| The precipitation inhomogeneity in red soil region of tropical and subtropical in China,which results in remarkable issue of soil & water loss and seasonal drought,is a main factor limiting agricultural production and development of this area.The soil moisture status of red soil slope affects the function of the agriculture and forestry ecosystem,and it is also the cause of seasonal drought.However,in recent years,the effect mechanics of the change of vegetation structure on the water dynamics principle of red soil are not clear.In this paper,the variation of soil moisture in dry and wet seasons of different kind of vegetation types was studied which is conducted to provide theory basis for ameliorating seasonal drought of red soil region.In this study,a typical red soil slope in Jianglitou country,Heshengqiao town,Xianning city of Hubei province(the slope area of about 13667 m2,the slope length is about 136 m,the average slope of about 4 degrees,the slope is oriented to the south,soil parent material is quaternary red clay)was selected and high frequency continuous positioning soil moisture monitoring test was carried out from April 12,2015 to October 3,2016.The experimental apparatus is set in the uphill,middlehill and downhill of this slope and the vegetation types are tree(Cunninghamia lanceolata(Lamb.)Hook.),dungarunga(Photiniaxfraseri),crop(Brassica campestris L.),4 EC-5 soil moisture probes are buried 20,40,60,90 cm below the ground to detect soil moisture content every 30 min in each vegetation plot,the monitoring of natural precipitation and soil moisture content are conducted simultaneously.3~5 soil cores were uniformly sampled along the longitudinal center line slope in each block on March 12,2016(before wet season)and the soil moisture content was determined by drying method.Plant roots digging and root scan experiment were conducted on April 21,2016 to obtain root length,root surface area and root volume.The results are as follows:(1)The different types of vegetation had different soil moisture macro charateristics.The waterlogging of wet season and drought of dry season in 2016 were more serious than that in 2015.The soil water storage capacity of tree was almost no difference between the two years,but the soil water storage capacity of dungarunga in the year of 2016 decreased significantly,and the soil water storage of the tree was significantly increased.The average soil moisture content of tree at 40 cm soil depth was the lowest during the two seasons,and the average soil moisture content of crop was the lowest in the wet season at the depth of 60 cm.The shallow soil moisture activity level of tree in the wet season(coefficient of variation)is larger and larger than the dry season,and the soil moisture activity level of dungarunga in the dry season was greater than that of the wet season.The soil moisture time stability in the depth of 60 cm(tree),20 cm(crop)in the dry season and the depth of 60 cm in the wet season,but possessed the high water activity level.There existed a small frequency but a concentration area with high water content in the soil depth of 60 cm,especially in the year of 2016.(2)Root characteristics of different types of vegetation were closely related to water uptake ability.The RLD(root length density)of tree and dungarunga at the soil depth of 40 cm was the largest,and the RLD of 60,90 cm decreased with the increase of soil depth and the RLD of the dungarunga at different soil depths was greater than that of the tree,especially the 40 cm depth of the soil.The RLD,RSD(root surface area density)and RVD(total root volume density)of crop in the 20 cm depth were the largest and decreased with the increase of soil depth.In the wet season,the relative root water uptake rate variation characteristic of the three types of vegetation along the soil profile was similar to that of fine root length distribution characteristic.The relative root water uptake rate change characteristic of tree and dungarunga during the drought period between 2015 and 2016 were almost the same,the relative water uptake of these two types of vegetation decreased in the early 2016 drought period,but in the later stage of drought,it tends to be stable and the change is consistent with the drought in 2015 which was mainly related to the duration of drought period.(3)The variation characteristics of drought intensity and drought degree of 20 cm soil depth in different type of vegetation were significantly different.In general,the soil drought intensity and degree of tree increased along with the increase of drought days.Although the soil drought degree of dungarunga and crop showed an increasing trend during the two years,the drought intensity of the two species had different performance in the two-year period drought,the drought intensity in 2016 was greater than that in 2015 and showed an overall downward trend.The soil drought of tree and dungarunga was greater than of crop in the initial stage of drought period,and the soil drought intensity of three vegetation manifested as crop > dungarunga > tree.The surface soil moisture content of crop and dungarunga decreased to the minimum at the early stage of the drought period and tree showed a lagging effect.(4)The response process of soil water and rainfall in different vegetation was different in different rainfall events.The soil moisture and rainfall response process of tree,dungarunga,crop were mainly affected by preferential flow(root channels or surface cracks)and subsurface flow(soil water supply from the uphill)in the heavy rain and hard rain events,the surface soil moisture fluctuation of crop during fallow period was not greater than that of tree and dungarunga.The effect of moderate rain and light rain on soil moisture of these three vegetation types was relatively reduced,however,the surface soil moisture response of crop during planting period to the moderate rain was most obvious and rapid,and the soil moisture fluctuation was much greater than that of tree and dungarunga.Therefore,the soil moisture and rainfall response process of arbor and small tree were similar in wet season,but the surface soil moisture fluctuation of crop was more obvious than that of tree and dungarunga during the growing period and the fallow period was just to the contrary.Tree had the least ability of water conservation regardless of soil drought degree in the dry season,but the ability of dungarunga and crop(fallow period)water conservation were highly affected by the degree of soil drought.To sum up,the temporal and spatial distribution of soil moisture in red soil slope is affected by vegetation and topography to a greater extent in wet season,the vegetation factor was still an importance influence in dry season but the influence of topography reduced. |
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