Soil water variation of different vegetation community in Taihang Mountain Area

Soil water storage and movement are highly heterogeneous across ecosystems. However, variation characteristics of soil moisture are not well understood at present, due to the high heterogeneity of environmental conditions. The Taihang Mountain region is an important water collection area of the North China Plain, and a functional area for water conservation in the national Beijing-Tianjin-Hebei coordinated development strategy. However, little is known about the soil water regime of the main plant species that inhabit this region. In this study, the soil water content for four representative vegetation types, Triticum aestivum/Zea mays, Themeda triandra var. Japonica, Vitex negundo var. heterophylla and Juglans regia, that are widely distributed in the semi-arid area of the Taihang Mountain, were observed using large scale weighing lysimeters, during the growing season (April to October) in 2018 and 2019. Weighing lysimeters systematically measured the soil water content, seepage, and evapotranspiration among different vegetation communities, and the collected data on the variation characteristic of soil moisture content for four vegetation types were analyzed by means of statistical analysis. The results showed that average soil water content for T. aestivum/Z. mays, T. triandra, V. negundo, and J. regia were 0.30 cm³·cm^-3, 0.35 cm³·cm^-3, 0.32 cm³·cm^-3 and 0.36 cm³·cm^-3 in 2018, and 0.28 cm³×cm^-3, 0.26 cm³·cm^-3, 0.23 cm³·cm^-3 and 0.31 cm³·cm^-3 in 2019, respectively. Similarly, the decrease of soil water content for T. aestivum/Z. mays, T. triandra, V. negundo, and J. regia were 0.05 cm³×cm^-3, 0.04 cm³·cm^-3, 0.09 cm³·cm^-3, and 0.05 cm³·cm^-3 in 2018, and 0.07 cm³×cm^-3, 0.13 cm³·cm^-3, 0.18 cm³·cm^-3, and 0.10 cm³·cm^-3 in 2019, respectively, compared to the soil water content at the end of growing season (December) of 2017. The decrease of the soil water content for V. negundo was greatest, and that for T. aestivum/Z. mays was the smallest, among the four vegetation types. The direction of vertical gradient of soil water content was consistent among the four vegetation types, with the soil water content decreasing as the depth of soil increased. However, the depth of water uptake from soil was discrepant. In T. aestivum/Z. mays fields, the main depth was between 45-100 cm. In T. triandra and J. regia fields, the main soil depth was between 100-150 cm. In V. negundo fields, the main soil depth was between 150-180 cm. These results may be due to differences in root distribution. T. aestivum/Z. mays and T. triandra are from Gramineae family, and their root systems are distributed in shallow soil. Although J. regia is arboreal, it has shallow-rooted plants. V. exnegundo has lateral roots radiating out from the main root crown, one or more deeply penetrating tap (sinker) roots, and can uptake water from deep soil. This indicated that V. exnegundo was the species with the greatest water consumption. The pattern of plant water consumption needs to be considered in plant species selection, ecological management, and restoration of semi-arid ecosystems in the Taihang Mountain region.