Root systems distribution and water use pattern of vegetation from abandoned croplands during dry and wet season in Loess Hilly Region

Abstract: Soil water is a key limiting factor for plant growth, species richness, primary productivity and grassland stability in arid and semi-arid area all over the world. Root system is the linkage between vegetation and soil for material circulation and energy transformation. Since the Chinese government implemented "Grain for Green" project in 1999, lots of annual croplands was converted to perennial native vegetation, which had significantly altered the eco-hydrological cycle in the Loess Hilly Region. The evaluations of root morphology and water-using characteristics of different vegetations abandoned from croplands are benefit to understand the effect of different vegetation communities on hydrologic cycle and provide theory guidance for vegetation restoration in semi-arid area. In this study, soil and root samples of 5 differents vegetation communities (Artemisia capillaris, A. sacrorum, Bothriochloa ischaemun, Lespedeza davurica and Pinus tabulaeformis) were collected in dry and wet seasons of 2015 in Wangmaogou watershed of Wuding River. The root morphological structures and signatures of stable isotopes were determined by using stable isotope technology. Each vegetation communities was arranged in the 2-3 plots as repeat, arranged 2 plots in Artemisia capillaris, A. sacrorum, Bothriochloa ischaemun and Lespedeza davurica, respectively, arranged 3 plots in Pinus tabulaeformis. Then 5 quadrats were established in each kind of plots. Ensured the same distances between collection points and the shrubs or trees when sampling in Lespedeza davurica or Pinus tabulaeformis, and to avoid the error caused by the different horizontal distances from the stems of the plant (For examples, root biomass, soil water contents, etc.). The results showed that the RLDs (Root length densities) of A. capillaris, A. sacrorum, B. ischaemun, L. davurica and P. tabulaeformis in dry season (16.3, 21.7, 17.3, 17.3, 6.0 mm/cm3) were 9.4, 10.2, 5.3, 2.9 and 1.4 times compared that in wet season (1.7, 2.1, 3.2, 5.9, 4.2 mm/cm3), respectively. The root systems of A. capillaris, A. sacrorum, B. ischaemun were more sensitive to seasonal variations of soil water than L. davurica and P. tabulaeformis. The soil water contents and δ18O of soil water in the surface soil layers were obvious difference between dry and wet season. A. capillaris could only use the soil water from 0-10 cm layer, and A. sacrorum, B. ischaemun mainly absorbed the soil water from 30-40 cm layer during the dry season, while the layers of soil water were moved up to 0-10 and 10-20 cm layers in wet season, respectively. L. davurica and P. tabulaeformis mainly used the soil water from below 60 cm layers in dry season, and absorbed soil water from 20-40 cm layer in wet season. In conclusions, the strategies of soil water uptake were significant difference between dry season and wet season. The root systems used soil water from deeper soil layers in dry season and more closer to surface layers in wet season. These results demonstrated that the changes of soil water and root systems distribution were consistent with the seasonal variation in water uptake patterns estimated by using stable isotope. The results of the present research can provide technical support for vegetation restoration and eco-hydrological process in the Loess Hilly Region.