| The frangibility of grassland ecosystem stability caused by grassland degradation is an important problem in China’s ecological environment.Songnen grassland is an important agricultural and animal husbandry base in Northeast China,and also an important ecological barrier of northern China.In recent years,due to the influence of climate change,soil salinization,the change of.Land use and land cover,and human activities,the degradation of Songnen grassland is also serious year by year,and the stability of the grassland ecosystem is becoming more and more unstable.Numerous studies have pointed out that the degradation of Leymus chinensis grassland is closely related to soil characteristic factors.However,it is difficult to clarify the main soil obstacle and mechanisms that affect the changes of L.chinensis communities.Therefore,we chose 5 original grasslands of western Songnen Plain:Chang Ling,Jiang Jiadian,Da’an,Zhen Lai,and Tu Muji.Through the investigation and analysis of the community characteristics,L.chinensis biological characteristics and mineral contents,and soil characteristics of L.chinensis community,we clarified the differences of the characteristics of L.chinensis community in different regions by ANOVA.We also explored and verified the main soil factors which influence grassland degradation and its mechanisms in Songnen grassland by correlation analysis,regression analysis,aggregated boosted tree,and structural equation model.The results show that:1.In Songnen grassland,there are obvious regional differences in the community features,biological and biochemical features of L.chinensis,and soil features(P<0.05).The community biomass of Da’an region was the highest,which significantly higher than that of Jiang Jiadian,Chang Ling,Zhen Lai and Tu Muji by 211%,112%,107%,60%(P<0.05);The average biomass of L.chinensis in Da’an region was 0.25 kg·m-2,and in Jiang Jiadian area was of 0.03 kg·m-2;There were no significant regional differences in plant height,aboveground P content,soil electrical conductivity and available P of L.chinensis in Chang Ling,Jiang Jiadian,Zhen Lai and Tu Muji regions;There were significant differences in soil moisture among different regions.Soil moisture in Da’an region was 14.80%,which is the highest one among the 5 regions;followed by Jiang Jiadian,Chang Ling,Tu Muji,and Zhen Lai regions,with average values of 7.67%,6.38%,4.28%,and 3.67%,respectively.There were significant differences between Da’an and the others,there was no significant difference among the characteristics of the L.chinensis community in Chang Ling,Jiang Jiadian,Zhen Lai,and Tu Muji regions.In a word,from the coefficient of variation of each characteristic of the L.chinensis community in Songnen grassland,the variability is not large,and the overall variation is medium.2.The correlation analysis indicated that there was a close relationship among community features,biological features and mineral contents of L.chinensis and,soil features.For example,there were significant positive correlations between community biomass and L.chinensis biomass,plant height,soil moisture,K+ion content,and available phosphorus content(P<0.05).The aboveground K+,N contents in L.chinensis were significantly negatively correlated with Na+ content in soil;the aboveground P content in L.chinensis was significantly negatively correlated with the content of soil available nitrogen(P<0.05).In addition,there was a positive correlation between the biological features and biochemical features of L.chinensis,and there was a significant negative correlation between the contents of soil nutrients and salinity features,such as available nitrogen and organic matter among pH,EC,Na+,and Cl-(P<0.05).3.Regression analysis indicated that community biomass was mainly affected by soil moisture and Na+ion content(R2=0.259);The biomass of L.chinensis was mainly affected by soil moisture(R2=0.158);The plant height of L.chinensis was mainly affected by soil Na+ content,soil moisture,SO42-、Ca2++Mg2+and K+content(R2=0.393);The abundance of L.chinensis was affected by the contents of K+and Na+ in soil(R2=0.150).As a whole,the changes of soil moisture and Na+ ion content have important effects on community biomass,L.chinensis biomass and plant height.4.The results of aggregated boosted tree analysis indicated that the relative influence of soil moisture,available phosphorus,and electrical conductivity were more than 10%for community biomass and L.chinensis biomass.Among them,the relative influence of soil moisture was 34.20%and 36.58%,and of soil available phosphorus were 10.77%and 11.82%,and of soil electrical conductivity were 10.66%and 9.48%,respectively.The relative influence of soil pH to plant height of L.chinensis was the highest,reaching 19.74%.The relative influence of soil moisture and available phosphorus content were 18.92%and 13.52%respectively.The relative influence of soil available nitrogen to the abundance of L.chinensis was the highest,accounting for 32.00%,followed by the relative influence of soil organic matter and K+ion content reached 22.92%and 18.06%,and the relative influence of other soil variables were less than 10%.5.According to the structural equation model,the change of community biomass is determined by the biomass of L.chinensis regulated by its elongation growth,the path coefficient between L.chinensis biomass and community biomass is 0.7.Soil moisture is a key index in the community-L.chinensis-soil relationship.Its change can not only affect the growth of L.chinensis and the biomass of the community directly,but also affect the community dynamics indirectly by adjusting soil salinity and available nutrients.6.According to the important influence of soil environmental factors on the biomass of L.chinensis,the critical value of soil characteristics when the reduction rate of L.chinensis biomass was 50%were predicted by calculating the percentage of L.chinensis biomass of each sample relative to the average biomass of more than 95%higher than the average L.chinensis biomass in the whole study area.That is,when the soil moisture decreased from 16.00%to 5.33%,the biomass of L.chinensis decreased by 50%or more;when the soil available phosphorus content decreased from 14.19 mg·kg-1 to 4.78 mg·kg-1,the biomass of L.chinensis decreased by 50%or more.In conclusion,taking community biomass as degradation index,the soil barrier factors affecting the of Songnen L.chinensis grassland were soil moisture,available nitrogen,available phosphorus,pH,K+,Na+,Ca2++Mg2+,and SO42-ion content.Among them,soil moisture and Na+ content play a crucial role,they not only affect the the community biomass indirectly by affecting the plant height or biomass of L.chinensis,but also by adjusting other soil features.In addition,soil moisture plays a more important role in the whole relation network.In other words,the degradation of the L.chinensis community in the Songnen grassland was affected by the changes of soil environmental factors dominated by soil moisture.And the dynamic change of community depends on the response of dominant species to the change of soil environmental factors.The results of this study provide an important theoretical basis for the restoration of degraded grassland communities in Songnen grassland. |
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