Characteristics of preferential flow dyeing morphology in the field-ridge transition zone of different land use types in low mountainous and hilly areas

Abstract: A complex soil-water transformation can be found under undulating topographical conditions in low mountain and hilly areas. It is also easy to cause the distribution of soil macropores and heavy preferential flow under the dry and wet alternative environment. This study aims to explore the characteristics of soil preferential flow patterns in the Jiangning District, Nanjing City, Jiangsu Province, China. A typical low mountain and hilly region was then selected as the study area. Three land use patterns were selected as the study objects, including the typical paddy field, dry land, and forest land. The outdoor dry tracer technology combined with indoor image processing was used to quantitatively analyze the differences in soil dye area ratio, dye path, and preferential flow type at different sites (inner field, transition zone, and ridge). The results showed that the stained area ratio (SAR) of land use types decreased with the increase of soil depth. Specifically, the average SAR of paddy fields was higher than that of dry land and forest land, particularly in the depth of 20-40 cm. There was also a significantly different average SAR of the three field sites. The overall performance was higher in the field than that in the transition zone and ridge. The number of staining paths (SPN) of different sites in various land use types first increased and then decreased with the increase of the soil depth. The average SPN of the whole profile was the highest in the forest land and its average SPN of each point was at most in the inner field, while the value in the paddy field and dry land was the opposite. The staining path width (SPW) of the 0-10 cm soil layer in the inner field and transition zone under different land use modes was in the range of 4-40 mm and more than 40 mm, whereas, the SPW of less than 10 cm was in the range of 4-40 mm. The water flow types are homogeneous flow or heterogeneous finger flow in 0~15 cm soil layer of the inner field and transition zone, and the highly interacting macropore flow is below 15 cm soil layer. The water flow types of ridges were highly interacting with macropore flow. The correlation analysis showed that the SPN and the proportion of SPW< 4 mm showed a positive correlation under different land use patterns, where the largest was found in the paddy field, followed by the dry land and forest land. Different sites showed a negative correlation with the SAR and SPW, indicating the maximum of SAR and SPW in the field, followed by the transition area, and the minimum of the ridge. Nevertheless, the proportion of SPW<4 mm was irrelated to the depth of the soil layer. The rest indicators decreased with the increase in the depth of the soil layer. The path of water leakage showed that the vertical and lateral leakage of paddy field ridges developed significantly. The ridge of dry land only developed the vertical seepage. There was a significant development of vertical leakage in the forest land ridge, instead of the lateral leakage. Some recommendations were proposed to improve the utilization efficiency of water and fertilizer for the reduced loss of water and soil. Specifically, the crack development can be controlled the soil moisture and reduced under the alternation of dry and wet conditions in the paddy field. The plough bottom was broken in the dry land. The weeds can be regularly removed inside and outside the ridge of the dry land and forest land. Particularly, animal holes can also be reduced in the typical low mountain and hilly areas in south China.