| Soil water palys an important role for plant growth and survival.It is important for the research of soil and water conservation,ecological protection and forestry management.Vegetation types change effects soil properties caused by different,especially soil bulk density,total soil organic carbon,water-stable aggregates,soil water-saturated conductivity,and fine root biomass,which are closely related to the soil water,and these factors lead to the changes in soil water content and preferential flow properties.Evergreen broad-leaved forest,coniferous and broad-leaved mixed forest,bamboo forest and tea garden are the most common vegetation types in subtropical forests,which are of great significance for soil and water conservation,ecological environment protection and sustainable economic development.The aim of this study was to explore the characteristics of soil moisture content and the law of preferential flow caused by vegetation differences under natural succession conditions or with little human disturbance.In this study,the methods of field investigation,artificial simulation experiment,spot observation and indoor analysis were conducted.In order to describe the variation characteristics of soil water content time series and preferencial flow properties in different vegetation types,the terms of "soil water content time index","soil water content variance index”,“relative root infiltration area”,“fine root infiltration ratio”,“root infiltration effect index” were defined.We found that:The vegetation type effected soil properties and fine root biomass significantly.With the increase of soil depth,the soil bulk density of four vegetation types showed an increasing trend,while the total organic carbon showed a decreased trend.In the evergreen broad-leaved forests and coniferous and broad-leaved mixed forests,the soil capillary porosity decreased with the increase of soil depth,while the change was not significant in bamboo forests and tea gardens.The soil non-capillary porosity decreased firstly and increased then in the evergreen broad-leaved forest,and it increased in the coniferous and broad-leaved mixed forest and bamboo forest,while it decreased in the tea garden.The fine roots biomass showed a decreasing trend in evergreen broad-leaved forests,coniferous and broad-leaved mixed forests and bamboo forests,while it decreased firstly and then increased in tea gardens.With the increase of soil depth,the content of large aggregates in the evergreen broad-leaved forests increased firstly and then decreased,and the content of large aggregates in the soil of coniferous and broad-leaved mixed forest decreased firstly and then increased.There was no significant difference in the content of large aggregates in the soil of bamboo forests and tea gardens.The contents of meso aggregates in evergreen broad-leaved forest and bamboo soil decreased firstly and then increased,while the contents of mesa aggregates in coniferous and broad-leaved mixed forest and tea garden increased firstly and then decreased.With the increase of soil depth,the contents of small water stable aggregates in the soil of evergreen broad-leaved forests and tea gardens decreased firstly and the increased,while it in the coniferous and broad-leaved mixed forest increased gradually,and it of in the bamboo soil remained unchanged.The main influencing factors of the composition of water-stable aggregates were soil total organic carbon,non-capillary porosity,and fine root biomass.The composition of water-stable aggregates was less affected by other factors.With the increase of soil depth,the saturated hydraulic conductivity of the evergreen broad-leaved forest showed an increasing trend.The saturated hydraulic conductivity of the coniferous and broad-leaved mixed forest and bamboo forest was not significantly different in all the soil layers,and it of the tea gardens decreased firstly and then increased.The effects of non-capillary porosity,soil water-stable aggregates,medium-sized water-stable aggregates and small water-stable aggregates effected more on soil saturated hydraulic conductivity,and the effects proportion were 51.2 %,17%,10.6% and 8.3%,respectively.In the evergreen broad-leaved forest,the soil water content of 0-10 cm and 10-20 cm soil layers were close,but the water content of 20-30 cm soil layer was significantly lower;in coniferous and broad-leaved mixed forests and bamboo forests,soil water content of all soil layers were close;intea gardens,it was similar to that of evergreen broad-leaved forest,but the water content of different soil layers was more significant.In 2014,2015 and 2016,the maximum water content of soil was in May to August,while the minimum one appeared in November to next January.Rainfall played the key role in the change of soil water content.The rainfall time was the most important positive factor affecting the " soil water content time index ",the diameter coefficient between the " soil water content time index " and the rainfall time was 0.699;the total rainfall was the most important factor that effectted the "soil water content variance index,and the diameter coefficient between them was 0.549." soil water content time index " and " soil water content variance index” both increased with the increase of soil temperature.With the increase of rainfall intensity,the preferential flow from the soil profile,riot root,1mm and 2mm fine roots gradually increased,and the variance increased also.Among them,under the same rainfall intensity,the preferential flow varied between the soil layers.When the rainfall intensity was 240 mm h-1,soil profile preferential flow of 10 cm soil layer was significantly greater than it of 20 cm and 30 cm soil layers.Under the same rainfall intensity,soil profile preferential flow and dead root preferential flow in 30 cm soil layer could be 10 times less than it of 10 cm,while the difference of 1mm and 2mm fine root preferential flow of different soil layers reached to 70 times and 20 times,respectively.“Fine root infiltration ratio”varied with rainfall intensity and soil depth.Under the rainfall of 150 mm h-1,200 mm h-1 and 240 mm h-1,there was a significant difference in“fine root infiltration ratio” between 1 mm fine root and 2 mm fine root system in 0-10 cm soil layer.In the soil layers of 10-20 cm and 20-30 cm,the difference was not significant.When the rainfall intensity was 100 mm h-1,150 mm h-1,200 mm h-1 and 240 mm h-1,the difference of the “relative root infiltration area” of each root was mainly in the 0-10 cm soil layer.In the other soil layers,the difference of ?relative root infiltration area?was not significant.The difference of ?root infiltration effect index? of different types of fine roots was mainly showed in the 0-10 cm soil layer under the rainfall intensity of 150 mm h-1,200 mm h-1 and 240 mm h-1,respectively.The SEM analisis showed that all the factors explained the 73.2% of the?root infiltration effect index?,in which the rainfall intensity and ?fine root infiltration ratio? have significant direct positive effect on the?root infiltration effect index.In the process of artificial rainfall and natural rainfall,it was found that there was a phenomenon that the flow gradually decreased or even stopped during the process of soil preferential flow.In this study,it was defined as "soil embolism".The elimination of "soil embolism" was called "soil embolism elimination".Under the condition of natural rainfall,the total amount of soil flow generated along the rhizosphere of the fine roots showed a tendency of decreasing.The number of fine roots along which preferential flow produced showed a tendency of ladder descent totally.The amount of roots was reduced generally,while for each observation,there was a phenomenon deccribes as "the fine roots of the pre-generated preferential flow were no longer generated,or the preferential flow along fine roots was not observed." Under the condition of artificial simulated rainfall,there were different degrees of "soil embolism" and "soil embolism elimination" in the process of preferential flow through the soil profile,dead root,1mm and 2mm fine roots.Under the large rainfall intensity,the soil profile,root,2mm fine roots were more likely to produce "soil embolism elimination". |
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