Effects of Oil Pollution on Soil Properties and Phytoremediation of Polluted Soils
Supervisor:wang yi quan
With the rapid development of the oil industry,the development and use of oil and its products is increasing,but the problem of oil pollution to the soil is extremely common.It is a serious threat and has negative impacts on crop production and ecological security.The ecological environment carries a huge pollution load.A series of ecological and environmental problems caused by oil pollution have made it one of the hotspots in soil science and environmental science research.The harmful effects of oil pollutants on crops and ecosystems are beyond doubt and will not be repeated here.However,the mechanism and pathways of oil pollution to plant growth are still negligible.The mechanism and extent of oil pollutants on different soil properties are still not clear enough.The damage of oil pollutants to soil during the degradation process is still rarely studied.This study investigated the impact of oil pollution on soil water retention,infiltration and migration performance,and explored the degree of damage and mechanism of oil pollution on drought resistance of plants in different texture soils in a field experiment.This study breaks through previous thinking mode of static monitoring and evaluation,and dynamically analyzes the influence and mechanism of oil pollutants on soil physical and chemical properties during degradation process;it further explores the bioremediation measures of oil polluted soils which is suitable to the regional environmental conditions,reveals the hazard pathways and mechanisms of oil pollutant to different texture soils,and provides sufficient evidence for accurate evaluation and scientific restoration of oil polluted soils.The research showed the following results:(1)Oil pollution significantly reduced the water holding capacity and soil drought resistance in different texture soils.According to soil water characteristic curves,curve model parameters and soil water constants,it shows that with the increase of oil pollution level,the water loss rate and slope of V-G curve n continued to increase in different soil types;field water holding capacity and the effective water content were significantly reduced,which indicates that oil pollution significantly reduces soil water holding capacity and increases the risk of soil drought.Soil residual saturation increased with the increase of the oil pollution level,making it difficult to achieve full soil saturation,thus directly affecting soil water storage capacity.The effect of different oil pollution level on soil water holding capacity has a stage characteristic.For oil pollution with the level of below 0.5%,it mainly affected the surface molecular water holding capacity of soil particles.When the oil pollution level increased to more than 1%,it mainly affected the structural pore water holding capacity of the soil.Different texture soils have different mechanisms and thresholds.(2)The influence of oil pollutants on the operation mechanism of soil capillary water was revealed by measuring the height of soil capillary water rise.Although oil pollution caused the surface of soil particles to be hydrophobized,there is still a rise in soil capillary water,which indicates that the impact of oil pollution on soil capillary water rise is limited.The contact angle between liquid water and the pore wall of oil-contaminated soil is an acute angle with a certain affinity rather than an obtuse angle with non-affinity.The oil pollution seriously inhibited the speed and height of the soil capillary water rise,and reduced the soil moisture content and reserves in area of the soil capillary water rise(the vadose zone).The soil capillary water rise has different responses to oil pollution in different texture soils,and the damage of oil pollution to Aeolian sandy soil was much greater than that of Lou soil and Loess soil.The oil pollution had relatively less damage to the height and velocity of the soil capillary water rise in soils with finer particles,larger surface and more complex pore structure,but serious impact on soils with coarser soil particles,smaller surface and single pore structure.The impact of oil pollution on capillary water operation was also one of the factors that might aggravate soil drought.(3)This study clarified the effects of oil pollution on soil permeability and wettability in different soil texture types.The saturated hydraulic conductivity of non-polluted soils showed significant soil texture effects.The saturated hydraulic conductivity of soil has a significant decreasing trend with the increase of oil pollution level.This was probably because that oil pollution had different effects on the dominant pores in different texture soils,and there is a certain difference in the decreasing extent of saturated hydraulic conductivity of each treatment soil.Under normal circumstances,due to the instability of soil aggregates,the saturated hydraulic conductivity showed a decreasing trend in soil water saturation process,but the decline of saturated hydraulic conductivity of oil polluted soil was relatively small,which proves that the oil has a certain protective effect on the stability of the aggregates.With the increase of oil pollution level,the unsaturated hydraulic conductivity of Aeolian sandy soil and Loess soil gradually decreased,but the unsaturated hydraulic conductivity of Lou soil with heavy clay gradually decreased in high water content(low suction)and gradually increased in low water content(high suction).Oil pollution had a weaker effect on water repellency in Lou soil with a certain amount of micro-aggregates,and had significant effect on water repellency in Loess soil and Aeolian sandy soil,which dominated by texture pores.(4)From the dynamic scale,it proved that the influence of oil pollution on soil physical and chemical properties during the degradation process was non-static,which showed a certain dynamic change law.Through a 70-day indoor biodegradation culture experiment,we found significant influence of oil pollution on soil pH,organic matter and nitrogen content in northern Shaanxi province.Even in calcareous soils with strong acid-base buffer capacity,oil degradation could reduce soil pH by an average of 0.22.As the pollution level increased,soil pH decreased.The degradation rate of oil pollutants in the soil was extremely slow in the first 3 weeks,and then rapidly increased,which was completely different from the degradation process of other fresh organic matters in the soil.The oil pollution rapid declined soil nitrogen content in the first 3 weeks,and then stabilized nitrogen content,indicating that nitrogen was the basic precondition for the degradation of oil pollutants.The depletion of nitrogen in the early stage was to prepare for the degradation of organic carbon in the later stage.Oil pollution would reduce the ability of the soil to supply nitrogen.Bioremediation requires the addition of a certain amount of nitrogen nutrients to regulate soil C/N in order to achieve the desired repair effect.It also shows that the evaluation of oil-polluted soil should not only be monitored in terms of status,but also be analyzed and evaluated in the process.(5)This study revealed the response of six native plants in northern Shaanxi province to oil pollution and their remediation capacity.It proved that oil pollution reduced the drought resistance of the soil,leading to plant wilting,which significantly inhibited the growth and development of plants.Oil pollution was more harmful to shrubs with high water demand,but the damage to herbaceous plants was relatively small.Considering the degree of plant growth inhibition and the amount of petroleum hydrocarbon degradation,indigenous plants with certain restoration potential for different levels of soil oil pollution should be screened out.Caragana and Erect milkvetch had the potential to repair oil-polluted soil with less than 0.5% oil pollution level;tall fescue,ryegrass,wheatgrass and alfalfa had the potential to repair oil-polluted soil with below 1% pollution level;ryegrass,tall fescue and wheatgrass had the potential to repair oil-polluted soil with below 4% oil pollution level.The results in this research can provide technical support for the establishment of a system to phyto-remediate oil-polluted soil in northern Shaanxi province.