Urbanization-induced site condition changes of peri-urban cultivated land in the black soil region of northeast China

Ecological Indicators 80 (2017) 215–223

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Ecological Indicators journal homepage: www.elsevier.com/locate/ecolind

Urbanization-induced site condition changes of peri-urban cultivated land in the black soil region of northeast China

MARK

⁎

Wenbo Li, Dongyan Wang , Hong Li, Shuhan Liu College of Earth Sciences, Jilin University, Changchun 130061, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Site assessment Urbanization Cultivated land Peri-urban agriculture Black soil region

The site condition of cultivated land greatly influences the utilization and management of cultivated land resources and is an element that is disturbed tremendously by urbanization. Since the rejuvenation of the northeast old industrial base strategy in 2003, urbanization in northeast China has progressed rapidly. The excessive urban sprawl has profoundly changed land use structure in the peri-urban area of the black soil region, and the subsequent site condition changes will impede the full utilization of cultivated land resources. This study used the suburb of Changchun Kuancheng District as an empirical case, and employed a patch-scale site assessment system to analyze dynamic changes in cultivated land site conditions at a typical rural-urban interface of the black soil region from 2004 to 2014. Cultivated land loss and land use changes were prominent in the study area and the land conversion rate was shown to be accelerating. Most of the occupied cultivated land was converted to urban areas such as industrial land or urban settlements. However, a part of the occupied cultivated land was left unutilized, which indicates how the urban sprawl is jeopardizing benefits of both urban development and cultivated land protection. Besides direct occupation of cultivated land resources, urbanization has led to a loss of cultivated land with good site conditions and a deterioration of the site conditions of unconverted cultivated land in the peri-urban area. Urbanization has fragmented peri-urban cultivated land, increased farming distance and brought more frequent anthropogenic disturbances. On the other hand, it has also improved transportation conditions and the local ecological environment. As site condition is believed to be closely related to both cultivated land loss and cultivation abandonment, the deterioration will aggravate the loss of cultivated land resources in a disguised form.

1. Introduction Over the past few decades, China has endured dramatic urbanization rates that have highlighted several land utilization issues, in particular, the urban sprawl rate continues to increase even though the population has a limited demand for urban space (Tan et al., 2016; Chen et al., 2015; Wei and Ye, 2014; Chen, 2007). The high urbanization rates have resulted in excessive conversion of cultivated land for construction purposes, and this phenomenon is especially significant in the black soil region of northeast China (Guo et al., 2015), which is one of core grain-producing areas in the country (Zhao et al., 2016; Liu et al., 2012). A direct effect of the urban sprawl has been the loss of high-quality cultivated land, and this has seriously threatened grain security (Kong, 2014; Song et al., 2015). The deterioration of cultivated land quality, which is an indirect effect of urbanization in China, poses another challenge to grain production. Researchers generally agree that urbanization-induced deterioration of cultivated land, which can be noticed through changes such as soil ⁎

Corresponding author. E-mail address: [email protected] (D. Wang).

http://dx.doi.org/10.1016/j.ecolind.2017.05.038 Received 13 February 2017; Received in revised form 10 May 2017; Accepted 13 May 2017 1470-160X/ © 2017 Published by Elsevier Ltd.

degradation (Roy, 2012; Davies and Hall, 2010; Pouyat et al., 2008; Chen, 2007) and irrigation water pollution (Maheshwari and Bristow, 2016; Adhikary and Dash, 2012; Wittmer et al., 2010), will seriously damage cultivated land resources and lower the quality of agricultural products. Furthermore, fragmentation and changes in land use within the cultivated area are reported to be additional obstacles for profitable agricultural production, as they can potentially lead to cultivation abandonment (Janus et al., 2016; Qian et al., 2016; Falco et al., 2010; Niroula and Thapa, 2007). This indicates that landscape metrics and relative location, namely the major site conditions of each cultivated land patch, are crucial indicators that reflect the quality of cultivated land as well as elements that have been significantly disturbed by urbanization. Site condition is a concept originally used in forestry that describes all of the external environment conditions that affect forest productivity (Matyssek et al., 2004; Pinard et al., 1996). However, the term, when applied to the field of agricultural land assessment, describes a collection of factors, except for natural quality factors, that influence

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were interpreted using SPOT multispectral and panchromatic fused images with a spatial resolution of 10m, SPOT panchromatic images with a spatial resolution of 2.5 m and IKONOS multispectral and panchromatic fused images with a spatial resolution of 1m. In accordance with the study requirements, land use was classified into eight categories (Fig. 2): cultivated land (dry land, paddy land and agricultural greenhouses), rural settlement, urban settlement, industrial land (land for industrial manufacturing, storage and mining), transportation land (highways railways and major rural roads), ecological land (garden plot, grassland and forest land has been specified as a green area or wind shelter), water (river and irrigation reservoir) and unutilized land (bare land without vegetation cover or construction).

land quality, such as the distribution, location and coordination of land use (Qian et al., 2016). The American LESA (agricultural Land Evaluation and Site Assessment) system was proposed in 1980s as a way to emphasize the role of site conditions in agricultural land quality evaluations (Dung and Sugumaran, 2005; Tyler et al., 1987). On the basis of this evaluation model, some Chinese scholars have combined traditional cultivated land classification methods with site assessment and applied the evaluation results to the demarcation of prime farmland (Qian et al., 2016; Qian et al., 2015; Bian et al., 2015). There is a general consensus that good and stable site conditions are one of the representative characteristics of high-quality cultivated land. Moreover, cultivated land location and cultivated landscape are considered to be indispensable features of peri-urban cultivated land quality (Bian et al., 2015), and they are the site condition indexes discussed in this study. Black soil is named according to the classification and codes for Chinese soil, and is defined as soils with evident humus accumulation process, similar to Mollisol in the American soil classification system (Li et al., 2017; Li et al., 2016). Characterized by cultivated soil rich in organic matter and superior tillage conditions, the black soil region of northeast China has always been an important area for the production of grain. However, since the rejuvenation of the northeast old industrial base in 2003, the urbanization rate in northeast China has increased (Zhang, 2013) and the need to protect black soil resources has become more apparent. There are noticeable changes in peri-urban land use within the black soil region (Sun et al., 2015; Yi et al., 2014), and the subsequent site condition changes may impede the optimal utilization of the cultivated land resources. The proper functioning of a strict cultivated land protection system requires patch-scale site assessments of peri-urban cultivated land, as well as measurements of spatiotemporal changes in site conditions. This study applies a patch-scale site assessment system for cultivated land to the Changchun Kuancheng District and then analyses dynamic changes in the cultivated land site conditions at a typical rural-urban interface of the black soil region from 2004 to 2014. This study aims to evaluate how urbanizationinduced land use change affects the external utilization conditions of peri-urban cultivated land in order to contribute to the optimization of peri-urban land utilization structure and conservation of black soil resources.

2.3. Modeling site condition for peri-urban cultivated land patches According to a previous peri-urban cultivated land site assessment study (Bian et al., 2015), the site condition of an individual cultivated land patch in this study includes the landscape and relative location of the cultivated land, both of which are directly affected by land use pattern. Shape index, area index and contiguity index of cultivated land patch were selected as landscape features, while transportation convenience, irrigation availability, farming distance, urbanization risk and ecological environment of cultivated land patch were selected to represent the location features. This selection of indexes reflects land use changes that are typical for peri-urban cultivated land. Detailed descriptions of the indexes are shown in Table 1. 2.3.1. Computational method for landscape indexes All the presented methods for the computation of patch-scale landscape indexes are from a previous study (Li et al., 2000).

Frac = 2log(P /4)/log(A) G = P/ A SI = 4/(Frac 2 × G )

(1)

Where P is the perimeter of the cultivated land patch; A is the area of the cultivated land patch; Frac is the fractal dimension, and the theoretical range of Frac is (1,2) when the cultivated land patch is compared to a square patch, with 1 representing a patch shape that is most similar to a square, and 2 representing the most complex shape under the same circumstances; G is the growing index, and G equals 4 when the patch shape is square; moreover, the higher G is, the more elongated the patch will be; SI is the shape index, and the theoretical range of SI is (0,1) when the cultivated land patch is compared to a square patch, with a higher SI indicating a more regular patch shape.

2. Materials and methods 2.1. Description of the study area A suburb of Kuancheng District (Fig. 1) was chosen as the study area for an analysis of the spatiotemporal changes in site conditions of a peri-urban cultivated land patch. The results from this study could then be extended to other peri-urban areas that are strongly influenced by urbanization. Kuancheng District is one of the major districts of Changchun City (124°18′E-127°05′E, 43°05′N-45°15′N), covering a total area of 23800 ha, and is located in the Jilin section of black soil region in northeast China. This region is characterized by high soil fertility and superior agricultural resources. However, Kuancheng District is one of the most developed areas in the black soil region, with the urban area expanding rapidly during the past decade. This has led to land use changes in areas along the urban-agricultural fringe. The selection of a suburb in Kuancheng District as the study area was motivated by the conflict between urban development and agricultural protection in this region, which is typical for urban-rural interfaces in the black soil region that are affected by urbanization. Therefore, the Kuancheng District case study provides evidence for how urbanization can impact cultivated land use in peri-urban areas of the black soil region.

AI = log(Ai-log Amin)/(log Amin-log Amin)

(2)

Where Ai is the area of the cultivated land patch i; Amin and Amax are the minimum and the maximum areas, respectively, of all cultivated land patches during the three years; AI is the area index, and represents the relative size of the patch. The range of AI is [0,1], with a higher AI indicating a relatively larger patch area. ' ' ' CI = (log Ai' -Amin )/(log Amax -log Amin )

(3)

All the cultivated land patches of each year were merged into one patch before the Contiguity Index was computed. Railways, highways, major rural roads, shelter belts and village boundaries were used as landscape corridors to segment the merged cultivated land into a new collection of patches. Where Ai' is the area of newly formed cultivated ' ' and Amax are the minimum and the maximum areas, land patch i; Amin respectively, of all newly formed cultivated land patches during the three years; CI is the contiguity index, which represents relative size of a certain area, namely the relative contiguity of an area in which the cultivated land patch is located. The range of CI is [0,1], with a higher CI indicating relatively higher patch contiguity.

2.2. Data processing Land use information for the study area from 2004, 2009 and 2014 216

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Fig. 1. Distribution of the black soil region in northeast China and the location of the study area.

Fig. 2. Land use maps of the study area in 2004, 2009 and 2014.

Thus, the distance from the center of each cultivated land patch to the source of each location impact factor was calculated. Logarithmic normalization, with AI and CI as references, was used to calculate each of the location indexes. UR was computed via a formula for a positive index, i.e. longer distances indicate better location, while TC, IG, FD

2.3.2. Computational method for location indexes Proximity analyses from the spatial center of each cultivated land patch to major rural roads and highways; water; rural settlements; urban settlements, industrial land and transportation land; and ecological land were performed to compute TC, IG, FD, UR, EE, respectively. Table 1 Site assessment system for peri-urban cultivated land patch. Index type

Index name

Abbreviations

Connotation

Landscape Index (LANI)

Shape Index

SI

Area Index

AI

Contiguity Index

CI

Transportation Convenience

TC

Irrigation Availability

IA

Farming Distance

FD

Urbanization Risk

UR

Ecological Environment

EE

SI indicates the shape complexity of the cultivated land patch. Fractal dimension and growing index were used to evaluate the complexity of the patch shape. A higher SI indicates a more regular shape, as it will be easier to manage and utilize the cultivated land. AI indicates the relative size of each patch. A patch with a higher AI has higher conservation value and production capacity. CI indicates the contiguous degree of multiple cultivated land patches that are closely grouped, and it is one of the most crucial indicators of cultivated land quality. A higher CI indicates that the cultivated land is more concentrated and has a higher degree of contiguity. TC indicates the relative distance from the spatial center of a cultivated land patch to the nearest highway or main rural road, and thus represents the transportation conditions for agricultural products or agricultural mechanical practices. IA indicates the relative distance from the spatial center of a cultivated land patch to the nearest water source, and thus represents the difficulty for obtaining irrigation water. FD indicates the relative distance from the spatial center of a cultivated land patch to the nearest rural settlement, and thus represents the cultivation convenience and accessibility to the cultivated land patch. UR indicates the relative distance from the spatial center of a cultivated land patch to the nearest urban settlement, industrial land or transportation land, and thus represents the human disturbance through urbanization. EE indicates the relative distance from the spatial center of a cultivated land patch to the nearest ecological land, and thus represents the ecological environment of cultivated land patch.

Location Index(LOCI)

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and EE were computed via a formula for a negative index, i.e. shorter distances indicate better location for cultivated land. PI = (log Di-log Dmin)/(log Dmax-log Dmin)

(4)

NI = (log Dmax-log Di)/(log Dmax-log Dmin)

(5)

Where PI is the computed value for positive indexes; NI is the computed value for negative indexes; Di is the distance from the center of cultivated land patch i to the source of the corresponding impact factor; Dmin and Dmax are the minimum and the maximum distances, respectively. The ranges of all location index values are all [0,1], with a higher value indicating better location. 2.3.3. Site assessment model LANI = a × SI + b × AI + c × CI LOCI = d × TC + e × IA + f × FD + g × UR + h × EE SQI = α × LANI + β × LOCI

Fig. 3. Cumulative land use conversion areas within the study area between 2004 and 2014.

(6)

Where LANI is the landscape index, and, according to a previous study (Li et al., 2000), a was set to 0.2, b was set to 0.3 and c was set to 0.5; definition of weights for the calculation of LOCI were obtained by the analytic hierarchy process (AHP), and the judgment matrix passed the consistency test (CR = 0.0009968 < 0.01), indicating a reasonable setting. d–h were set to 0.127, 0.297, 0.322, 0.137 and 0.117, respectively; SQI is the site quality index, and with a site assessment of peri-urban cultivated land (Bian et al., 2015) as reference, α and β were both set to 0.5.

ecological land. Water represented the most stable land use type in the study area, and the slight decline in water body area is mainly attributed to the absence of ground irrigation reservoirs. It can be concluded that cultivated land within the study area is under severe pressure from surrounding urban sprawl, and that the rate of urbanization is accelerating, as shown by the scale of land conversions between 2009 and 2014 surpassing that between 2004 and 2009.

3. Results and analysis

3.2. Site assessment for cultivated land patches

3.1. Land use change from 2004 to 2014

The calculated LANI, LOCI and SQI for cultivated land patches in 2004, 2009 and 2014 were analyzed based on the site assessment model with ArcGIS (10.1, Environmental Systems Research Institute Inc., Redlands, CA, USA). To increase the spatiotemporal differentiation for the assessment of results, each of three indexes of all studied years (2004, 2009 and 2014) were classified into five levels (I, II, III, IV and V, with the index values decreasing from level I to level V) by Jenks natural breaks classification method. The classifications are based on measurements of relative changes in each index between different time periods. The spatial distribution, as well as distribution of classifications, for LANI, LOCI and SQI are shown in Figs. 4 and 5. When spatial distribution (Fig. 4), as well as evident quantity loss of cultivated land from 2004 to 2014, are assessed, the landscape and relative location of cultivated land at the rural-urban interface are affected to different extents, and the impacts of urbanization on the landscape of peri-urban cultivated land are relatively significant. The land use maps (Fig. 2) show that many artificial corridors have been created to split the continuous cultivated land, and this will undoubt-

A statistical analysis of land use within the study area (Table 2 and Fig. 3) showed that the land use structure had changed significantly over the study period. The loss of cultivated land resources and rural settlements is broadly apparent from 2004 to 2014, a finding that reflects the rapid non-agriculturalization process in the study area. The area of converted cultivated land from 2004 to 2009 is 1295.94 ha (8.59% of total cultivated land area) and the converted area rose to 2750.80 ha (19.94% of total cultivated land area) from 2009 to 2014 (Table 2). Industrial land, urban settlement, transportation land and unutilized land were the most common land use types that cultivated land was converted to during the study period (Fig. 3). In addition, more than half of the unutilized land in 2004 remained unconverted in 2014, and the total area of unutilized land increased significantly between 2004 and 2014. Moreover, to response to an appeal for more ecological areas, the construction project of Beihu Park, which began in 2010, has resulted in a large conversion of cultivated land into Table 2 Land use area and percentage changes within the study area between 2004 and 2014. Land use type

Cultivated land Rural settlement Urban settlement Industrial land Transportation land Ecological land Water Unutilized land

Area (in ha)

Area changes (in ha)

Percentage changes (%)

2004

2009

2014

2004–2009

2009–2014

2004–2009

2009–2014

15088.82 1897.02 0.00 101.74 196.55 1053.91 576.07 146.49

13792.88 1938.69 56.14 529.07 407.63 1286.71 567.10 482.38

11042.08 1581.38 408.65 1146.00 807.73 2529.61 539.70 1005.45

−1295.94 41.67 56.14 427.33 211.08 232.80 −8.97 335.89

−2750.80 −357.31 352.51 616.93 400.10 1242.90 −27.40 523.07

−8.59 2.20 – 420.02 107.39 22.09 −1.56 229.29

−19.94 −18.43 627.91 116.61 98.15 96.60 −4.83 108.44

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Fig. 4. Landscape index(LANI), location index(LOCI) and site quality index(SQI) assessment results for cultivated land.

edly result in the fragmentation of cultivated land and increase the frequency of urbanization-induced anthropogenic disturbances, such as industrial discharge, household refuse and traffic emissions. Meanwhile, a slight increase in the cultivated land area that falls under levels I and II in LOCI (Fig. 5) indicates that there is potential regional improvement of the location conditions for cultivated land under urbanization pressure. However, the area of cultivated land that falls under level V in LOCI also increased from 2004 to 2014. The percentage of level I areas under SQI decreased from 40.70% to 16.63% during the 2004–2014 period. The percentage of level II areas under SQI fell from 39.60% to 36.50% during the 2004–2014 period, and the area percentages of all the lower-quality level areas increased during the study period. There is a general descending trend in cultivated land site quality due to the land conversions between 2004 and 2014.

Table 3 The total areas of each site level that were either converted to or from cultivated land during 2004–2014. Level

I II III IV V In total

Converted to cultivated land (in ha)

Converted from cultivated land (in ha)

LANI

LOCI

SQI

LANI

LOCI

SQI

11.41 91.53 150.27 95.80 12.71 361.72

39.79 111.54 47.46 54.60 108.33 361.72

23.93 91.01 114.12 97.80 34.86 361.72

2942.91 898.76 438.99 112.56 15.24 4408.46

104.68 427.66 1161.31 1902.44 812.37 4408.46

2004.66 1620.96 620.79 157.41 4.64 4408.46

of surrounding areas contributing to cultivation difficulty. On the other hand, cultivated land that was converted to other land uses between 2004 and 2014 had relatively better site conditions, and it is clear that most of the cultivated land that was lost during urban sprawl had better tillage conditions, which is in accordance with results reported by Liu et al. (2013). The landscape index(LANI), location index(LOCI) and site quality index(SQI) assessment results were used to identify unconverted cultivated land during different time intervals (from 2004 to 2009, from 2009 to 2014) and analyze the spatiotemporal variation of

3.3. Site condition changes of the cultivated land from 2004 to 2014 The total area of cultivated land that was converted between 2004 and 2014 was over 12 times the area of cultivated land that was added to the study area during the same interval (Table 3). When the converted cultivated land (Table 3) is analyzed, a majority of the land that is converted to cultivated land has relatively poor site conditions. Furthermore, the cultivated land that was added was generally smallscale, irregular in shape and not concentrated, with the land use change

Fig. 5. Accumulative histogram for the percentages of level I, II, III, IV and IV areas in LANI, LOCI and SQI.

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Fig. 6. The level changes of unconverted cultivated land at different time intervals based on LANI, LOCI and SQI results.

Furthermore, residual cultivated land patches in urban fringe areas have worse site conditions than patches in other areas (Fig. 6). In summary, the pressure of urbanization is leading to a loss of cultivated land with better site conditions and a deterioration in the site conditions of unconverted cultivated land in peri-urban areas, while the site conditions of the cultivated land that is added are rather inadequate.

cultivated land site conditions (Fig. 6). Level changes were classified to three types: promotion (from a lower index level to a higher index level); constant (the assessment result stays within the same level); and demotion (from a higher index level to a lower index level). The results for unconverted cultivated land (Fig. 6) showed that peri-urban cultivated landscape mostly deteriorated during land use changes in terms of location features, and the urban fringe, where land was often converted, was a hot spot for cultivated land fragmentation. The cultivated landscape of the central part of the study area mostly deteriorated between 2004 and 2009, whereas the landscape of eastern and western fringe areas mostly deteriorated between 2009 and 2014. LANI showed slightly more intense changes than LOCI, yet some regional improvements in the relative location of peri-urban cultivated land can be observed from Fig. 6. Moreover, the loss of rural settlements in the east of the study area can be easily noticed in all three indexes. It can be concluded that urbanization contributes to the deterioration of site conditions for peri-urban cultivated land, as 26.264% of unconverted cultivated land from 2004 to 2009 and 27.826% of unconverted cultivated land from 2009 to 2014 showed a demotion in SQI.

3.4. Variations in site condition index during urbanization The statistics for individual indexes (Fig. 7) demonstrate that the average of SI, AI and CI decreased from 2004 to 2014. This indicates the fragmentation of peri-urban cultivated land, namely, that the shape of cultivated lands grew more complex, the average area of cultivated land was decreased and cultivated land pieces that were contiguous became segmented. All of these changes make the development and utilization of cultivated land resources more difficult. In addition, the spatial distribution characteristics identified in the site assessment (Figs. 4 and 6) show that cultivated land patches located at the urban fringe, where 220

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cultivated land mostly consider the probability of irrigation, transportation accessibility, land use regime, farm distance, relative location to major farm markets or towns, and the landscape features of a cultivated land patch (Qian et al., 2015; Bian et al., 2015). Hence, the spatial form of a plot, as well as the relationship with the land use of surrounding plots, constitute the main factors that are considered during site assessment for cultivated land. Moreover, unlike other external conditions of cultivated land use that can be subjective, such as labor force input and farm investments, the site conditions of cultivated land discussed in this paper are directly determined by the regional land use pattern. Index selection in this study was closely related to the actual land use structure in the peri-urban area. Location indexes that measured distance to major farm markets or towns were excluded because of the crop production and sales mechanisms in the black soil region along with the small-scale nature of this study. Grain is the staple crop in most of the black soil region, and a distinct mechanism of grain collection and storage in China determines normal farm markets and puts less pressure on regional grain production. Site condition stability for cultivated land has been emphasized by many scholars on account of its role in maintaining a system of prime farmland and healthy grain production (Bian et al., 2015). However, rapid land conversions around urban fringe areas make it nearly impossible to maintain stable site conditions in peri-urban areas. Under such circumstances, site assessment has been heralded as an efficient instrument for demarcating prime farmland around the urban fringe. This method could be very beneficial for protecting cultivated land resources in peri-urban areas and restricting disordered urban sprawl, as assessments could specify prime farmland areas, which, under Chinese law, cannot be occupied and transformed into industrial land for urbanization (Bian et al., 2015; Liu et al., 2013). Thus, the assessment and monitoring of the regional site conditions of cultivated land is paramount in the context of rapid urbanization, and is especially important for cultivated land that is located in the peri-urban area.

Fig. 7. Statistics for all indexes in 2004, 2009 and 2014. Total patch numbers in 2004, 2009 and 2014 were 1918, 2084 and 2108, respectively. The error bars indicate the standard deviation.

land conversion is more common, comprise an area characterized by the heaviest fragmentation. The results illustrated in Fig. 7 also indicate that the main benefits of urbanization are improvements to transportation conditions and ecological environment. Improvements resulting from land use changes within the study area (Fig. 2) could mainly be attributed to the development of highways and major rural roads, as well as the restoration of the regional ecological environment. The increase in UR from 2004 to 2014 indicates that urbanization has increased the utilization risk for cultivated land, reflecting an intensification in anthropogenic disturbance and potential soil pollution, both of which will negatively impact the surrounding cultivated land (Biro et al., 2013; Ouyang et al., 2013; Emadi et al., 2009). A slight change of irrigation availability for cultivated land was noticed, and this was based on the stability of water (Table 2) during the study period. There was an increase in small rural settlements between 2004 and 2009, but a substantial decrease in the total area of rural settlements between 2009 and 2014 (Table 2), which can be attributed to the loss of rural settlements in the southwest region of the study area. This change has substantially increased the farming distance for cultivation in this area, a dynamic that may deter people from cultivating land in this area.

4.2. Cultivated land loss in peri-urban area Peri-urban agriculture is an important source of local farm produce and a vital element of any metropolis, accounting for a great proportion of the world’s food supply (Maheshwari and Bristow, 2016; Pribadi and Pauleit, 2015). The peri-urban area is characterized by an aggregation of small-scale agricultural production, scattered industrial manufacturers and flowing residential settlements (Li et al., 2017; Lin et al., 2016; Dossa et al., 2015; Ives and Kendal, 2013), and the land use structure is unstable. For this reason, the peri-urban area is the place where cultivated land is most affected by the urban sprawl. Research discussing the impacts of urbanization on cultivated land resources in China has always attracted controversy. Although occupation by urban sprawl is one of the primary pathways that leads to cultivated land loss, some have argued that the total amount of cultivated land was only slightly affected due to new additions of cultivated land that were converted from forests, grassland or other land use types (Song and Pijanowski, 2014; Deng et al., 2006). However, peri-urban agriculture and cultivated land resources around the urban fringe have indeed suffered from urban sprawl (Wästfelt and Zhang, 2016; Li, 2015). An empirical case of suburban Changchun Kuancheng District indicates that there is significant occupation of cultivated land by urban land types including urban settlements, industrial land and transportation land, along with ecological land developed for urban green spaces. Furthermore, it was shown that the urban sprawl rate is gradually accelerating. In addition, an increase in unutilized land also indicates that a part of occupied cultivated lands in peri-urban area will become unused vacant lots after the conversion and remain either unused or underused for a long time. The disjunction between cultivated land occupation and land utilization management results in wasted urban space, which will jeopardize the benefits of both urban development and cultivated land protection.

4. Discussion 4.1. Site assessment of cultivated land and its importance Site condition originates from forestry, and is defined as a comprehensive embodiment of all the environmental factors that influence the formation and development of forest, such as landform, soil properties, hydrology, and biology, among others (Matyssek et al., 2004; Pinard et al., 1996). The extension of the concept of site condition to site assessments of land use was first systematically proposed in the land evaluation and site assessment (LESA) by Soil Conservation Service, USA (Tyler et al., 1987). The land evaluation portion of LESA was used to assess the agricultural potential of land plots, with soil properties and agricultural infrastructure as the main determinants. In contrast, the site assessment portion relied heavily on external conditions for cultivated land use, such as location and farming convenience (Qian et al., 2016; Qian et al., 2015; Tyler et al., 1987). Thus, it integrated conventional quality evaluations of agricultural land, which value soil properties, with utilization conditions. Moreover, in comparison with conventional assessment indexes for agricultural land, site assessment, which has long been neglected, considers the external utilization conditions of cultivated land (Liu et al., 2013). Site assessment and its related indexes of cultivated land are now widely applied in the planning and management of cultivated land resources in China, with concentrated and continuous land considered to favor cultivation and site conditions used to represent the characteristics of prime and high-quality cultivated land (Qian et al., 2016; Qian et al., 2015; Bian et al., 2015). Current site assessment systems for 221

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settlements has increased the farming distance for the cultivated land that is located around the urban fringe, and rural-urban migration and population outflow in northeast China has reduced the labor force input (You et al., 2017; Liu et al., 2015; Zhang, 2013). When this is considered along with severe fragmentation, the cultivated lands located in the urban fringe zone have an even higher risk of being abandoned by landowners. Thus, the potential risk of abandonment, which has been triggered by urbanization-induced land fragmentation and changes in farming conditions, may have a larger impact that the quantity of cultivated land lost.

As one of the most crucial bases of grain production in the country, the black soil region in northeast China is nevertheless experiencing rapid urbanization since the rejuvenation of the northeast old industrial base strategy in 2003. It is distressing that the urbanization modes have mostly been reported to be excessive and inefficient (Guo et al., 2015; Zhang, 2013). This type of land conversion will increase the pressure on agriculture development and cultivated land conservation in peri-urban areas of black soil region. It would be important to involve the quality of cultivated land in assessments of the urbanization impacts (Song and Pijanowski, 2014). An equilibrium between quality and quantity needs to be achieved to ensure a safe supply of food because currently the urban sprawl is occupying high-quality cultivated land in peri-urban areas and the cultivated land that is added is of insufficient quality, i.e. remote, polluted, or difficult to manage (Song and Pijanowski, 2014; Kong, 2014).

5. Conclusion The urban sprawl has exerted a far-reaching influence on peri-urban agriculture and cultivated land in the black soil region of China, shown through the occupation of cultivated land resources for urban purposes. In the context of rapid and excessive urbanization in northeast China, the monitoring of cultivated land resources appears to be vital to the conservation and utilization of cultivated land under the pressure of urbanization. However, unlike soil properties or other internal influence factors, site conditions of cultivated land have been described as crucial, yet are rarely mentioned in most assessments and dynamic monitoring of cultivated land quality. Moreover, the unstable land use structure in peri-urban area jeopardizes the stability of cultivated land site conditions, and, in this way, the external utilization conditions for cultivated land are altered. Thus, the assessment and monitoring of regional site conditions of cultivated land is important, in particular for cultivated land located in peri-urban areas. This study quantified a patch-scale site assessment for peri-urban cultivated land in a suburb of Changchun Kuancheng District and measured spatiotemporal changes in this site. The results showed that cultivated land loss in the study area was prominent, and most areas were converted to urban areas, for example, industrial land or urban settlements. However, not all of the occupied cultivated land was put to good use after the conversion, and inefficient utilization of the land created by urban sprawl is jeopardizing the benefits of both urban development and cultivated land protection. Furthermore, the cultivated land loss demonstrated a deterioration process for external site conditions, particularly the landscape of cultivated land, with the urban fringe identified to be a hot spot for the deterioration. Urbanization has exposed peri-urban cultivated land to severe fragmentation, increased the farming distance, and intensified anthropogenic disturbance. On the other hand, it has also improved transportation conditions and the local ecological environment. The patch-scale site assessment and its monitoring uncovered the process of cultivated land fragmentation and variations in the fundamental agricultural production environment, both of which underlie the urbanization-induced site condition changes in peri-urban cultivated land, which have been reported to lead to cultivation abandonment, increase tillage costs, and make cultivation more difficult (Latruffe and Piet, 2014; Falco et al., 2010; Niroula and Thapa, 2007). When the reduction in the labor force caused by rural-urban migration and population outflow in northeast China is also considered, the loss of cultivated land resources will pose more of a threat to food security than the direct occupation of cultivated land by urbanization in the black soil region.

4.3. Site condition change under the pressure of urbanization Two crucial issues underlie site condition changes of cultivated land under the pressure of urbanization, cultivated land fragmentation and variations in the fundamental agricultural production environment. Cultivated land fragmentation is an inevitable side-effect of urban sprawl; along with cultivated land loss, land patches will be separated by the newly-created corridors like highways or greenbelts. Cultivated landscape metrics, which are different from the concept of landscape ecology and employ a different methodology, emphasize how fragmentation impacts cultivated land use (Pei et al., 2014). Fragmentation has long been considered to be a barrier for cultivation, as it leads to potential harvest losses (Latruffe and Piet, 2014), increases organization and labor costs (Latruffe and Piet, 2014; Falco et al., 2010), reduces farm profitability (Falco et al., 2010) and land use inefficiency (Manjunatha et al., 2013; Deininger et al., 2012). Moreover, it has been shown that landscape metrics for site assessment are closely related to cultivated land loss, and the patch size and shape index of cultivated land are positively correlated with the loss (Liu et al., 2013). According to an actual field survey of the study area in 2015 and 2016, cultivated lands around urban fringe with deteriorating site conditions are also the underused cultivated land (Li et al., 2017). The landowners at the urban-rural fringe are more inclined to outsource cultivated land at a low rent and transfer to the city as migrant rural workers. As a result, nominal crop cultivation dominates in these areas, and this dynamic has the potential to reduce outputs of grain production. According to interviews with local farmers, this phenomenon arises from the imbalance between extra investment into land management and poor output from small-scale agricultural production. Meanwhile, these fragmented patches are also in close proximity to the urban builtup area, and are thus more likely to be expropriated for urban construction. Moreover, the underuse can also be attributed to the risk of land loss and crop removal prior to harvest, which is another drawback of highly fragmented cultivated land located in the urban fringe. Hence, the functioning of fragmented cultivated land decreases due to the growing unwillingness of cultivation, which then intensifies the potential loss of cultivated land resources under impacts of urbanization. Apart from the physical form change of cultivated land, relative location is another major physical change that arises from variations in land use pattern. The land use of surrounding plots forms one of the determinants that affects the utilization of cultivated land (Qian et al., 2016; Bian et al., 2015; Abelairas-Etxebarria and Astorkiza, 2012). However, it is difficult to determine how changes in the surrounding land use impact cultivated land. On the one hand, the construction of highways and green belts surely improves transportation convenience of agricultural products and ecological environment for cultivation. On the other hand, vehicular exhaust, pollution and the chemicals used for cleaning the roads are all potential hazards for the surrounding cultivated land (Chambers et al., 2016). In addition, loss of rural

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