Abstract:

This research investigated effect of slope on the performance of highway embankment soil properties post construction， as there is a demand for technical data of this nature to underpin the design of highway revetment work in alpine meadow regions of China. A section of national road G248 in Northwest Sichuan Alpine Grassland， built 10 years ago， was identified as a study site and plots were marked out for sampling on five slope classes： gentle slope， 6°-15° （GS）； moderate slope， 16°-25° （MS）； steep slope， 26°-35° （SS）； very steep slope， 36°-45° （VS）； extreme slope， >45° （ES）. Undisturbed flat natural grassland >10 m distant from the roadbed was used as the control （CK）. Measurements conducted included determination of particle size composition， physical and chemical parameters of the soils， vegetation cover and the proportion of the plot area exhibiting degradation or desertification. The relative reduction rate of organic matter and total nitrogen was also determined， and differences in soil water content and fertilizer retention capacity were assessed. It was anticipated these data would provide a basis for the construction of ecologically sound slope protection structures in alpine regions. It was found that： 1） Increased slope significantly decreased the proportion of clay and silt particles and increased the proportion of coarse sand and small stones （P<0.05）. The greater the slope， the greater the soil bulk density and pH value， and the lower the soil water content. 2） Slope facilitated leaching of soil carbon fractions with the leaching intensity of the fractions ranking soil organic matter （SOM）>total carbon （TC）>inorganic carbon （IC）>dissolved organic carbon （DOC）. For example， SOM levels were， respectively， 115， 92， 72， 30， 9 and 3 mg·kg^-1 for CK， GS， MS， SS， VS， and ES slope classes while DOC levels ranged from 35.4 mg·kg^-1 in CK to 9.2 mg·kg^-1 in ES. 3） The ranking for the effects of slope on various soil fertility measures was： effective N>alkali-hydrolysable N>total N>available K>available P>total K>total P. Thus， available N was more easily lost than total N and alkali-hydrolysable N. Available N levels declined progressively across slope classes from 236 mg·kg^-1 in CK to 6.3 mg·kg^-1 in ES， while total N and alkali-hydrolysable N showed marked loss between GS and VS slope classes. A slope threshold for sharp increase in loss of total P and available P was observed between GS and MS slope classes. With increase in slope gradient， the total K increased， while the available K content decreased. 4） With respect to the influence of slope on the degree of desertification： for CK and GS >90% of the plot area was classed as non-desertified； for GS 79% of the plot area was moderately or severely desertified； for SS， VS and ES， 79， 86 and 94%， respectively， of the plot area was severely or extremely severely desertified. The proportion of severe desertification increased markedly at >15° slope （i.e. MS） and the proportion of extremely severe desertification increased markedly at >25° slope （i.e. SS）. 5） When the slope effects were expressed in relative terms， the relative increase in desertification was high and changed little at slopes >15°， while the relative decreases in organic matter and total nitrogen were high and changed little once slope exceeded 25°. As slope angle increased， relatively low rate of total coverage percentage and powder particles <0.02 mm and the relative increase rate of sand particles >0.05 mm increase significantly. In summary， as slope of the study plots increased， fine sand， silt and clay particles were progressively lost， soil fertility progressively declined， and the degree of desertification increased markedly. Therefore， the technical plan for ecologically sound slope protection in highway construction should be formulated according to the steepness of the slope.

Key words: highway slope protection, alpine grassland, slope gradient, soil desertification, desertification performance