Study On Soil Water And Nutrient Changes In Deep Soil Of Alfalfa Grassland On Loess Plateau

The ecosystem in the Loess Plateau is fragile,with serious soil erosion by water and wind,barren land,scarce precipitation,and intense soil evaporation and vegetation transpiration.Soil water is the main factor for plant growth and vegetation restoration,as well as an important factor for vegetation succession.Alfalfa is one of the herbage widely planted in this area.Because alfalfa is a perennial leguminous crop with deep roots,continuous planting for many years leads to soil water and phosphorus deficit,nitrogen and soil organic carbon increase,which changes the soil water and nutrient environment.The Loess Plateau has a deep soil layer,and the influence of alfalfa on deep soil resources is not quantified.Therefore,this study selected perennial alfalfa grassland in Xifeng and Huanxian county in different rainfall areas of the Loess Plateau as the research object and annual farmland as the control to study the dynamic of alfalfa hay yield,agronomic traits and the changes of soil roots,water,available nutrients,total nitrogen and organic carbon in 0-1000 cm soil layer.The following results are obtained:（1）The yield of alfalfa increased year by year in the early growth stage of alfalfa,but decreased sharply in the late stage.The highest yield of M6(15.8×10³kg hm^-2)and the lowest yield of M19(5.8×10³kg hm^-2)are found in Xifeng region.Huanxian County had the highest yield(9.3×10³kg hm^-2)under M4 and the lowest yield(6.0×10³kg hm^-2)under M12.The fresh-dry ratio,plant height and leaf area index changed with the growth age and the number of harvesting stubble,and the change trend was similar in different harvests.With the increase of planting years,the first crop increased and then decreased,and the first crop was significantly higher than the later crop.（2）After 4 years of planting,the roots of M4 were planted in 1000 cm soil layer,but mainly concentrated in the shallow layer（0-200 cm）.The dry root weight and the fine root length density decreased exponentially in the shallow layer,and slowly in the middle and deep layer.In the shallow layer,the average fine root length density increased rapidly in the early ages,and remained stable after that.In the middle and deep layers,there was no significant difference between different treatments.（3）In 0-1000 cm soil layer,soil water decreased with the soil depth and alfalfa planting years.The soil moisture of Xifeng and Huanxian grasslands decreased sharply in the first 6 and 4 years of alfalfa planting,and the lowest values were 13.5%and 10.4%after 19 and 12 years planting,38.0%and 47.6%lower than that of reference annual crop field（RF）,respectively.In shallow,middle and deep layers,soil water depletion increased exponentially with the planting years.The soil water depletion rate firstly increased and then decreased with the increase of alfalfa age.In Xifeng plots,the soil water storage deficit degree in shallow layer was the largest,while that in deep layer was the smallest.The soil water storage deficit degree of Huanxian was the lowest in deep layer,the most serious in shallow layer during the first 4 years of alfalfa planting,in middle layer during 4～8 years,and the deficiency degree of shallow layer and middle layer was similar during 8～12 years.In 0-1000 cm soil profile,the soil water depletion under M19 was the highest（1049 mm）in Xifeng,M12 in Huanxian County was the highest（1123 mm）.（4）In 0-1000 cm soil profile,soil available nitrogen content decreased with the soil depth,and increased with the planting years.The content of available P decreased with the soil depth and the planting years.The available nitrogen storage in shallow,middle and deep soil was higher than RF.The highest ammonium nitrogen storage(166.9kg hm^-2)and nitrate nitrogen(1502.2 kg hm^-2)under M19 were 68.7%and18.7%higher than RF,respectively,and the lowest available phosphorus storage(720.1 kg hm^-2)was 58.8%lower than RF in Xifeng.The available nitrogen storage of alfalfa field in Huanxian was also higher than RF.The ammonium nitrogen(125.2 kg hm^-2)and nitrate nitrogen(1225.1 kg hm^-2)in M12 were the highest,47.3%and29.5%higher than RF,respectively,and the available phosphorus(580.5 kg hm^-2)was the lowest,47.1%lower than RF.（5）In 0-1000 cm soil layer,soil total nitrogen and organic carbon increased with the planting years,soil total nitrogen decreased with the soil depth,soil organic carbon decreased in 0-200 cm soil layer,and kept relatively stable below 200 cm.Soil total nitrogen and organic carbon increased with the planting years in shallow,middle and deep soil layer（P<0.001）.The highest values of total nitrogen and organic carbon storage was found in M19 and M12 in Xifeng and Huanxian,respectively.In Xifeng,total nitrogen and organic carbon were 20.4%and 17.4%higher than RF,respectively,and in Huanxian was 13.7%and 10.7%higher than RF.（6）There was a linear relationship between the fine root length density and soil water depletion,total nitrogen and organic carbon storage incrementalists.The soil water,total nitrogen and organic carbon in the Xifeng plot were significantly increased with the fine root length density in three soil layers（P<0.05）.But there is no difference in Huanxian.In conclusion,long-term planting alfalfa on the Loess Plateau will lead to soil water and available phosphorus depletion in deep soil layer,which will destroy soil water environment and nutrient balance,and seriously affect the sustainable land use.However,planting alfalfa can increase the soil available nitrogen,total nitrogen and organic carbon in deep soil,and reduce the application of nitrogen fertilizer and carbon dioxide emission.Considering the dynamics of alfalfa yield,soil moisture,nutrients and organic carbon,it is suggested that the planting years of alfalfa in the sub-humid area should be less than 6 years,and that in the arid and semi-arid areas should be less than 4 years on the Loess Plateau.In order to increase soil organic carbon and nitrogen storage and reduce soil water depletion,short-term alfalfa and annual crops should be rotated.