Effects Of Precipitation Variations On Soil Moisture And Water Use Efficiency Of Grassland Communities In The Loess Hilly Region

Global changes lead to changes in precipitation patterns,while in the loess hilly regions the ecological environment is fragile.The main source of soil water is the atmospheric precipitation,and the vegetation growth is controlled by precipitation,so the future precipitation changes will affect the soil moisture content and the development of grassland communities.Based on the regional precipitation data over the past 30 years,this paper presents a preliminary analysis of the precipitation variation trend in Loess Hilly Region in Ansai County,northern Shaanxi.Precipitation control experiments were set up to analyze the response of soil water and their use efficiency in grassland communities to precipitation changes through the investigation of grassland community biomass and soil moisture measurements under different precipitation gradients.The main points as follows:(1)Precipitation of Ansai fluctuates in a decreasing trend from 1987 to 2016,and the frequency of dry years was increasing.In the level of precipitation events,the days of light rain and moderate rain showed a decreasing trend,and the days of heavy rain and torrential rain showed an increasing trend;the rainfall intensity of light rain,moderate rain and heavy rain all showed a not obvious decreasing trend,and the rainfall intensity of torrential rain showed an increasing trend;the contribution rate of light rain,heavy rain and torrential rain showed an increasing trend,the medium rain contribution rate showed a decreasing trend,that means the probability of a strong precipitation event was increasing.The precipitation in 2017 is close to the average of the past 30 years,but no torrential rain events have occurred,and the distribution of precipitation coincides with the trend of multi-year average distribution.(2)Soil moisture dynamics show a "W" shape during the growing season.In the natural restoration communities,the soil moisture content in the 0-30 cm soil layer has the biggest changes,belonging to the active layer of soil moisture,and the 30-60 cm soil layer is the secondary active layer of soil moisture,and the 60-120 cm soil layer is the relatively stable layer of soil moisture.In typical grassland communities,0-60 cm soil layer belongs to the sub-active layer of soil moisture,and 60-120 cm soil layer is a relatively stable layer of soil moisture.Precipitation and vegetation mainly affect soil moisture in soil layers of 0-60 cm.The dynamic analysis of soil moisture indicated that the soil moisture content of each layer in the end of the growing season was greater than the beginning.(3)Precipitation gradient had a significant effect on the soil moisture content of grassland community in different months.The soil moisture content increased first and then decreased with the increase of precipitation gradient.Of these,the soil moisture content maintained a high level with-20% precipitation conditions,meaning drought treatment helps maintain high soil moisture content.The soil water content was in a logarithmic relationship with the precipitation,and the soil moisture content in all soil layers increased and then stabilized with increasing precipitation,but the typical grassland community was less affected by precipitation than the natural restoration community.(4)In the typical precipitation events,the response of soil moisture dynamics of the two communities to the precipitation tends to be the same.They both increase first and then decrease with time.The precipitation of 30 cm soil layers in each level of precipitation event peaks on the second day,while the 30-60 and 60-120 cm showed different degrees of delay,and the peak appeared in the 3rd to the 6th days after the precipitation.Precipitation levels,pre-precipitation and post-precipitation,and soil depth have extremely significant effects on soil moisture.The rainfall of medium-rain level has the greatest effect on the soil moisture content in 0-30 cm soil layer,and the precipitation of heavy rain and torrential rain has the greatest impact on soil moisture content in 30-60 cm and 60-120 cm soil layers.(5)The seasonal changes of soil water storage during natural growth and typical grassland communities were consistent with the soil moisture content,and the soil water storage of the natural restoration communities in the growing season was smaller than that of typical grassland communities.With the increase of precipitation gradient,the aboveground biomass of the natural restoration community showed an increasing trend,while the aboveground biomass of the typical grassland community showed an increase first and then decrease trend.With the increase of precipitation gradient,the water use efficiency of the natural restoration communities was decreasing insignificantly,while the water use efficiency of the typical grassland community was decreasing significantly.The water use efficiency between the two vegetation types was significantly different at-80%?+40%?+60% and +80%.The water use efficiency of community was affected by aboveground biomass,water consumption,soil water storage at the beginning of the growing season,and was significantly negatively correlated with aboveground biomass and water consumption.