| Jujube(Ziziphus jujuba) is a perennial fruit tree in arid and semi-arid areas in central and Eastern Asia. Since the initiation of the large-scale ecological rehabilitation project(known as “Grain for Green” project) by the Chinese central government in 1999, the area of jujube orchards has been increased rapidly on the Loess Plateau mainly for conserving soil and water and improving the economic income. Over the past decade, the area of jujube plantations on the Loess Plateau has been reached 122,376 ha. With the growing of jujube trees, there should be a corresponding increase in water consumption. This increase, however, would result in the decrease of soil water in the rain-fed jujube plantation. Here we investigated the distribution of root-zone soil water variation and fine roots with a range of stand ages(2, 6, 10, and 15 years) in a rain-fed jujube plantation on the Loess Plateau, also we used stable hydrogen and oxygen isotope to distinguish the water using strategies, and grasped the regulation of jujube transpiration and evapotranspiration during the jujube growth period. On this basis, we used HYDRUS-1D mode to simulate the water dynamics and sensitivity analysis in the jujube orchard, the applicability of empirical root distribution functions and the effect of empirical root distribution functions on soil moisture dynamic change in the jujube orchard were also verified and analyzed by comparing of measured and simulated values of soil moisture using the HYDRUS-1D model. Research results are as follow:(1)There were different monthly dynamic of soil water content in study sites during the growth period between 2014 and 2015. In 2014, soil water contents were the lowest in June(from May to October), which were increased greatly in August and reached the maximum value in October, whereas there were declining tendency of soil water contents from May to September in 2015. Soil water contents of 0-0.6 m layers in all study sites were greatly affected by precipitation and evapotranspiration, which increased clearly following apparent rainfall events, and mean soil water content in 2-year-old jujube tree was consistently greater than that in other stand ages in the 0.6-1.8 m soil layers, and there were significant differences in soil water contents among different stand ages(p<0.05), this mean that the soil water content had been utilized by roots in the 0.6 m-1.8 m soil layers, and the water depleted increased gradually with stand age. Soil water contents in the deep soil layers(1.8 m-3.0 m) were weakly affected by precipitation, which showed almost constant values at all study sites, and soil water contents in the deep soil layers were approximated permanent wilting point(7%) in the mature jujube plantation(10-, and 15-year old jujube trees). It can be observed that in June 2014, before the typical rainfall, the mean soil water contents in the 2-, 6-, 10-, and 15-year old jujube trees were 15.76%、13.38%、12.93%、12.36%, respectively, which were all in low humidity(less than 60% of field capacity). However, soil water contents increased dramatically in the 0-0.8 m soil layers in 2-, and 6-year-old jujube trees following 90.4 mm precipitation(June 8, 2014-June 11, 2014), while the soil water contents were significantly increased only in the 0-0.6 m soil layers in 10-, and 15-year-old jujube trees under the same conditions. Jujube mainly absorbed soil water from the middle and deep soil layers(60~200 cm) in the dry season, while it absorbed soil water from dense root layers(20~60 cm) in the rainy season, however, few soil moisture were absorbed from surface soil layers(0 ~ 20 cm) during the whole growth period.(2)The fine root length density increased with increasing the stand age, and the surface soil had a high root density with the highest fine root length density values in the 0-0.6 m surface soil layer in all sites. Stand age had a significant effect on specific root length(p<0.05), and the specific length of fine roots in young stands was higher than in other age classes. The fine roots of the 2-year-old jujube plantation were distributed mainly in a range of 1.5 m, horizontally, within a thickness of 1.6 m, vertically; while those the 10- and 15-year-old jujube plantations have exceeded 1.5 m in the radial direction and 3 m in the vertical direction, and a high-density region of fine roots were observed in the middle of the inter-row in 10-, and 15-year-old jujube trees. The fine root length density decreased with increasing soil depth, and there is no difference of fine root length density in the radical distribution in the mature jujube plantations(10- and 15-year-old). There was obvious linear relationship between root length density(except 2-year-old) and soil moisture in the jujube plantation, and the same principle was applies to specific root length and soil moisture.(3)The water consumption of the rain-fed jujube plantation increased with increasing stand age in normal year(2014), and it was higher than that in drought year(2015), in addition, the water consumption of the rain-fed jujube plantation decreased with increasing stand age after 6-year-old. The amount of rainfall in growth period in 2014 was sufficient for jujube growth, whether it was not sufficient for normal growth in 2015 with the growth period delay about 20 days. The variation of jujube sap flow during the growth period was unimodal distribution, and it was low at the beginning and the end of the growth period, respectively. While it was high during key water requirement period, and the peak value of jujube sap flow in day time were observed delayed with jujube growth. The jujube sap flow in cloudy day were lower than that in sunny say with irregular variation.The variation of transpiration can be divided into the slow increase of sprout leaves period, quick increase of blossom and fruit period, high water consumption in blossom and fruit period and low water consumption in fruit ripe period. The transpiration of jujube plantation in 2014 was higher than that in 2015(p<0.05), this means the transpiration of jujube orchard could increase with the rise of soil moisture, while it was decreased under water stress condition. The irregular variation of proportional change in transpiration and evaporation in total water consumption were presented during the growth period.(4)Using HYDRUS-1D model, we calculated sensitivity analysis of parameters which influenced soil moisture. The most sensitive parameters which affected soil moisture in 30~50 cm soil layers were soil hydrologic parameters, meteorological parameters and crop parameters. Soil saturated water content, the aperture index, soil saturated hydraulic conductivity and precipitation were the most sensitive parameters which effected the variation of soil moisture in the 0~10 cm soil layers, and soil moisture of the deeper soil layers(90cm) were more sensitivity in temperature, total light radiation flux, leaf area index, root depth and tree height. Prasad distribution could describe the practical root distribution of jujube trees under non-stress growth condition. Comparing the soil moisture simulation results with measures values, and found that HYDRUS-1D model can well simulate the soil water dynamic in root zone of the jujube plantation with stand age, the values of determination coefficient(R2) between the simulated and measured water moisture in different soil layers were all larger than 0.6, the values of root mean square error(RMSE) were below 0.05, and the values of relative error(RE) were below 0.05. |
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