Changes Of Vegetation And Soil In Succession Of Alfalfa Grassland In Loess Hilly Region Of Ningxia

Planting alfalfa(Medicago sativa)is one of the means of animal husbandry development and ecological construction in China.However,due to the backwardness of cultivation and management mode and the low level of modern machinery,alfalfa grassland degenerates after a certain period of time,which restricts the development of animal husbandry.In order to clarify the succession process of alfalfa grassland and the change characteristics of vegetation and soil with succession in loess hilly region,the method of spatial gradient instead of time gradient was used in this study.Nine alfalfa fields with different planting years(1 year,5 years,7 years,10 years,15 years,20 years,30 years,40 years and 50 years)in Longde County of Ningxia were selected as the research objects,The vegetation characteristics,soil physicochemical properties,soil enzyme activities and soil microbial characteristics of alfalfa grassland with different planting years were analyzed:1.The succession process of Alfalfa Grassland in Ningxia loess hilly region can be divided into three stages.The first stage is alfalfa artificial grassland stage(1～10 years);The second stage is the transition stage(10～30 years):the transition stage from alfalfa artificial grassland to natural grassland;The third stage is the natural grassland stage,and the dominant species is Miscanthus sibiricus(more than 30 years old).2.Planting time significantly affected the production performance of alfalfa,in which plant height,stem diameter and grass yield reached the maximum at 7 years of planting,which increased first and then decreased.3.In the process of alfalfa grassland succession,the soil bulk density decreased with the extension of planting years,and the soil bulk density was the highest in the stage of Alfalfa artificial grassland;The field water capacity and soil porosity showed an increasing trend,which was the largest in the natural grassland stage.The results showed that soil moisture in alfalfa grassland stage>Natural Grassland stage>transition stage.Soil pH was in the transition stage>Natural Grassland stage>alfalfa grassland stage.The characteristics of soil nutrients were alfalfa grassland>transition stage>Natural Grassland stage.The content of total potassium and alkali hydrolyzable nitrogen in soil reached the maximum in one year.Soil available potassium,organic matter and total phosphorus reached the maximum in 5 years,and available phosphorus reached the maximum in 10 years.4.The activities of soil urease,invertase and phosphatase increased with the succession of alfalfa grassland.The activities of various enzymes in surface soil(0～20 cm)were higher than those in 20～40 cm soil layer.5.With the increase of planting years,there was no significant change in soil microbial community and dominant species,and the microbial diversity index increased first and then decreased with the increase of planting life.The soil bacterial and fungal community structure was the most affected by the total phosphorus content and available phosphorus content in alfalfa grassland succession.6.Based on the purpose of production,alfalfa reached the maximum yield after 7 years of planting in Loess Hilly Region of Ningxia,then began to degenerate,and could be regenerated or replanted in about 10 years;For the purpose of ecological construction,the natural succession of alfalfa field is conducive to the improvement of soil properties,and the transition stage(10-30 years)is the best.