Study On The Preferential Flow Characteristics And Influencing Factors Of Red Soil Under Different Vegetation Covers

The red soil developed from the quaternary red clay parent material has a large area(approximately amount to 22.7%of the total area of the country)in China,mainly concentrated in the south of the Yangtze River,and is one of the important soil resources for the development of agriculture,forestry and fruit industry in south China.Although the regional water and heat conditions are superior,the seasonal drought problem has been very serious.The seasonal drought problem is highly related to the soil moisture dynamics and the main driving factors.Rainfall and vegetation are important driving factors for the dynamic distribution of soil moisture.The preferential flow is a response feature of soil moisture to rainfall,the rainfall response pattern of soil moisture in the clayey red soil slope under the interaction of rainfall conditions and vegetation in the monsoon area has certain regional characteristics.The presence of preferential flow can promote water migration,increase the deep water storage capacity in the soil,and alleviate the seasonal drought;under different vegetations,the degrees of preferential flow are different,and the abilities to alleviate seasonal drought are also different due to the differences in soil properties and root characteristics.The research site is located on a typical red soil gentle slope in Heshengqiao Town,Xianning City,Hubei Province.Five vegetation plots of the China fir(11 y),the China fir(6 y),the Photinia glabra,the Soapberry,and the Osmanthus fragrans were selected as the research objects.High-frequency dynamic monitorings of meteorology and soil moisture during 2017-2019 were carried out to obtain the dynamic changes of soil moisture and the characteristics of soil preferential flow response to natural rainfall under different vegetations;at the same time,the field staining tracer tests were conducted and the physical and chemical properties of the soil and the root index were acquired to analyze the preferential flow morphological characteristics and the influencing factors under different vegetation covers on the gentle slope of red soil.This study was aimed to clarify the developments of clayey red soil preferential flow characteristics and their main influencing factors,as well as the impacts on soil water movement,distribution and storage.Conclusions are shown as the following:(1)All the preferential flow types under the five vegetation plots belonged to the macropore flow.Among them,the preferential flow types under the China fir(11 y),the China fir(6 y),the Photinia glabra and the Soapberry mainly belonged to the hole flow,while that under the Osmanthus fragrans belonged to the fissure flow.The morphological characteristics of the preferential flow in different vegetation plots and different soil depths were different,which indicated that the preferential flow had a certain spatial variability.Among the five vegetation plots,the total stained area of the China fir(11 y)plot was the largest(1556 cm~2),the boundary between the preferential flow zone and the matrix flow zone(at a depth of 20 cm)was the most obvious,and there were a large number of open connected pores in the surface soil layer.The total number of stained paths(1031)and the number of paths with stained path width(SPW)<10 mm(701)of the Osmanthus fragrans plot were the most,and the preferential flow paths had the best connectivity;which are followed by those in the China fir(6 y)plot.The preferential flow path connectivity of the Osmanthus fragrans and the China fir(6 y)plots were the best among the five vegetation plots,and the degree of preferential flow development was also the best.The path connectivity of the Photinia glabra was the worst,and the development of preferential flow was also poor.(2)Heavy rain events and their higher level of(≥25 mm)rainfall events were easy to cause the preferential flow phenomenon on the red soil slope,and the preferential flow phenomenon occurs at all soil depths in the red soil.Under heavy rain events,shallow-soil(40 cm and 60 cm)preferential flows were generally triggered,while storm rainfall will trigger deep-soil(90 cm)preferential flows.Under the condition of a certain amount of rainfall,the influence of rain intensity on the preferential flow was significant,and higher rain intensity will lead to higher frequency of preferential flow.Under the five vegetation plots,40 cm and 60 cm were the two soil depths which are most likely to generate the preferential flow.(3)Among the soil properties,organic matter had the most significant influence on preferential flow,which were concluded from the significant positive correlation between organic matter and preferential flow found in all five vegetation plots.The increase of organic matter content can promote the occurrence of preferential flow in soil.The influence of soil texture on the preferential flow was mainly reflected in the plots of the Soapberry and the Osmanthus fragrans.The Soapberry plot appeared obvious texture stratification,above 40 cm depth the silt content was high,the soil was harder,poor soil permeability,and inhibited the migration of the preferential flow,the preferential flow was mainly distributed at a depth of 0-40 cm;in the Osmanthus fragrans plot there was a very significant positive correlation between the preferential flow and clay content,the presence of clay content led to obvious swelling and shrinking of the soil,wet and dry cracks were prominent.The effect of soil bulk density on preferential flow was not particularly obvious.(4)The root system had a significant influence on the preferential flow,and there was a significant positive correlation between the preferential flow.Among them,plant roots of different diameters and root hole formed by root decay were the main paths of preferential flow.However,the phenomenon of desiccation crack in the Osmanthus fragrans plot were very obvious,and the impact on the preferential flow was much greater than that of the root system,there was no significant correlation between the preferential flow index and the root system in this plot.The preferential flow was obvious in the clayey red soil slope,and its development characteristics were the result of synergistic effects of rainfall conditions,soil organic matter,clay content and the plant roots of different diametral grades.The presence of preferential flow can not only change the spatial and temporal distribution of water in the sloping red soil,but also increase the deep water storage in the soil.These results in this study can enlighten people when they adopt vegetation measures to regulate the water content of red soil slopes and alleviate the seasonal drought.