barth equation (estimated SR) for the test soils. The measured SR varied among the treatments as a result of different characteristics of soil parent materials (PM) and different temperature and soil moisture at the interface between the soil and the parent materials. The trends (the order of increase or decrease) of measured and estimated soil formation rates were the same, however, for the soil type and vegetation treatments, but not for the soil depth treatments. The measured SR values were 800 Mg km(-2) yr(-1) for J(3)s purple soils and 1200 Mg km(-2) yr(-1) for J(2)S and J(3)p purple soils in the study region. The estimated SR values were closely related to runoff volumes but were substantially lower than the measured values. It was concluded that the Barth equation is not a reliable prediction model for estimating the soil formation rate for short-term and plot-scale observations. Therefore, T values should be determined by the measured SIR values in the experimental region.