| Other Abstract | 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.
Keywords: soil water content, soil water availability, soil texture, evaporative demand |
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