Dynamic Characteristics Of Root System And Source Apportionment Of Soil Water Of Plant Community In Abandoned Farmland In Loess Hilly Region

Loess Hilly Region belongs to the area of seasonal drought,soil water supplied by natural rainfall is the limiting factor of vegetation restoration in the project of returning farmland to forest.Plant uptake of water from the soil-root-stem pathway is a major component of the eco-hydrological process,and its determine the evolution direction and ecological function of soil vegetation system.Root system is the main carrier of material circulation and energy flow in vegetation and soil.Therefore,it is important to study the distribution of vegetation root system and the characteristics of water use of the vegetation.In this study,the plant communities in the loess hilly region were studied,the distribution of plant roots,soil water content and stable hydrogen and oxygen isotopes were analyzed by sampling,reveal the distribution of them in different seasons,and the Iso-source model of stable isotope mass conservation was used to quantify the contribution of plants to different soil depths.The main conclusions are as follows:(1)The dynamic distribution characteristics of root system of the vegetation community were analyzed.The root length density(RLD)reached the maximum value in 5~6 months,The average of RLD decreased gradually,main root zone gradually increased from 0~37.5 cm to 0~120 cm,according to the root extinction coefficient,the proportion of roots in deep soil was gradually increased in the 0~30 years of succession;The root length fractal dimension increases with the soil depth in the 0~22 years of succession,then the fractal dimension of RLD decreases with soil depth when the succession to 30 years;The spatial variability of soil water is the main reason for the difference of spatial distribution of root system.(2)The dynamic characteristics of soil water distribution in the vegetation communities were analyzed.There were seasonal differences in soil water content in different succession stages,the soil water content in dry season is less than that in wet season,and increased with the succession;The soil water deficit gradually changed from mild deficit to no deficit,and was not affected by root system in the 0~22 years of succession,then the soil water content was seriously deficient,and it was strongly affected by root water consumption when the succession to 30 years.(3)The isotopic characteristics of different water sources and their recharge patterns were revealed.The variation range of ?18O of rain water,snow water,soil water and plant water was,-13.15~-1.81 ‰,-19.24~-10.48 ‰,-13.84~2.88 ‰ and-12.12~0.04 ‰ respectively;?D was-97.30~-11.54 ‰,-117.55~-71.51 ‰,-119.72~-14.03 ‰ and-116.06~-21.42 ‰ respectively.Based on the analysis of the ?D and ?18O values of winter snow and soil water in April,it was found that the abundant soil water in the April dry season was caused by the melting of snow in winter,in May to November,soil water was supplied by rainfall,and it was affected by the secondary evaporation after infiltration;The analysis of ?D and ?18O values in the soil profiles of dry and wet seasons showed that with the succession of plant communities,the range of unstable soil water decreased from 0~90 cm to 0~50 cm;Analysis of the slope plant communities in water hydrogen and oxygen isotope fitting line showed that in early successional plant communities are sensitive to the change of soil water,and can be quickly absorbed by the supply of rainfall,the response time of plants to soil water increased gradually with succession.(4)The contribution rate of water source and its influencing factors were analyzed.In the early stage of plant communities,can only uesd the soil water which from 0~20 cm,water utilization increased to 0~100 cm when the succession to 22 years;There was obvious seasonal variation in water use,the water source was deep,can reach 80 cm in dry season,then the main water source moved up to 0~20 cm soil in wet season.There was significant correlation between water use and root distribution,root length percentage and root water uptake had a significant positive correlation(p<0.01).The average soil water content of 0~20 cm increased from 6.28% to 15.08%,and the average root water uptake ratio increased from 16.67% to 57.90% from dry season to rainy season.