Soil resistance to concentrated flow and sediment yields following cropland abandonment on the Loess Plateau, China

The main objective of the present study was to investigate the effects of cropland abandonment on soil erodibility as measured by soil resistance to concentrated flow and sediment yield. Through the method of space-for-time substitution, undisturbed soil samples were collected at depths of 0-15 and 15-30 cm in the fields of different abandonment stages (including cropland for control, about 10 years for juvenile abandonment stage, 20 years for intermediate abandonment stage and 30 years for mature abandonment stage). Laboratory experiments simulating concentrated flow were conducted with a flume (200 cm long, 10 cm wide and 5 cm deep) to investigate soil resistance to concentrated flow and sediment yield. Soil physical properties (bulk density, porosity, aggregation, mean weight diameter, shear strength and disintegration rate) were measured. After each scouring test, roots were collected and root characteristics were measured. Compared with control, soil resistance to concentrated flow increased following cropland abandonment. For the upper (0 to 15 cm) soil layers, the increments of soil resistance to concentrated flow ranged from 5.0 times in the juvenile stage to 9.9 times in the intermediate stage, and it dropped to 2.5 times in the mature stage. For the lower (15 to 30 cm) soil layers, it increased from 1.3 times in the juvenile stage to 2.1 times in the intermediate stage, and it reached up to 4.1 times in the mature stage. In comparison with the total sediment yields of control, 66.0 and 49.3% reductions were averagely observed in the upper and lower soil layers of the abandoned soils, respectively. The increasing soil resistance to concentrated flow indicated the decreasing sediment available for scouring and this observation may largely be attributed to the ameliorated aggregation, shear strength, disintegration rate and the extended dense root network following cropland abandonment. Laboratory experiments simulating concentrated flow showed that, within 33 years of cropland abandonment, sediment yields greatly reduced and soil resistance to concentrated flow increased constantly and significantly. Soil aggregate, shear strength, soil disintegration rate and root surface area density were determined to be the key factors in the strengthening of soil resistance to concentrated flow on the abandoned cropland in the hilly Loess Plateau.