| The agriculture-pastoral ecotone in Northern Yinshan Mountains in Inner Mongolia is located in the middle and northern part of China’s northern agriculture-pastoral ecotone,which is the link between the central east and west of China,and is an important ecological barrier between the eastern monsoon climate region and the northwest arid region,and is also one of the most ecologically fragile and poor areas in the whole agriculture-pastoral ecotone.Desert grassland and typical grassland are the main zonal vegetation in the region,which have been degraded over the years due to unreasonable use by humans and excessive farming.The "Grain for Green Project" is an effective policy to reverse the loss of biodiversity and improve ecological functions.However,changes in vegetation and soils during the restoration succession of abandoned croplands in the grassland areas and their relationships with ecosystem functions,especially ecosystem multifunctionality,are still unclear.Based on this,our study was conducted on desert and typical grassland abandonment sites in Inner Mongolia,using spatial instead of temporal approach,and takes natural grassland as the reference,through field sampling and indoor analysis for different years of abandoned grassland to determine plant community structure,soil physicochemical properties,analyze soil bacterial fungal abundance and diversity using high-throughput sequencing technology,and calculate ecosystem multifunctionality by combining with each ecosystem function index,investigated the mechanism of "plant-soil-microorganism" interactions in the process of abandonment succession and the drivers of ecosystem multifunctionality,to reveal the process and mechanism of restoration of abandoned croplands in different grassland types,and provide theoretical basis and practical guidance for ecological restoration of abandoned croplands in grassland areas.The main results obtained in this study are as follows:1.The vegetation characteristics changed differently during the succession of abandoned grassland types,among which the aboveground biomass,total belowground biomass,species richness,and species diversity of desert grassland and typical grassland showed an increasing trend,and the total aboveground biomass of desert grassland recovered to the level of natural grassland at 20-y(P>0.05),and the species richness and diversity had a significant positive correlation with belowground biomass(P<0.001).The above-ground biomass and total below-ground biomass of typical grassland recovered to the level of natural grassland at 20-y(P>0.05),species richness,and diversity was significantly and positively correlated with above-ground biomass(P<0.01),and species richness was significantly and positively correlated with below-ground biomass(P<0.01).The plant community composition and species occupying larger ecological niche widths in desert grassland and typical grassland at 20-y of abandonment were the same as those in natural grassland,with Stipa breviflora,Agropyron cristatum,Stipa krylovii in the former and Carex korshinskyi,Stipa krylovii,and Leymus chinensis in the latter.2.The soil water content,saturated water content,water holding capillary,saturated hydraulic conductivity,soil capillary porosity,organic carbon,total phosphorus and available phosphorus increased gradually with abandonment years in the two grassland types,while soil bulk density,ammoniacal nitrogen and nitrate nitrogen showed a decreasing trend with the increase of abandonment years.Soil p H showed different trends in the two grassland types,with a decreasing trend in desert grassland and an increasing trend in typical grassland with the increase of abandonment years.The RDA redundancy analysis showed that the soil physical and chemical properties that contributed the most to the recovery of desert grassland vegetation were soil organic carbon,and the soil physical and chemical properties that contributed the most to the recovery of typical grassland vegetation was soil p H,and the soil physicochemical factors changed differently during the recovery process of abandoned succession in different grassland types,and the soil factors affecting vegetation recovery were different.3.The patterns of soil bacterial diversity with abandonment succession varied across grassland types.The diversity and richness of soil bacteria in desert grasslands increased significantly with abandonment succession(P<0.05),but were still significantly lower than those in natural grasslands at 20-y;and there were no significant differences in soil bacterial diversity and richness indices between years of abandonment in typical grasslands,and no differences with natural grasslands(P>0.05).The β diversity of soil bacterial communities changed with the abandonment in desert grasslands showed that there were significant differences in the bacterial community structure between 5-y and15-y,20-y and natural grassland(P<0.05),and there were no significant differences between 15-y,20-y and natural grassland(P>0.05).The β diversity of soil bacterial communities was not changed significantly by different years of abandonment in typical grasslands,but there were significant differences between abandoned croplands and natural grasslands(P<0.05).Redundancy analysis showed that soil ammonium nitrogen,total phosphorus,non-capillary porosity,saturated water content,and plant richness were the key factors affecting the soil bacterial community in desert grassland,while soil organic carbon,nitrate nitrogen,and plant richness were the key factors affecting the soil bacterial community in typical grassland.For soil fungi,there were no significant differences in the soil fungal diversity and richness among desert grassland at different abandonment years(P>0.05),and in typical grassland,the fungal diversity and richness were highest at 15 years of abandonment and higher than those of natural grassland(P<0.05).In contrast to bacteria,the variation in β diversity of fungal communities was smaller,and there was no significant difference in β diversity of soil fungal communities in desert grassland and typical grassland with different abandonment years(P>0.05).Redundancy analysis showed that total soil nitrogen and forb biomass were the key factors influencing soil fungal communities in desert grasslands,while plant diversity,aboveground biomass,p H,and saturated hydraulic conductivity were the key factors influencing soil fungal communities in typical grasslands.4.The ecosystem multifunctionality in desert grassland and typical grassland gradually increased with the abandonment years,but was still significantly lower than that of natural grassland at 20-y(P<0.05).In desert grassland restoration succession,plant richness,soil bacterial diversity and soil fungal diversity were significantly positively correlated with ecosystem multifunctionality(P<0.05),while in typical grassland,soil p H,plant richness and plant diversity were significantly positively correlated with ecosystem multifunctionality,and soil bacterial diversity was significantly negatively correlated with ecosystem multifunctionality(P<0.05).For individual ecosystem functions,the water regulation function in desert grassland and typical grassland gradually increased with the abandonment years,and the water regulation function in desert grassland recovered to the level of natural grassland at 20-y(P>0.05),while it was still lower than that of natural grassland at 20-y in typical grassland(P<0.05);the nutrient cycling function in desert grassland and typical grassland increased and then decreased with the abandonment years,and was still significantly lower than that of natural grassland at 20-y(P<0.05);the productivity function in desert grassland decreased and then increased with the abandonment years,while it gradually increased with the abandonment years in typical grassland,and neither of the two grassland types reached the level of natural grassland at 20-y(P<0.05).The driving mechanisms of ecosystem multifunctionality during restoration succession in different grassland types were different.The increase of plant diversity and richness in desert grassland led to the increase of ecosystem multifunctionality;while the increase of soil p H in typical grassland led to the increase of ecosystem multifunctionality.In addition,the increase in soil p H reduced soil bacterial diversity,which led to the increase in ecosystem multifunctionality.Overall,the process of vegetation and soil restoration in abandoned croplands differed across grassland types.Although aboveground biomass,plant community composition and species occupying larger ecological niche breadth,soil water content,saturated water content,capillary water holding capacity,total nitrogen,saturated hydraulic conductivity,capillary porosity,p H,soil fungal diversity,richness and βdiversity in desert grassland reached the level of natural grassland at 20-y,belowground biomass,plant diversity,soil bulk density,organic carbon,total phosphorus and available phosphorus still did not reach the level of natural grassland at 20-y.In typical grassland,belowground biomass,aboveground biomass,soil water content,saturated hydraulic conductivity,aboveground biomass,plant community composition and species occupying larger ecological niche breadth,capillary porosity,total phosphorus,available phosphorus,bulk density,p H,soil bacterial diversity,richness,β diversity,fungal diversity,richness and β diversity reached the level of natural grassland at 20-y,but plant diversity,soil saturated water content,capillary water holding capacity,organic carbon and total nitrogen still did not reach the level of natural grassland at 20-y.The multifunctionality and other individual functions of the abandoned croplands in both grassland types did not reach the level of natural grassland at 20-y,except for the water regulation function in desert grassland,and the driving mechanisms of ecosystem multifunctionality in the restoration succession of different grassland types were different. |
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