| Ecosystem sustainability is the basis for life,economic and social sustainability,the most ecosystems on Earth have been influenced and changed by humans,and their development and evolution mechanisms are important research topics in the field of ecology.In this paper,based on the emergy theory as the main method,the functional characteristics and emergy metabolism characteristics of different ecosystems under the same environmental conditions were discussed in the agro-pastoral ecotone of northern China;taking the mazie and wheat ecosystems in China as the research object,the rule of emergy metabolism of the same ecosystem under different environmental conditions was expounded.Through the above research,the mechanism of ecosystem evolution is analyzed..A combination of field tests and a questionnaire survey were carried out between June and August 2015,relying on the Guyuan National Grassland Ecosystem Field Station,Hebei Province.The ecosystems studied included natural grassland,artificial grassland,food crops and commercial crops,four major local land use patterns.These four ecosystems were further subdivided into a total of ten ecosystems.Natural grassland was divided into free-grazing and mowed ecosystems;artificial grassland consisted of oat(Avena sativa),Chinese leymus(Leymus chinensis)and corn silage(Zea mays);food crops included naked oats(Avena chinensis),flax(Linum usitatissimum)and wheat(Triticum aestivum);-and commercial crops consisted of cabbage(Brassica rapa pekinensis)and potatoes(Solarium tuberosum.).The emergy metabolism analysis of the national corn and wheat ecosystem is based on the relevant statistics of China in 2014.The main results are as following:1.The higher biomass was measured in cabbage,corn silage,potatoes and oat plots,and the least biomass was measured in the natural grassland,nitrogen addition can increase grassland system biomass to a certain extent.The greatest fresh:dry ratio of biomass and water content in cabbage were 24.2 and 155.4 t/hm2 respectively.Potatoes,corn silage,and oats had higher water contents,which were 72.4,66.3 and 37.0 t/hm2,respectively.Groundwater has always been the core resource for ecological security and social development of human life,especially in arid regions.Driven by economic interests,vegetables and potatoes that were grown in the local area used a large amount of water and were mainly sold to the other parts of the country or even abroad.The high water usage is a threat to groundwater resources and the local ecological security.2.TN content,TP content and N:P in plant tissue varied significantly among different ecosystems(P<0.05).In general,the average soil organic carbon(SOC,8.01 g kg-1),TN(1.05 g kg-1)and TP(0.33 g kg-1)content in the 0-30cm soil layer in potatoes was significantly lower than that in other ecosystems(P<0.05).The average C:N in 0-30cm soil layer depths,did not showed significant differences(P>0.05)across the ten ecosystems;the average C:P in potato sites was lower than that in cabbage(24.64 vs 33.17,P>0.05),and both were lower than that in other ecosystems.The average N:P in nine ecosystems did not showed significant differences(P>0.05),but were higher than that in potatoes(P<0.05).Above all,N was the limitation for biomass production in local ecosystems.Soil C:P and N:P values decreased significantly with increasing land use intensity.Our findings suggest that properly increasing the use of nitrogen fertilizers and protecting soil organic matter will contribute to the sustainable development of local ecosystems.3.The purchased emergy inputs of the ten ecosystems ranged from 3.53 to 147.67×1014 seJ/ha/yr and that the rain input of 4.78×1014 seJ/ha/yr constituted the highest renewable natural resource emergy.Groundwater is the most important non-renewable resource for ecosystems that need irrigation.The emergy investment ratios(EIR)for potatoes(27.81)and cabbage(19.03)were higher than those of the other ecosystems,but mowed and artificial Chinese leymus grassland had the higher emergy self-sufficiency rates(ESR).Natural grassland,artificial Chinese leymus grassland,corn silage and traditional grain crops had a low environmental load and high sustainability,whereas potatoes,cabbage and oat had a high environmental load and low sustainability.Rain-fed artificial grassland has a high development potential in the local area from the perspective of environment and primary productivity.4.For the maize ecosystem,the sum of renewable natural resources(R)ranged from 0.62×1014 seJ/ha in Xinjiang to 6.34×104 seJ/ha in Guangxi;the sum of nonrenewable natural resources(N)ranged from 3.95×1014 seJ/ha for Henan to 9.46×1014 seJ/ha for Jilin;the sum of purchased renewable resources(PR)ranged from 2.97×1014 seJ/ha for Heilongjiang to 26.14×1014 seJ/ha for Gansu;the sum of purchased nonrenewable resources(PN)ranged from 14.89×1014 seJ/ha in Sichuan to 33.00×1014 seJ/ha in Gansu.The R value of the wheat ecosystem varies from 0.53×1014 seJ/ha in Xinjiang to 3.91 ×1014 seJ/ha in Hubei;the N value is from 3.42×1014 seJ/ha in Henan to 7.61×1014 seJ/ha in Heilongjiang.PR value increased gradually from 3.90×1014 seJ/ha in Heilongjiang to 21.63×1014 seJ/ha in Ningxia;the lowest PN value was 9.90×1014 seJ/ha in Heilongjiang,and the highest was 31.34×1014 in Inner Mongolia.In addition,the environmental loading ratio(maize)in Heilongjiang was the highest,followed by Inner Mongolia(4.79),Jilin(3.95),Liaoning(3.52),the lower values in Guizhou,Chongqing,Guangxi,Yunnan and Sichuan.Similarly,the emergy sustainability index in the provinces of southern China were higher than other provinces.Maize and wheat ecosystem in most provinces are developed but also have a high environmental loading ratio.5.The proportion of natural and purchased emergy input of the maize ecosystem ranged from 13.65%vs 86.35%in Xinjiang to 33.70%vs 66,30%in Heilongjia-ng,which were close to 30%vs 70%,25%vs75%,22%vs 78%,20%vs 80%and 15%vs 85%for Northeast of China(NE),Southwest of China(SW),Loess Plateau(LP),Huang-Huai-Hai Plain(HP)and Northwest of China(NW)respectively.Furthermore,the proportion of natural energy and purchased energy in the wheat ecosystem is close to 40%vs 60%(NE),27%vs 73%(SW),23%vs 77%(the mid and lower reaches of the Yangtze River,YR),18%vs 82%(LP),15%vs 85%(NW)and 14%vs 86%(HP).The natural energy in the maize or wheat ecosystem influenced the quantity and proportion of purchased energy.Overall,natural energy is the basic force for sustaining ecosystem development,and artificial energy input is the direct-cause of ecosystem evolution.Different combinations of natural and purchased emergy were coupled to maintain the same ecosystem under the different environmental conditions.In general,certain natural resources correspond to specific natural ecosystems.With the disturbance of human energy,the natural ecosystem is separated from the original system development model and forms a new ecosystem. 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