Effect of terrace forms on water and tillage erosion on a hilly landscape in the Yangtze River Basin, China

In long-term agricultural practices, cultivated land on long slopes in hilly areas is sometimes terraced to diminish soil loss caused by water and facilitate farming operations. In the Upper Yangtze River Basin, the forms of the terraces vary with the landform, soil resources and traditional techniques used in different areas. This study assessed the pattern of soil redistribution and soil particle size redistribution for two major types of terraces: with and without embankments. Samples of 137Cs and soil texture were collected at the upper and lower parts of terraces, and at an interval of 5 m along the transect of the toposequence of individual terraces. The non-embankment terrace (NET) landscape showed increasing 137Cs downslope, whereas the embankment terrace (ET) landscape exhibited an abrupt change in 137Cs at the embankment. Tillage erosion dominated the soil redistribution, with smaller contributions made by water erosion in the ET landscape. In addition to tillage erosion, water erosion played an important role in the soil redistribution in the NET landscape, resulting in a net soil loss at both the upper and lower parts of the terrace. Soil fine particle fractions exhibited a trend of gradual increase along the transect of the toposequence in the NET landscape, and a similar fraction of fine particles was found at the upper and lower parts of the terrace. The establishment of embankments at the lower end of the terrace obstructed the formation and development of overland water flow, thereby creating a line of null downslope soil transport leading to tillage-induced soil accumulation on the upslope side of the embankment with little granulometric sorting. In the NET landscape, soil loss seemed to increase as a result of tillage erosion (an important delivery mechanism for water erosion), accelerating the on-site soil loss. The embankments at the lower end of the sloping terrace played a crucial role in the soil redistribution patterns of the toposequence, resulting in a shift from a water-dominated erosion process to a tillage-dominated process and a distinct pattern of soil particle size distribution.