Characteristics Of Soil Macropore On The Loess Plateau And Their Effects On Soil Water

With the advent of large-scale restoration of vegetation in the Loess Plateau,northwest China,there has been an increase in concern about the suitability of loess soil to support permanent vegetation cover.The substantial increase in vegetation cover had a marked effect on properties of the soil,including deepsoil profile water content.This has,however,hindered the sustainable development of vegetation in the plateau region.Because of this,efforts to increase the infiltration and storage of rainwater in the soil have become more urgent.Natural vegetation recovery could improve considerably soil pore characteristics.On the other hand,a substantial increase in vegetation could promote the development of soil-dependant animals by providing abundant food and suitable habitats.The physical activities of soil-dependant animals could dramatically increase the amount of soil macropores.Soil macropore are closely related to water storage and regulating capacity of the soil water reservoir.The quantification of soil macropore characteristics and studying the relationship between the macropore and soil water could be critical in revealing the regulating mechanism of soil macropore on the rainfall and could in turn be used to devise strategies for sustainable recovery of vegetation in this region.In this research,we characterized the macropores associated with different types of vegetation and evaluated the effects of soil macropores on saturated hydraulic conductivity(Ksat)by collecting undisturbed soil cores for four treatments: areas dominated by Quercus liaotungensis(QLI)and Pinus tabuliformis(PTA)in Fu county and by Caragana korshinskii(KOP)and Medicago sativa(MSA)in Shenmu county in Shaanxi Province on the Loess Plateau.Two methods of scanning soil cores with X-ray CT and filling subterranean nests with thin slurry of orthodontal plaster in the field were used to generate three-dimensional images of ants nest(Camponotus japonicus)and their effects on water infiltration and soil evaporation in the Liudaogou catchment.And we assessed the main factors that could affect the characteristic of ants nest.The main conclusions of this study are as follows:1.A medical computed-tomography(CT)scanner and the ImageJ software were used to obtain the macropore data(2D and 3D)in the cores collected from soil under QLI and PTA in Fu county,KOP and MSA in Shenmu county.The 2D characteristics of soil macropore included macroporosity,largest pore area,amounts of macropores,circularity and fractal dimension.The 3D characteristics of soil macropore included macropore surface area density,branch density,junction density and connectivity.Except for the cicularity,the values of all macropore parameters for the four treatments were in the order: QLI > PTA > KOP > MSA.Soil macropore characteristics improved with the increasing time.Soil under KOP40 treatment had better macropore network than that under KOP22.In addition,fractal dimension was significantly(P<0.001)correlated with macropore number(R2=0.928)and macroporosity(R2=0.838).2.The saturated hydraulic conductivity(Ksat)varied greatly among four treatments.The mean value of Ksat was highest in the QLI treatment(0.073 cm/min),followed by PTA(0.063 cm/min)and KOP(0.054 cm/min)treatments.Soil under MSA treatment had the lowest Ksat(0.018 cm/min).The Ksat in KOP40 treatment(0.054 cm/min)was higher than that in KOP22(0.048 cm/min).CT measured macropore parameters(macroporosity,amounts of macropores and fractal dimension)were positively(P<0.001)correalated with Ksat.The fractal dimension accounted for 73.7% of the variation in Ksat.As a single variable,bulk density was negatively correlated with Ksat and accounted for the best variation(80.1%).Bulk density and macroporosity were the best combination of parameters for determining Ksat and accounted for 86.2% of its variation.Macropore 3D parameters correlated positively with Ksat,however,the coefficients of determinations between 3D parameters and Ksat were lower than that of 2D parameters.Moreover,the soil organic matter and soil texture were not significantly(P>0.05)correlated with Ksat.3.The ant nest in the field was composed with vertical tunnel connecting a series of flattened horizontal chambers.Ant nest could reach 60 cm depth in the field,and the diameter of nest tunnels ranged from 4.1 to 6.6 mm.The nest architecture in the field was highly developed in both horizontal and vertical directions.However,the PVC cylinders restricted the development of ant nest in the horizontal direction.The area of ant nest in the soil cores at each depth ranged from 17.8-2117.4mm2,which made a mean value of 362.7 mm2.There are many colonies of Camponotus japonicus living in the loam and loessal soil,however,they prefer not to live in the dry sand in the Liudaogou catchment.Excessively wet or dry conditions would lead to the collapse of ant nest.Sandy soil with proper soil moisture,which is relatively stable,could support a big nest.On the other hand,soil moisture is an important factor that affect the burrowing activities and fertility of the queen ants.With proper soil water content(6%-20%),the queen ant can burrow soil and spawn in the nest.And the number of spawns increased with the increasing of soil water content.When the water content higher than 25% or lower than 0.2%,the queen ant can not burrow soil and will die in three weeks.Soil bulk density can also greatly affect the structure of ant nest.The length,branch number,node and total volume of ant nest would decrease with the increase of soil bulk density.However,the diameter of nest tunnels didn’t varied greatly under different soil water contents or soil texture.The tunnel diameter mainly deponded on the body size of ants.4.The difference of soil water contents between inside and outside the nest varied greatly under different rainfall.With little rain,the soil water content stayed at a low level,there was no difference of soil water contents between inside and outside the nest.When the rainfall was 24.8mm,soil in the nest tended to have higher water contents in the 0-30 cm soil depth than that outside the nest,however the difference was not significant(P>0.05).When the rainfall reached 47.9mm,the difference of soil water contents between inside and outside the nest turned to be more obvious,and the difference were significant(P<0.05)in some soil depth.The initial infiltration rate with a nest present was significantly(P < 0.01)higher than that in the soil without a nest.In contrast,the infiltration rates with nests present in the area were more than 20 times higher than those in soil without ants.The dyeing depth in the soils with nests were significantly(P < 0.01)greater than those without nests.Moreover,the dyeing depth in the areas with nests in the loam soil was deeper compared with that in the sand.The effects of a nest on soil moisture existed between 0 and 120 cm depths,in which the soil moisture in the soil without ants nest had a higher variation than the treatment with ants nest.The reduction of water storage in the soil without ants nest was always higher than that in the soil with ants nest during the experiment period.From April to September,the water storage reduced 72.3 mm in the soil without ants nest,however,the value was only 57.1 mm in the soil with ants nest.5.With burrowin the nest,worken ants could make soil aggregates with a mean diameter of 1.6 mm and transport them onto the surface.Soil aggregates on the surface formed a new soil layer which have high porosity.The aggregate mulch also exhibited a positive effect on reducing evaporation.The number of aggregates on the soil surface significantly(P < 0.001;R2=0.8927)increased with the increasing scale of the ant colony.With identical levels of soil water content,the surface evaporation reduction capacities were positively correlated with the thickness of aggregate mulches from 0 to 9 mm.In the daytime,high water vapour and pores in the aggregate mulches significantly(P<0.05)reduced the soil evaporation.When the solar radiation was at a low level at night,no significant difference was observed in the evaporation rate between soil with and without mulches.Moreover,the effects of aggregate mulch on soil evaporation disappeared when the aggregates disintegrated.Soil macropore characteristics under different plants and ants nest were researched in this study.Our work broadened the scope of studies on soil macropores in the Loess Plateau.The main factors that affects Ksat were analysed and we quantified the percentages of different factors on Ksat.The characteristics of preferential flow in the ants(Camponotus japonicus)nest and the effects of ant nest on the water distribution in the soil profile were assessed.The results could deepen our understanding of the relation between soil macropore and soil moisture.The findings of this research might provide developing strategies for improving the infiltration of rainfall and the restoration of vegetation in the Loess Plateau.