| Preferential flow in slope land soils could cause lots of disastrous effect, such as nutrient loss in deep soil, crops reduction, groundwater pollution, river eutrophication and so on. The study of preferential flow in slope land soils has important significance for realizing the mechanism of nutrient loss in deep soil and the effect of preferential flow on agricultural industry, and alleviating the groundwater pollution and improving the quality of the environment.Brilliant blue dye tracing and soil column infiltration methods were applied in soils of three land use types (waste land, maze field, citrus land). Based on the image analytic, spatial point pattern analysis method, the quantity and distribution characteristics of preferential flow paths, the morphological characteristics and the influencing factors of preferential flow, characteristics of preferential flow and nitrate nitrogen transport were studied, respectively. A systematic study has been done on the characteristics of preferential flow and its effect on nitrate nitrogen transport. The main results were as follows:(1) Point pattern analysis was used to reveal the spatial distribution of preferential flow paths in slope land soils of the Three Gorges Reservoir area. The results showed that in the horizon dye profiles, the maximum quantity of preferential flow paths was in the waste land, and the maximum quantity was in the citrus land in the vertical dye profiles. The distribution of prefer ential flow paths with different diameter grade of the three kinds of planting patterns showed t hat the maximum quantity was the≤1mm preferential flow paths, and the minimum quantity was the>10mm preferential flow paths, and most of the5-10mm preferential flow paths distri buted in the0-20cm soil. In the horizon dye profiles, except for most of the distribution of th e>10mm preferential flow paths was random distribution, others were aggregate distribution or trended to uniform in the waste land, and the distribution of the preferential flow paths were aggregate distribution in the maze field and citrus land; In the vertical dye profiles, the distribu tion of the>10mm preferential flow paths was significant random distribution, others were wa s significant aggregate distribution. This suggests that the spatial distribution of preferential flow paths in the vertical dye profiles has consistency.(2) Dye coverage ratio was extracted by image analytic and the morphological characteristic of preferential flow was studied. In the horizon dye profiles, the dye coverage ratio showed significant decrease with soil depth, the dye coverage ratio in the waste land was larger than other treatments, which suggest that there are more preferential flow paths; In the vertical dye profiles, there was obvious preferential flow phenomenon in the citrus land, the water could even move to the top of the basement. But there was no significant preferential flow under the top10cm soil in the maze field. In the horizon dye profiles, most of the variation tendency of dye coverage ratio showed multi peak pattern, which suggest that there is significant differentiation of the morphological characteristics of preferential flow and the preferential flow paths show dense distribution. So the moisture conduction capacity of the soil profile. In the vertical dye profiles, the preferential flow mainly moved to the vertical direction, but there also existed curve and lateral movement. This maily related to root extension, soil texture and connectivity of the preferential flow paths.(3) Preferential flow occurrence in slope farmland soil was attributed to environmental factors such as soil physicochemical property and crop root system and so on.25environmental factors were selected for the spearman correlation analysis. The results showed that there was a significant negative correlation between the dye coverage ratio and soil density, volumetric water content and clay content (p<0.01), a significant positive correlation with total porosity, the3-5mm root length density and the quality of root holes (p<0.01), a significant positive correlation with the root weight density,<1and3-5mm root length density (p<0.05), but was uncorrelated with the silt content, total nitrogen, available nitrogen, total phosphorous, available phosphorous, total kalium, available kalium, aluminum content, iron content and>5mm root length density. This illustrates that the preferential flow in the soils of three land use types of the Three Gorges Reservoir Area is affected mostly by the soil basic physical properties and the crop root system with diameter less than5mm.(4) The three-dimensional undistributed and packed soil column and the two-dimensional packed soil column were used respectively to observe the characteristics of preferential flow and its effect on nitrate nitrogen transport. The results of the three-dimensional undistributed and packed soil column experiment showed that there existed preferential flow in all the treatments. In the same time, the cumulative outflow amount of the undistributed soil column was significant higher than the packed soil column, which illustrate that the water movement in the two columns is different from each other. The outflow velocity of the preferential flow had instability and the breakthrough curve had asymmetry and tailing phenomenon. The preferential flow resulted in mass and fast transport of nitrate nitrogen, which account for40.9%-48.8%of the total transport amount. The results of the two-dimensional packed soil column experiment showed that the outflow velocity of O-C (macropore open at the soil surface-closed at the bottom of the column) treatment was1.6times higher than CK (column without macropore) treatment. The water in the O-C treatment moved faster than CK treatment from the top of the macropore to its bottom. The water in the O-C treatment moved toward to horizontal direction in the early time, but moved toward to vertical direction in the CK treatment. The barycenter of the brilliant blue distribution map of the O-C treatment was deeper than the CK treatment, and the brilliant blue diffused to the soil on either side of the macropore through the bottom of the macropore. |
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