Losses of Soil and Nutrients from a Purplish Soil on Slopping Lands as Affected by Rain Intensity and Farming Practices

Abstract

Increasingly aggravated nonpoint source pollution resulting from intensified farming activities has drawn wide concerns from the society (Quan et al., 2002; Zhu et al., 2005). Among the farming activities, overdose of chemical fertilizers is blamed the most. Sichuan Basin, home of the so-called purplish soil, is one of major grain production basis in China where farming on sloping lands is prevailing and intensive. The purplish soil, a unique soil type developed from an array of easily-weathering purplish parent materials, widely spreads in the basin and dominantly forms the sloping lands. The soil is usually characterized with low contents of organic matter and plant nutrients with poor structure, course texture, high saturated infiltration rate, low conserving ability of soil moisture and nutrients and thus, is very erosive, resulting in shallow top soil layers of less than 20-30 cm lying directly on the underneath parent materials. During rainstorms, the soil is easily saturated with rain water and then quickly forms runoffs discharging into ground waters with rich nutrients, nitrogen (N) and phosphorus (P) in particularly (Zhang, 1992). This has put great pressure on the widespread water eutrophication. Previous studies documented in literature have well addressed the patterns of soil erosion and surface runoff with which how and how much soil nutrients are lost from the sloping lands. Most of the research was focused on N loss in surface runoff from farmlands (Hamsen and Djurhuus, 1996; Cookson et al., 2000; Havis and Alberts, 1993; Torstensson and Aronsson, 2000; Bergstrom and Kirchmann, 1999), in subsurface runoff from uplands induced by rains and/or irrigation in north China (Wang et al., 2005; Wang et al., 2006), from paddy fields (Yu et al., 1999; Wang et al., 1996; Wang et al., 1999) and from uplands of the red soil regions (Sun et al., 2003; Ji et al., 2006) in southern China. The factors considered in the related literature included climate (rainfalls, rain intensity, and so on), soil property, land use, etc., under artificial rains impacting on bare soils to study the patterns of nutrient losses (Fu et al., 2003; Kang et al., 2003; Ma et al., 2002). Nevertheless, the workers seldom considered the total nutrient losses from both surface runoff and subsurface runoff in one study but more often in a separate way. There is little, if any, information available on cultivation practices, fertilizer techniques such as fertilizer source, rate, timing and placement affecting nutrient losses from farmlands.