Studying On The Soil Water Availability To Typical Plants On The Loess Plateau

Water resources shortage is very severe in the Loess Plateau of China. The climate warming and drying in this area is leading to a further decline of soil water reserve, which challenges the ecological and agricultural sustainable development. Studying on the dynamics of soil water availability for different plants in different textured soils, and under different weather conditions could guide for the vegetation restoration and the development of the dryland farming. In this study, three typical textured soils (loamy clay, clay loam and sandy loam) collected from different area in the Loess Plateau were selected for growing maize (Zea Mays L.), winter wheat(Triticum Aestivum L.), and black locust (Robinia Pseudoacacia L.) under six different constant soil water levels in the pots and under three water treatments in the plots. The plant growth and transpiration and photosynthetic parameters were measured in different soil water treatments. The effects of soil texture, plant and weather conditions on soil water availability were systemically analyzed based on these measurements. With the Hydrus-1D model, the factors affecting the response of root water uptake to soil water availability were discussed on a theoretical basis. The main conclusions of this study were as follows:(1) The responses of plant growth and transpiration of maize and winter wheat to soil water stress were compared between the pot and plot experiments. Characteristic responses of the various plant indices to changes in the soil water content obtained in the pot experiment were applicable to the field, although large differences were found between the plants grown in the pots and plots when considering their absolute plant growth and total transpiration. (2) The dynamic aspects of soil water availability were compared among different physiological indices. With the linear-plateau function, the dynamic aspects of soil water availability for various physiological indices showed the same trend with a threshold response. The threshold values for the indices over the transient time scale were lower than that over the daily and seasonal scales. The threshold values for indices related to growth were lower than that related to transpiration. Higher threshold value was obtained for daily transpiration before the filling stage, however, no significant difference were obtained between the threshold values for indices related to growth before and after the filling stage.(3) The dynamic aspects of soil water availability for various physiological indices of maize, winter wheat and black locust were also compared among different soils. The average threshold values of relative soil water content for maize were larger for the loamy clay than clay loam and sandy loam; near the broken capillary moisture, soil water availability for maize was the largest for the clay loam, the medium for the sandy loam and the least for the clay loam. The average threshold values of relative soil water content for winter wheat were the largest for the sandy loam, the medium for the loamy clay and the least for the clay loam; near broken capillary moisture, soil water availability for winter wheat was the largest for the clay loam, the medium for the loamy clay and the least for the sandy loam; The average threshold values of relative soil water content for black locust were the largest for loamy clay, the medium for the sandy loam and the least for the clay loam; near broken capillary moisture, soil water availability was the largest for the loamy clay, the medium for the clay loam and the least for the sandy loam. The capacity of draught resistance was the largest for black locust, the medium for winter wheat and the least for maize.(4) By comparing the dynamic aspects of soil water availability under different weather conditions, we found that soil water availability in the loamy clay was insusceptible to humidity variation under high temperature, but increased with the humidity increasing under low temperature; soil water availability in the sandy loam increased with humidity increasing under high temperature and decreased with humidity increasing under high temperature; The effects of evaporative demand on the dynamic aspects of soil water availability were different among different textured soils and plant types. For maize, the threshold values of soil water content increased with the evaporative demand increasing, but no significant difference could be found among different ET0 levels. For winter wheat, the threshold values of soil water content increased with the evaporative demand increasing, and significant higher threshold values were obtained when ET0 > 3.0 mm d-1. For black locust, the threshold values of soil water content in the sandy loam increased significantly when ET0 > 3.0 mm d-1 as the evaporative demand increasing, however, in the loamy clay and clay loam, the threshold values of soil water content decreased significantly.(5) The Hydrus-1D model was selected to investigate the mechanism of the influence of soil texture, plant growth and weather conditions on the dynamic aspects of soil water availability. The plant selected for simulating was maize growing with different leaf area index, root depth and root density distribution. Results showed the threshold values of soil water content were quite different under different soil, plant and weather conditions, among which root depth and root density distribution also had significant influence on the shape of the dynamic curve of the soil water availability to plant. However, the response of soil water availability to leaf area index and weather condition was different among different soils. In general, the critical soil water content declined with soil texture in the order: loamy clay > clay loam > sandy loam, increased as evaporative demand increasing and decreased as root depth and root density distributed in the deep depth increasing.The results showed that, with the normalized plant physiological indices, the soil water availability to plant in the pot experiment could be applied in the field; the soil water availability would be different when the plant indices used were different in time scales, growth stages or related physiological processes; soil water availability was the plant responses to soil water variation under the comprehensive effects of soil texture, weather condition and plant growing state.