Spatiotemporal Variation Of Soil Moisture And Its Response To Rainfall In Two Typical Grasslands In The Northern Loess Plateau

Soil water is a key factor to maintain the stability and sustainability of ecosystem restoration in the Loess Plateau.Soil moisture controlled by vegetation is the main influencing factor of its change in many semi-arid ecosystems.Medicago sativa L.and Stipa bungeana Trin.are typical legumes and grasses,res.Studying the spatial and temporal distribution and heterogeneity of soil moisture in these two grassland is helpful to fully grasp the soil moisture quantity and variation characteristics of different grassland types under space-time scale,and to formulate more scientific and effective vegetation layout and field management measures.The study of soil moisture changes caused by different rainfall intensity can provide theoretical basis for optimizing grassland management and maintaining the sustainability of grassland ecosystem function.In the Medicago sativa L.and Stipa bungeana Trin.plots of Shenmu Liudaogou watershed,24 observation points were set up to observe the temporal and spatial dynamic characteristics of soil moisture in different soil layers for a long time.The main conclusions are as follows:(1)Using classical statistical and geostatistical methods,the spatial variation characteristics of soil moisture in the 0~5 m profiles of Medicago sativa L.and Stipa bungeana Trin.were analyzed.The soil moisture of both grassland increased first and then decreased with the increase of soil depth,and then tended to be stable.The soil moisture content and spatial variation coefficient of the two grassland in the vertical direction were significantly different(p <0.01).The mean values of soil moisture variation coefficient of 0~5 m profiles of Medicago sativa L.and Stipa bungeana Trin.were 0.22 and 0.27,respectively.Generally,Medicago sativa L.caused excessive consumption of deep soil moisrye,the soil water content below 0.7 m was obviously lower than that of Stipa bungeana Trin.,its high water consumption characteristics weakened the spatial variation intensity of soil water in each soil layer in the vertical direction.Therefore,Medicago sativa L.vegetation strengthened the consumption of soil moisture in the vertical direction,and led to the negative balance of deep soil moisture,which made the distribution of soil moisture in the horizontal direction of sample plot scale tend to be homogenized.(2)The temporal stability of soil moisture at each observation point of Medicago sativa L.was stronger than that of Stipa bungeana Trin.,but Stipa bungeana Trin.had more stable spatial pattern in time.0~1 m layer of soil moisture followed the pattern of rainfall variation,and the deep soil tended to be more stable.Medicago sativa L.caused soil water retention deficit.Under different observation times,the more similar the soil moisture,the more stable its spatial pattern.In the range of 0 ~ 5 m,the average rank correlation coefficients of Medicago sativa L.and Stipa bungeana Trin.,were 0.65 and 0.74.The rank correlation coefficients of the soil layers below 1 m showed significant differences(p <0.01).The spatial pattern of deep soil moisture was more stable.The temporal stability of soil moisture was affected by the combination of vegetation and soil layer depth.The relative standard deviations and average absolute deviations of each soil layer in Medicago sativa L.were smaller than those in Stipa bungeana Trin.,which had significant differences in individual soil layers.The number of measuring points with time stability increasesd with the increase of soil depth.The best representative measuring points of each soil layer in Medicago sativa L.were A18,A10,A10,A4,and A7 in Stipa bungeana Trin.were B5,B1,B17,B12,and B15,respectively.In determining the best representative measurement point,various determination methods should be fully considered and verified with accuracy parameters.(3)Soil moisture was significantly positively correlated with its variability during most of the observation time and soil layer.The combined effect of vegetation and soil depth complicated the relationship between soil moisture and its variability.During the observation period,the soil moisture change had a strong dependence on rainfall,but the sensitivity of different depths of different vegetations to rainfall varies.The soil moisture in the 0~1.8 m soil layer of Medicago sativa L.had a significant positive correlation with rainfall.There was a significant positive correlation between the 0 ~ 0.7 m soil layer of Stipa bungeana Trin..Light rain and moderate rain had no obvious effect on supplementing soil moisture,while heavy rain and rainstorm had significantly increased soil water content of 0~0.5 m.A certain amount of rainfall can quickly replenish surface soil moisture content,it is the spatial distribution of soil moisture tended to be homogenized,thereby weakened the spatial variability of soil moisture.When rainfall intensity increased further,the variation pattern of soil moisture changed.Rainfall increased the sill,nugget,and nugget-sill ratio of each soil layer in the two grasslands of 0~0.5 m,and part of the soil water content changed from strong spatial autocorrelation to moderate autocorrelation.The range of soil surface water after rainfall increased and the variability decreased.