| As a main barrier to the sustainable development of agricultural ecological environment in the Three Gorges Reservoir Area, soil erosion on slope farmland in Chongqing has been paid more attentions. In order to reduce soil erosion on slope farmland and to reasonably utilize of the land resources, the transformation of slope farmland and the ecological reconstruction have become the main trend of ecological agriculture development in the region. For the sake of reasonable using of terrace bank land resources formed after the transformation of slope farmland and of improving the ecological environment conditions around slope farmland, the application of bio-embankment measure on terrace land could effectively improve the resistance ability of seasonal drought on slope farmland, reduce soil erosion, maintain terrace bank stability.Taking the bio-embankment on slope farmland in Purple Hilly area and Rocky Desertification as the research object, this paper thoroughly studied the effect of bio-embankment measure on soil anti-erodibility and soil resisting shear and make a comparative analysis on the difference in mechanical properties of roots and soil for bio-embankments through for the of soil physical properties determination indoor or outdoor method combined with plant roots biomechanical testing; the paper also evaluated comparatively soil water storage effect for bio-embankments, and then de a further study on the effect of bio-embankment measures on soil anti-scouribility by use of soil reservoir and the method of undisturbed-soil trough scouring; the paper analyzed thoroughly the mechanism of soil reinforcement and conservation on slope farmland through the study of soil anti-erodibility, soil anti-scouribility and soil shear strength for bio-embankments with different types and management patterns, combined with the characteristics of bio-embankments root bi-mechanics, which could provide theoretical basis for the suitability evaluation on bio-embankment measures. The main conclusions were as follows:(1) Soil aggregates structure stability characteristics obviously differed from different types and management patterns of bio-embankments. Tree embankments showed more obvious effect on protecting riser and more advantage in improving soil aggregates stability of embankments and riser compared to grass embankments, nevertheless, farming activities had great impact on soil aggregates stability of bio-embankments; the content of soil water stable aggregates (>0.25mm) for bio-embankments with different types characterized by prickly ash (69.29%)> mulberry (59.97%)>alfalfa (47.92%)>natural grass (41.32%), however, that for bio-embankments with different management patterns were natural grassland> weeded land> fannland, and the content of soil water stable aggregates of mulberry increased as the tree neared slope farmland; the fractal dimension of both soil dry aggregates and wet screened aggregate for farmland with mulberry embankments were the minimum, which were2.358and2.910respectively; soil aggregate stability index for different bio-embankments characterized the order as prickly ash (3.61)>mulberry (2.83)>natural grass (2.05)>alfalfa (2.01)while that of different management patterns were farmland(1.42)<weeded land (1.50)<natural grassland (1.71); tree embankments presented good soil structure and soil stability because of its loose soil structure and fine permeability, meanwhile, farmland had relatively poor soil aggregates stability caused by effects of anthropogenic activities.(2) The distribution characteristics and stability of soil aggregates for bio-embankments under3kinds of treatment methods based on Le Bissonnais method were different. The proportion of large aggregates (5-1mm) under fast wetting (FW) treatment was the least while that of small aggregates (<0.25mm) was the highest, and the disintegration rate of soil aggregates for bio-embankments under fast wetting treatment were the biggest; soil aggregates stability of bio-embankments after3kinds of treatments presented the trend of FW<SW<WS, soil aggregate MWD arranged such order as FW (0.84)<SW (1.44)<WS (1.87) and soil aggregate GMD manifested by FW (0.35)<SW (0.85)<WS (1.23), meanwhile, the stability under fast wetting treatment was the weaker, which could draw the conclusion that the main collapse mechanism of soil aggregates of bio-embankments was the compressed air pressure in soil aggregates.(3) Root morphology and its spatial distribution characteristics were obvious deferent in tree embankments and grass bio-embankments. The root system of tree embankments developed into oblique root type, the root system of alfalfa was vertical root type. Within0-40cm soil layer, root distribution of bio-embankments decreased with the increasing of soil layer depth and the horizontal distribution of roots system presented obvious ladder shape, moreover, root biomass, root length and root surface area were significantly higher than that of grass embankments (P<0.01); within0-20cm vertical and0-60cm horizontal soil space, the root of bio-embankments were dominated by those fine roots with D<1mm and D=1-2mm; thick roots with D>2mm played a role in supporting and firming soil and demonstrated the main mechanical properties during the collapse and landslide of terrace bank, which maintained terrace bank stability.(4) Biomechanical characteristics of bio-embankments were closely related to root diameter, while soil shear strength characteristics of bio-embankments associated with root content. Stress-strain curves of roots characterized by unimodal curve without apparent necking and the curves of roots with smaller diameter presented more smooth, moreover, the maximum stress of roots for three bio-embankments showed the order as prickly ash (14.50MPa)>mulberry (12.42MPa)> alfalfa (9.13MPa); the average tensile strength of roots for bio-embankments manifested as prickly ash (73.91N)>alfalfa (68.07N)>mulberry (61.88N), meanwhile, the maximum tensile force of roots had positive correlation with diameter; root tensile strength decreased with the increasing of root diameter and was manifested by prickly ash> alfalfa>mulberry, whose values were16.52MPa,16.08MPa and13.02MPa, respectively; both root tensile strength and root tensile force had a power function relationship with root diameter, namely f=axb (both a and b are constant); bio-embankment measures enhanced soil shear strength and had obvious soil reinforcement and conservation effect, furthermore, soil shear strength had significantly positive correlation with root length density and root weight density and was manifested by alfalfa (0.382kg/cm2)> prickly ash (0.260kg/cm2)>mulberry (0.228kg/cm2).(5) Bio-embankment measure could effectively improve soil infiltration and soil water storage capacity, and significantly increased soil anti-scouribility. Soil stable infiltration rate of bio-embankment characterized by mulberry (1.26mm/min)> prickly ash(1.07mm/min)> alfalfa(0.56mm/min)> natural grass(0.52mm/min); Soil available water storage of all bio-embankments was obviously different (P<0.05) from natural grass, which presented as the bund planted with tree>grass>natural grass, nevertheless, soil available water storage of mulberry embankment in Rocky Desertification ranked as the order weeded land> natural grassland> farmland and its values were1235.71t/hm2,1187.38t/hm2and937.84t/hm2, respectively, which had closer relationship with soil porosity and soil infiltration of bio-embankments. Soil erosion process for bio-embankment could be divided into rapid erosion (0-3min), slow erosion (3-20min) and steady erosion (20-28min)3stages, soil anti-scourability decreased with the increasing of slope and the relationship between anti-scourability (ANS) and slope could be described by the equation y=61.235x-14826.(6) Terrace bank on slope farmland is subject to three forces, namely soil eco-hydrological effect, its gravity and root biological force, bio-embankment measure obviously intensified soil anti-scourability, strengthened Soil anti erodibility and soil structure stability, also increased soil shear strength by roots reinforcement mechanism and root anchoring mechanism, moreover, the effect of soil reinforcement and conservation was remarkable, which maintained the terrace bank stability. This paper also constructed12indicators for suitability evaluation of bio-embankment, which included3levels (target layer, criterion layer and indicator layer) by analytical hierarchy process (AHP), furthermore, obtained the weight of each indicator combined with expert scoring and consistent matrix. Among those indicators, the weight of root tensile strength for bio-embankment (C8) was the maximum (0.2646), followed by soil shear strength (C6) with0.2167weight, moreover, such indicators as soil bulk density (Cl) represented soil compaction status, landscape aesthetics degree (C12) represented the landscaping effect of bio-embankment, soil aggregate water stability index (C4) represented soil anti-erodibility, soil organic matter (C2) represented soil fertility of bio-embankment had lowed weight and its values were0.0322,0.0213,0.0212and0.0114, respectively, the indicator system could be used to evaluate the comprehensive effects of bio-embankment measures on slope farmland and its appropriateness. |
PDF Full Text Request