| Black soils in NE China have long played an essential role in food and ecological security strategies.The continuing encroachment and assimilation of urbanization on cultivated black-soil land in urban–rural transition zones is becoming a contentious issue.The coordination of urban development space,grain production space,and ecological service space poses a severe challenge to the scientific utilization and management of cultivated land in urban–rural transition zones.Scientific understanding of cultivated-land functions is a prerequisite for its protection and provides a basis for building urban–rural ecological security barriers.There has been a lack of clarity regarding the evolution of black-soil urban–rural transition zones,and restrictions on the use of cultivated land functions with changes in eco-geological conditions under urbanization have become increasingly evident;the development of assessment systems for cultivated land multifunctionality at local scales which can serve the fine agriculture is ongoing.Both urban and cultivated land are acknowledged as being the lithosphere transformed by modern human technology and are important components of the eco-geological system.Geochemistry provides crucial indices of the productive capacity and environmental changes of cultivated land,recording lithospheric information and the shaping effect of human activities on eco-geological conditions,and providing an effective means of investigating cultivated-land multifunctionality.This study was undertaken in Changchun City,a typical black-soil region of Songliao Basin in NE China,where urban–rural transition zones were identified from 1990 to 2020using the land-use information entropy model and central gravitational agglomeration method,with spatial evolution patterns being tracked through long-term trends.A representative urban–rural transition area of 15×16 km with continuous cultivated land was selected as the study sample area.Based on systematic field investigation and soil/crop sampling and analysis,the spatial heterogeneity of soil/crop elemental geochemistry was described,together with the consideration of urbanization mechanisms.Evaluation indices and evaluation models for cultivated-land multifunctionality were developed on the basis of ecological geochemical characteristics.Trade-off and synergy analyses of cultivated-land multifunctionality and coordinated development under urbanized land-use patterns were undertaken using a bivariate local Moran’s I method.Through these analyses,proposals for the utilization,protection,and management of cultivated land in urban–rural transition zones were developed by integrating results according to spatial heterogeneity,correlation,and dependence of cultivated land functions under pressure of urbanization.The overall aim was to verify existing theoretical frameworks and methodology for multifunctional cognition and to provide practical references for cultivated-land protection in black-soil regions,alleviating the conflict between areas for urban development,grain production,and ecological services.The main conclusions of the study were as follows.（1）The dynamic evolution of urban–rural transition zones has a major impact on the reshaping of spatial patterns of eco-geological conditions.The urban–rural transition zone of Changchun City is a typical ring–belt system with outreach features.Spatial changes over the last 30 years have been characterized by“inner-circle subtraction”,“internal-hollow filling”,“leaping expansion”,and“extension spreading”.The zone has exhibited an“increase–decrease–increase”trend,especially during 1995–2000 when the area-change ratio and expansion rate peaked together at 33.14%and 66.81 km2 yr-1,respectively.With the continuous expansion of the zone,spatial patterns became increasingly clustered with frequent transformations between cultivated,forest,and construction land,but the structure of cultivated land remained relatively stable,with its proportions in stable,expanded,and reduced urban–rural transition zones remaining above 51%,65%,and 46%,respectively.Landscape patterns displayed highly fragmented,complex,discrete,and heterogeneous characteristics,especially in stable and expanded urban–rural transition zones.Urbanization promoted internal reduction and external expansion of the zone while continuously shaping its land use and landscape structure,resulting in the continuous reconstruction of original eco-geological conditions.（2）There has been an elemental geochemical response to changes in external eco-geological conditions in the urban–rural transition zone.Spatial variations in nutrient indices for N,Mg,Fe,Mo,Mn,and AP displayed obvious urban–rural trends.Soil heavy-metal（HM）concentrations varied widely in the soil–crop three-dimensional system,with a spatial mismatch between total,available,and accumulated HM concentrations.Total As,Cr,Ni,Cu,and Zn and available Cr and Ni concentrations in soils and accumulated concentrations of Hg and Pb in crops tended to decrease along the urban–rural gradient,with higher enrichment near urban areas.Gradient analysis indicated that spatial differences and variations in elemental geochemical indices were distinct,with a 10 km distance between urban and rural boundaries the principal variation nodes of nutrient and HM indices.SEM regression analysis indicated that land-use types such as recent transportation land;the Yitong River;urban and rural settlement;land for food,medical,and chemical industries;building materials and machinery manufacturing land;and industrial service land were closely related to elemental geochemical changes of cultivated land.Urbanization thus had a major impact on elemental concentrations and spatial distribution patterns while reshaping the external eco-geological conditions of cultivated land.（3）The ecological geochemical evaluation system quantitatively identifies cultivated-land multifunctionality in the urban–rural transition zone.The black-soil urban–rural transition zone is a gray-box system for quantitative identification of cultivated-land functions,where the availability of function-evaluation data is poor and does not reflect differences in cultivated land at local scales because of administrative boundaries.Taking advantage of elemental geochemical indices for productive functions of cultivated land,an entropy–TOPSIS model based on a minimum data set（MDS）for soil nutrient elements was constructed to evaluate the productive functions of cultivated land.The reliability of the evaluation system was assessed by linear fitting between MDS evaluation result and the total data set evaluation result.Results indicated that the productive-function value varied from 0.194 to 0.556,with the production capacity of the studied cultivated land being predominantly of low–moderate grade（gradesⅢandⅣ）.The spatial fluctuation of productive function had a clear urban–rural imbalance,with hot spots occurring mainly within 2–10 km of the urban periphery.Based on crop-output maintenance and environmental contamination by HMs,a three-dimensional comprehensive evaluation system of the cultivated-land ecological function in soil–crop system was developed using the catastrophe progression method and a fusion algorithm.The reliability of the system for cultivated-land ecological function was tested through spatial comparison and correlation analysis of trinity evaluation results for ecological function,and conventional assessment methods for cultivated-land environmental quality such as the pollution load index,potential ecological risk index,Nemerow comprehensive index,and impact indices of comprehensive quality for soil and crop.The evaluation of ecological function indicated that cultivated land in the study sample area had sound ecological environmental conditions at moderate–high quality levels,whereas the ecological function for cultivated land was relatively low near urban areas,increasing toward rural areas.（4）Spatial correlations of cultivated-land multifunctionality are clear,with patterns depending on external effects of urbanization.Results of the bivariate Moran’s I analysis indicated that in 54.86%of the cultivated area,productive and ecological functions were spatially correlated with a strong trade-off relationship（high–low（H–L）and L–H）,9.12 times the strength of the synergistic relationship（H–H and L–L）.High productive-function–low ecological-function（H–L）areas were mainly in central and southern parts of the study area,and L–H areas in northern and southeastern parts,whereas synergistic domains were scattered across the area.The spatial-response relationship between cultivated-land functions and urban land-use types demonstrated that the externality of urbanization has affected the coordinated-development pattern of the two cultivated-land functions to different degrees while changing eco-geological conditions in the urban–rural transition zone.In particular,cultivated land near the city had high levels of agricultural-production nutrients and favorable geographical conditions,coinciding with a high productive-function coordinated development area.However,this area also included a low ecological-function coordinated development area—a contradiction between urbanization and cultivation.In contrast,cultivated land near rural areas was subjected to relatively little urbanization pressure,with high ecological-function coordinated development and low productive-function coordinated development areas being concentrated there.（5）Zoning utilization and differentiated management of cultivated land in the urban–rural transition zone is a feasible path to sustainable development of regional black soils.The clarification of spatial heterogeneity patterns and mutual feedbacks of cultivated-land multifunctionality,starting from characteristics such as subjectivity,multiple appropriateness,and conflict association,cultivated land in the study sample area was partitioned using multiple spatial relationships such as hot and cold spots of cultivated-land functions,trade-off and synergy of cultivated-land functions,and spatial coordination of cultivated-land functions and urbanization.Zoning principles for functional utilization of cultivated land were formulated on the basis of sustainable development of peri-urban agriculture and clarification of the functional characteristics of each cultivated-land unit,including the encouragement of synergistic development of cultivated land,promotion of positive dependence of multifunctional hot spots on urbanized land-use patterns,reduction of unnecessary multifunctional trade-offs,and enhancement of the ecological use of cultivated land in the urban–rural transition zone.Cultivated land was divided into three agricultural-use zones,namely an agricultural production zone,a recreational agricultural development zone,and a potential functional exploitation zone.Considering each type of zone,programs such as promotion of intensive,efficient,and clean agricultural production;development of organic and leisure agricultures;and graded prevention,control,and adaptive adjustment of peri-urban cultivated land give full consideration to relative advantages of the regional cultivated-land functions.In the context of the contradiction between cultivated land protection and regional development in a black-soil region,this study provides an effective means of alleviating the contradiction between agricultural production,urban development,and ecological services,stabilizing the security of urban and rural ecosystems. |
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