Soil Conservation Effect Of Caragana Intermedia Introduced Into Desert Steppe

The introduction of artificial shrubs to natural grassland in arid areas is a widely existing ecological management measure.In theory,shrubs can help restore natural grassland by preventing wind and fixing sand to stabilize the surface environment.Caragana intermedia is a common shrub used in desertified grassland management in the study area.Previous studies have shown that C.intermedia plays an important role in the restoration of desertified grassland.However,most of the studies remain in the comparative comparison of artificial C.intermedia forests with different introduced years on local scale,and lack of systematic and continuous studies,especially multi-scale comparative studies.Based on the survey data and remote sensing data of 37 spatial grid and 148 experimental plots in the desert steppe region of central Ningxia,we studied the changes of vegetation cover and soil texture in this region in recent 20 years,and compared the differences of vegetation cover and soil texture between artificial C.intermedia forest and natural grassland.Combined with the forest ages and NDVI extracted from remote sensing and the spatial partition of soil texture,the attribution analysis was carried out.The main research results and conclusions are as follows:(1)The average value of NDVI in the study area from 2000 to 2019 ranged from 0.18 to 0.40,at the middle and low coverage level,with the maximum value in 2018 and the minimum value in 2000.With the increase of years,NDVI generally showed a slow fluctuating upward trend,with an average annual growth rate of 0.74%.The soil texture of natural grassland was mainly reflected in the distribution of soil particles.The spatial distribution of clay was the highest in the south,which was basically 52.94%～71.99%,and showed a decreasing trend from south to north.The silt did not show a significant north-south trend,but formed a low value channel along the northeast to southwest direction,and the silt on both sides of the channel was relatively high,which basically ranged from 35.62%to 43.34%.The sand content showed a significant trend of zonal decrease from north to south,and the content was the highest in the northwest,basically ranging from 60.73%to 71.64%.(2)The soil conservation effect of C.intermedia was analyzed on shrub scale and patch scale respectively,characterized by the relative enrichment ratio of clay.On the shrub scale,the clay enrichment ability was relatively strong,with an average enrichment rate of 0.11 and a maximum value of 1.09.The overall positive effect was dominant,especially in the northern part of the study area,while the clay enrichment ability was significantly weakened in the southern part.On the patch scale,the enrichment ability of sand is relatively strong,but the average enrichment ratio is only 0.08,and the average value of enrichment ratio is shown as sand>silt>clay.The enrichment of clay showed obvious spatial zoning characteristics.The negative effects were concentrated in the central area of the east-west direction,running through the north and south,and the positive effects were mainly on the east and west sides.Compared with the natural grassland,the clay concentration of the artificial C.intermedia forest on the patch scale was not obvious.The introduction of C intermedia only significantly promoted the spatial redistribution of soil clay matter on small scales(inside and outside shrub).(3)Combined with the factors of soil desertified degree,forest age and vegetation coverage in 148 sample plot,the influencing factors of soil conservation of C.intermedia were further analyzed.First,the relationship between soil content of C.intermedia forest and adjacent natural grassland at different ages showed the same changing trend,which showed that clay content showed a gradually increasing trend with the increase of forest age,silt content had a small change,and sand content showed a gradually decreasing trend.With the increase of forest ages,the enrichment ratio of clay on shrub scale generally decreased,while the enrichment ratio of sand increased.On the patch scale,with the increase of forest ages,the clay had no enrichment effec,but the sand had a better enrichment effect.Second,with the increase of sand content in natural grassland,the clay enrichment ratio increased on shrub scale,decreased on the patch scale.Thirdly,the results of redundancy analysis showed that the sand content and forest age had the greatest influence on the clay enrichment ratio,and were the important factors affecting the growth status and soil conservation.C.intermedia had a better effect on soil conservation in the grassland with a higher degree of desertification,at the age of 20～25a had the best effect on soil fine particles retention.>20～25 years,sand content increased while clay content decreased.For the grassland with low desertification and degradation,the introduction of C.intermedia may produce negative effects.In conclusion,the relative conservation effect of C.intermedia introduced into desert steppe for a long time is still not obvious at the regional scale due to the comprehensive influence of various factors.The forest ages of C.intermedia and sand content background are two key factors affecting soil conservation of C.intermedia,and soil conservation of C.intermedia has obvious scale effect and space effect.The introduction of C.intermedia did change the spatial distribution pattern of small scale soil particulate matter(enrichment)inside and outside the shrub.This effect increased with the increase of grassland desertification background,especially in the northern area,but decreased with the increase of forest age.However,the introduction of C.intermedia may have some negative effects on the grassland with slight desertification or no significant degradation.These are the problems to be paid more attention to in the future.