| The semi-arid region of Loess Plateau is one of the most serious soil erosion areas in China and even in the world.Restoring and increasing vegetation coverage is an effective way to solve this problem.Legumes(such as alfalfa)is widely cultivated in arid and semi-arid area in Loess Plateau,it has been widely used in the process of vegetation reconstruction in the region because of higher biomass and vegetation coverage,nitrogen fixation ability and good environmental adaptability.In 2003,we selected three farmland to abandon with different landscape positions(sunny slope and semi shady slope,top slope)at the dryland agroecological experiment station of Lanzhou University on the Loess Plateau.At each position,with natural revegetation as control,Alfalfa(Medicago sativa L.),sweet clover(Melilotus officinalis L.)were introduced as the two treatment to promote the vegetation restoration of long-term monitoring experiment.Our aim is to monitor the long-term dynamic change of vegetation and soil properties after the introduction of the leguminous plants.Based on the former experiment results,we measured SOC,TN,TP and AP in 0-20 cm,20-60 cm,60-100 cm and 100-150 cm of soil during 2012-2016 after growing season with old experiments,and got the following conclusions.1.In the three landscape positions of 2003,the 1.4-5m soil moisture was significantly difference of the initial stage of experiment,semi shady slope > top slope > sunny slope;In time series,the soil moisture of upper 0-1.4m was in larger fluctuation than below,the 1.4-5m soil water content of decreased obviously,especially in Alfalfa.2.In the range of 0-1.5m profiles,soil organic carbon(SOC),total nitrogen(TN),total phosphorus(TP)and available phosphorus(AP)content decreased as the depth increased in general.3.After years of alfalfa and sweet clover introduction,the SOC of soil 0-150 cm was that,in the semi shady slope,alfalfa was significantly higher than the sweet clover and natural abandonment.In the sunny slope,alfalfa and natural abandonment were significantly higher than the sweet clover,and in the top slope,the sweet clover was significantly higher than the natural abandonment and alfalfa.From the profile level,after the introduction of different legumes,SOC was significantly higher in the four sampling depths than in the other two treatments in the semi shady slope.The difference of SOC between sunny slope and top slope is mainly reflected in 0-20 cm and 20-60 cm,and there is no difference in 100-150 cm.4.After years of introduction of alfalfa and sweet clover,the TN of soil 0-150 cm is that,in the semi shady slope,alfalfa is higher than the sweet clover and natural abandonment.In the sunny slope,alfalfa > Natural abandonment > sweet clover,and in the top slope,the sweet clover is significantly higher than the natural abandonment and alfalfa.From different soil depths,after the introduction of different legumes,TN was significantly higher in the four sampling depths than the other two treatments in the semi shady slope.Also,the difference of TN between sunny slope and top slope is mainly reflected in 0-20 cm and 20-60 cm,and there is no difference in 100-150 cm.5.After years of introduction of alfalfa and sweet clover,the TP of soil 0-150 cm was performanced as follows: in the semi shady slope,alfalfa > sweet clover > natural abandonment,with significance difference.In the sunny slope,alfalfa and natural abandonment were slightly higher than the sweet clover,and in the top slope,the sweet clover was slightly higher than the natural abandonment and alfalfa.From different soil depths,after the introduction of different legumes,TP was significantly higher in the four sampling depths than in the other two treatments in the semi shady slope.While on the sunny slope and the top slope,there was almost no significant difference in the depth of TP.6.After years of alfalfa and sweet clover introduction,the AP of soil 0-150 cm was performance as follows: in the semi shady slope,alfalfa was significantly higher than the sweet clover and natural abandonment.In the sunny slope,in 2015,alfalfa was significantly higher,while in 2016,the content of natural abandoned AP was higher.And in the top slope,the AP of sweet clover was higher than that of natural abandonment and alfalfa.The AP content of 0-20 cm in the soil surface was mainly influenced by precipitation and aboveground biomass(AP=0.10precipitation-0.13 Aboveground biomass,R^2=0.14**).The growing season mainly absorbs AP below 20 cm,and this part of AP would be recovered in the non growing season,and the difference of AP content between different treatments in different landscape positions is also mainly reflected in 20-60 cm.7.We set the aboveground biomass and soil C,N and P indices in 2012-2016 of different landscape locations and restoration methods as a whole.The results of grey correlation analysis showed that,in the semi shady slope,the relationship between aboveground biomass and SOC,TN of 20-60 cm,60-100 cm was the highest.On the sunny slope,0-20 cm and 60-100 cm TN,20-60 cm TP were highest.In the top slope,100-150 cm TP,100-150 cm AP,20-60 cm AP and 100-150 cm SOC have a strong correlation with aboveground biomass.The soil C and N play an imporyant role in the formation of the aboveground biomass in semi-shady and sunny slope while soil P in top slope.Therefore,based on the data analysis of 0-150 cm SOC,TN,TP and AP,we conclude that,because of different landscape location has different microenvironment and the adaptability of plants to the environment is different.So the strategies for vegetation restoration in this area can not be generalized.The introduction of Alfalfa in the semi shady slope;the sunny slope with natural abandonment and alfalfa,introduction of sweet clover on top slope can restore soil C,N and P. |
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