Characteristics of land use conversion in soda saline-alkali soil region of central Northeast China

ZHANG Haobin ,YANG Changbao ,GUO Huali and HOU Guanglei

1.College of Geo-Exploration Science and Technology,Jilin University,Changchun 130026,China;

2.Northeast Institute of Geography and Agroecology,Chinese Academy of Sciences,Changchun 130102,China;

3.Shaanxi Satellite Application Center for Ministry of Natural Resources of the PRC,Xi'an 710054,China

Abstract:Since the 1970s,the soda saline-alkali soil region,which is located in the semiarid-arid region of the central Northeast China,has experienced drastic climate variability.Meanwhile,human activities and socio-economic development have led to an increase in water consumption.These factors have brought out considerable land use change and a lot of soil salinize-alkalization.The land use types were obtained from remote sensing images interpretation based on Landsat MSS in 1970s,TM,ETM in 1990s and 2010s.The characteristics of land use conversion and its response to climate change and influence of human activities were explored in the study area during 1970-2010.The results include:(1) The land use types had changed significantly,especially farmland and grassland,with annual growth rates of 0.31% and 0.35%,respectively.In terms of the saline-alkali land,the area increased greatly during 1970-1990,and its increase was mainly from grassland.There was a decrease from 1990 to 2010,and the loss of saline-alkali was replaced by farmland and grassland.(2) Climate change in the study area is obvious,showing a trend of decreasing precipitation and increasing temperature.The change of saline-alkali land area is closely related to the precipitation,with R2=0.78,passed a passing the 0.01 significance test.(3) The correlation of land use degree index and human activity index with saline-alkali land was not obvious,and the changes in these indices only reflected the land use patterns.Under sufficient rainfall,the saline-alkali land was developed into farmland by humans,which caused the decrease of the saline-alkali land.Therefore,climate change is the main driving force of land salinization in the study area.

Keywords:land use change;soda saline-alkali soil region;climate change;land use degree index

Introduction

Land-Use and Land-Cover Change (LUCC) as a basic parameter,was used to quantify the changes in natural environment and human activity and considered as different types of transformation by human activity on the Earth’s surface (Fu,2003;Costanzaetal.,2014;Ellis,2013;Arsanjaniet al.,2015).It affects climates,biogeochemistry and biological diversities profoundly (Salazaret al.,2015;Zhanget al.,2018;Hao &Ren,2009;Seitzingeret al.,2015;Karaliet al.,2011;Song &Deng,2017).These impacts would be far beyond spatial and temporal scales (Shaoet al.,2016).The response of the eco-environment to LUCC became the core problem in the fields of global change,especially in semi-arid and arid regions,because those eco-environments are very fragile (Baiet al.,2014;Jianget al.,2015).The soda saline-alkali soil region,located in semi-arid and arid of the central Northeast China,experienced rapid economic development and population growth and drastic climate variability,and the influence of multiple drivers has led to a dramatic change in land use types in this region (Zheng &Sun,2011).

Large-scale changes in land use over long time scales can induce significant variations in soil physicochemical properties (Gamboa &Galicia,2012),particularly in the arid and semi-arid regions.The central Northeast China is one of the three largest soda saline-alkali soil regions in the world.The rapidly growing population is under high pressure to supply the required food,and the climate conditions have changed dramatically over the past decades.These pressures have led to a change in land use,which infulences soil properties (Zheng &Sun,2011).Therefore,it is important to timely detect land use conversion in saline-alkali soil region,which affects the severity and extent of saline-alkali soil and assess the infulence of the climate change and human activity.

In this paper,we first analyzed the LUCC changes in soda saline-alkali soil region of the central Northeast China from 1970 to 2010,and then assessed the relationship between the LUCC and driver factors including climate factors (annual precipitation and average annual temperature) and human activity indices (Comprehensive index of land use degree and Human activity intensity index).The ultimate objective is to improve the understanding of important factors influencing the change of saline-alkali soil and try to provide a scientific basis for the development and protection of land resources in soda saline-alkali soil region of the central Northeast China.

The central Northeast China is the most severely saline-alkali soil region,which consists of a piedmont area in the west and a central low plain,including parts of Heilongjiang,Jilin,Liaoning and Inner Mongolia Autonomous Region.Meanwhile,these regions are the most important food production areas and commodity grain bases.Controlled by the East Asian monsoon,the mean annual precipitation ranges from 300 to 450 mm,but the precipitation varies greatly within the annual cycle,mainly concentrating in the folod season (June to September),and only 10%-20% of the precipitation occurring in spring.The evaporation is between 1 400-2 000 mm,more than 40% of which occurs in the spring (Wang &Ripley,1997;Zhanget al.,2011).The long duration and large area of drought cause great losses to agricultural production and economic development in the study area.

1 Materials and methods

1.1 Materials

1.1.1 LUCC data

The land use data were derived from better-quality cloudless images in Landsat MSS in 1970s,Landsat TM in 1990s and Landsat TM,ETM in 2010s.Through geometric correction of Landsat images,the root mean square (RMS) error was controlled below 0.5 pixels,and the relevant coefficient method was used to perform radiometric calibration of the selected Landsat images.Considering that saline-alkali land is an important land use type in the study area,we extracted it as a separate category.Based on ENVI and ArcGIS software,Landsat images were supervised and classified to obtain land use classification information,and six land use types were extracted from Landsat images,including farmland (FL),forestland (F),grassland (G),wetland (W),saline-alkali land (S),and other land type (O) (sand,residential land,etc.).And the previous Google Earth map was used to verify the classifciation accuracy,and the overall accuracy of the classification is 89.3% in 2010.

1.1.2 Climate data

The temperature and precipitation data were obtained from 20 meteorological stations of the China Meteorological Administration in the study area (Fig.1).The meteorological data were downloaded,and the average annual temperature and precipitation were calculated for every station.

Fig.1 The location of the study area

1.2 Methods

1.2.1 Regression analysis

The regression analysis method to detect the annual trend changes in temperature and precipitation and explore the relationship between climate change and saline-alkali land change by analyzing the changing trend.

1.2.2 Comprehensive index of land use degree

The comprehensive index of land use degree is used to refelct the natural attributes of land use,which also reflects the comprehensive effects of human factors and the natural environment.This method divides the land use types into four indicators of different levels according to different land use patterns (Wang,2000).Therefore,the farmland,grassland and wetland are the main sources and destinations of the salinealkali land change.We divided the land use types into three levels:The first grade is saline-alkali land;the second grade is forestland,grassland and wetland;the third grade is farmland.The calculation equation of comprehensive land use index is as follows:

WhereLDis the comprehensive index of land use degree;Aiis thei-th land use grading index;Ciis the grading area percentage of land use degree;nis the grading index of land area utilization degree.

1.2.3 Human activity intensity index

The human activity intensity index was used to evaluate the impact of human activities on LUCC in the soda saline-alkali soil region of the central Northeast China (Shi &Ehlers,1996).It is used to reflect how much human activity affects land use change,and usually represents the socio-economic index.The index is calculated as follows:

WhereUis the total area converted from one land use type (forestland,grassland,water body and unused land) to another land use type;Ris the total area of that land use type;dtis the time interval between two phases.In this paper,we use one year as the unit of the time interval.

2 Results and discussion

2.1 Land use conversion

In the past 40 years,the land use in soda salinealkali soil region of the central Northeast China experienced dramatic changes.Table 1 showed that from 1970 to 2010,the change in land use in different periods,the increased areas of farmland,forestland and saline-alkali land were 9 827.59 km2，1 382.19 km2and 668.18 km2,respectively.The annual increase ratios of the three types were farmland (0.31%) ＞forestland (0.14%) ＞ saline-alkali soil (0.12%).The increase in the farmland was mainly concentrated in 1970-1990,about 6 626.21 km2,while the increasein forest land was mainly concentrated in 1990-2010,about 1 120.41 km2.The reason for this change was that the rapid population growth requiring more food,leading to an increase in the farmland during 1970-1990,while the forestland increase was due to the implementation of the protection forest project from 1990 to 2010 (Wanget al.,2010).Furthermore,those changes were also in line with the law of national economic development.During 1970-1990,agriculture played a major role in local economic development,which caused an increase in farmland.Since 1990,more attention has been paid to the protection of ecological environment,and a lot of ecological lands like wetland,grassland were strictly forbidden to convert to farmland.The increased area of saline-alkali land was mainly in 1970-1990,about 2 311.49 km2.Though there was a little decrease in the saline-alkali land from 1990 to 2010,the total area increased by about 668.18 km2.

Table 1 LUCC for three periods

The change in grassland,water body and other type land showed a decreasing trend during 1970-2010,and the annual reduction ratio was 0.35% for grassland,0.26% for wetland and 0.15% for other type land,respectively.The grassland decreased similarly in two periods,about 4 695.66 km2during 1970-1990 versus 4 234.28 km2in 1990-2010.The area of wetland continued to decline during 1970-2010,with about 2 239.30 km2loss.The area of the other type land decreased in 1970-1990 and increased in 1990-2010,but total area lost was 708.77 km2.

Fig.2 shows six land use classification for three periods in the study area.The saline-alkali land was concentrated in the center part of the study area in 1970s.Subsequently,it expanded to south and west during 1990-2010.Saline-alkali land expansion resulted in landscape fragmentation of the farmland and grassland.On the other hand,wetland in the center of the study area rapidly reduced,which caused salinity in the lower layer of the soil to infiltrate into the surface,and those were the main source of the salinity,leading to the increase of saline-alkali land.

Fig.2 Land use type classification in three periods

To clearly show which land use types were converted to saline-alkali land and where the saline-alkali land was lost,we built transfer matrix to discuss the LUCC for two periods.Table 2 and Table 3 showed the amount of change in 1970-1990,1990-2010,respectively.The values on the diagonal of the matrix were those land use types with no area change (persistence)during that period,and the off-diagonals showed the gains,losses,net income (gains-losses NI) and trajectories of the conversions of the land use types.

From Table 2,the area of farmland and salinealkali land increased rapidly,while the area of grassland and wetland decreased.The increased area of saline-alkali land was from farmland,forestland,grassland,wetland and other type land,about 749.82 km2,732.6 km2,2 215.43 km2,820.49 km2and 1 607.07 km2,respectively.On the other hand,the saline-alkali land lost to farmland,grassland,wetland,about 737.06 km2,2 198.33 km2，878.53 km2，respectively.There was no change from saline-alkali land into forestland and other type land.

Table 3 showed that land use changed drastically during 1990-2010.The areas of farmland,forestland and other type land increased by about 3 201.39 km2,1 120.41 km2,1 950.48 km2,respectively.Simultaneously,the decreased area included grassland,wetland and saline-alkali land,about 4 234.28 km2,404.04 km2,and 1 643.31 km2,respectively.During this period,thesaline-alkali land experienced a drastic conversion.About 468.33 km2of farmland,3.87 km2of forestland,2 214.12 km2of grassland,1 052.72 km2of wetland,2.58 km2of other type land were transferred to salinealkali land.Meanwhile,the saline-alkali land transferred to farmland,forestland,grassland,wetland and other type land,about,774.52 km2,1 087.99 km2,1 547.64 km2,388.28 km2,1 586.51 km2,respectively.

Table 2 Transfer matrix of LUCC during 1970-1990

Table 3 Transfer matrix of LUCC during 1990-2010

The changes in saline-alkali land types showed opposite characteristics in 1970-1990 and 1990-2010,indicating an increase in 1970-1990 and a decrease in 1990-2010.Based on the administrative division data,we calculated the saline-alkali land area in four regions(Jilin,Liaoning,Heilongjiang,and Inner Mongolia Autonomous Region) (Fig.3).Among them,the area of saline-alkali land in Jilin Province was more than than in the other three regions,and the changes of saline-alkali lands in the study area showed the same trajectory,increasing during 1970-1990 and decreasing in 1990-2010.Although there was a great reduction of saline-alkali in 2010,it was still more than that in 1970.

Fig.3 Area changes of saline-alkali lands during 1970-2010 in the study area

3.2 Driving forces for LUCC in saline-alkali land region

Drastically changes in LUCC over a long time can induce significant variations in soil physicochemical properties.In the central Northeast China,soil salinization is a serious ecological problem.Both natural factors and human activity were the main causes of the LUCC in the past 40 years,and in most cases,their interactions were driving forces for the LUCC in the study area.Climatic conditions were the restraining factor to the land use structure.Moreover,human activity has also obviously affected LUCC.For example,wetland reclamation brought an increase in farmland,and overstocking caused the loss of grassland.It is necessary to explore the infulence of the two driving forces on LUCC in the soda saline-alkali soil region.

3.2.1 Natural driving factors

Because the soda saline-alkali soil region is located in semi-arid and arid zone,the land use conversion was greatly controlled by the amount and spatial distribution of precipitation.The amount of rainfall decided the growth of vegetation and spatial distribution.Since most parts of the study area are plain,the topography had little impact on land use.In this paper,we calculated the annual precipitations and average annual temperatures from 1970-2010,and trend analysis was used to explore the relationship between changes in the climatic factor and area of saline-alkali land.

Fig.4 showed the temporal variation of average value of 20 meteorological stations in the soda salinealkali soil region during 1970-2010.The temperature of the study area had an increasing trend with an annual increment of 0.031℃,but in recent 20 years,this trend slowed down.We explored the average annual temperature changes for the two time intervals,i.e.,1970-1990 and 1990-2010,respectively.During 1970-1990,the increment of the average annual temperature of the study area was 0.045℃ per year,while the slope of the average annual temperature was 0.012℃ per year from 1990 to 2010.Though the average annual temperature kept increasing,the increment in the recent 20 years obviously slowed down.

Fig.4 Trends change of temperature and precipitation in study area

The annual precipitation in the study area had a decreased trend,but in recent two decades,this trend was not obvious.We divided the study period into two time intervals (1970-1990 and 1990-2010)and analyzed the difference in trend changes.During 1970-1990,the annual precipitation had a decreased trend with 3.95 mm per year,while in 1990-2010,the decrease ratio is only 1.5 mm per year.Considering the extreme precipitation in 1998,the changing trend of precipitation during 1990-2010 was slightly increased by 1.05 mm per year if precipitation in 1998 was eliminated as an outlier.The increased precipitation effectively reduces soil salinity.On the one hand,the increase in rainfall produced surface runoff and infiltration,resulting in the salinity of the soil surface entrainment and infiltration.On other hand,the amount of precipitation supplied to vegetation caused the increase in vegetation area.That was the major reason why the saline-alkali land area had decreased in recent 20 years.

Considering the soil saline-alkali was sensitive to climate factors,especially to annual precipitation,the decrease in precipitation is certainly harmful to natural vegetation,exacerbating local salinization.Meanwhile,the warming of average annual temperature promoted the increase of water evaporation in soil which resulted in the migration of salinity from the deep of the soil to the surface.This is why the changing trend of salinealkali land area was in accordance with the climate variation,especially the precipitation.In order to clearly and quantitatively explain how much the climate factor affects the saline-alkali land,a model was built to explore the relationship between the precipitation and the saline-alkali land area.The saline-alkali land area of the four provinces was normalized to build the model.Fig.5 shows the cross plot of the salinealkali land are and the precipitation,withR2=0.78 and passing the 0.01 significance test.

Fig.5 Relation between precipitation and area of saline-alkali land in the study area

3.2.2 Human activity driving forces

It is a long term for the change of land use which is driven by climate change,but human activities may have immediate impacts on LUCC,especially in the implementation of certain social activities and national policies.In this paper,we used a comprehensive index of land use degree and human activity intensity index to explore the influence of the change of saline-alkali land.Fig.6 showed the comprehensive index of land use degrees in different provinces of the study area.The composite land use extent index increased in Jilin,Heilongjiang,Liaoning,and Inner Mongolia during 1970-2010,with increments of 31.1,23.14,10.89 and 14.14,respectively.The increase comprehensive index of land use degree shows that more other type land,forestland and grassland were lost while farmland land was expanded,which is consistent with the LUCC in part 2.1.

Fig.6 Land-use degree in the study area

Compared with the natural landscapes like salinealkali land and wetland,farmland is closely correlated with human activity.In order to explore the influence of human activity on the change in land use well,we tried to model the relationship between the comprehensive index of land use degree and saline-alkali soil and farmland,respectively.Because the comprehensive index of land use degree is a dimensionless parameter,we normalized the area of saline-alkali soil and farmland in the study area.Fig.7 shows the relation between land use degree and normalized saline-alkali land and farmland area.The land use degree has a linear relationship with the area of saline-alkali soil,and the coefficientR2=0.55.It indicates that the correlation between saline-alkali land and the degree of land use degree is weak and human activities are not the most important factor in the expansion of saline-alkali land.However,the farmland had a significant relationship with land use degree,the coefficientR2=0.71 and passing the 0.01 significance test,which indicates the land use degree is most closely related to farmland.

Fig.7 Relations between land-use degree and normalized saline-alkali land (a) and farmland (b) areas

To quantitatively characterize the impact of human activities on the LUCC,we calculated the human activity intensity index for different regions in two periods,1970-1990 and 1990-2010.Table 4 shows the human activity intensity index in the two periods for four regions.Except for Liaoning,the index of the other three regions in 1990-2010 were more than that in 1970-1990,indicating that the degree of human activities had been increasing during the past 40 years.

Table 4 Human activity intensity index in the study area

Similarly,we modeled the human activity intensity index and the area of saline-alkali land and farmland.Fig.8 shows the relationship between human activity intensity index and normalized saline-alkali land and farmland area.The human activity intensity index had a linear relationship with the area of saline-alkali soil,and the coefficientR2=0.12,indicating that the saline-alkali land and human activity intensity index had no obvious correlation.Nevertheless,the farmland had a significant relationship with the human activity intensity index,with the coefficientR2=0.76,and passing the 0.01 significance test,indicating that the change of farmland had a close relationship with the human activity intensity index.

Fig.8 Relations between human activity intensity index and normalized saline-alkali land (a) and farmland (b)areas

The land use types in thd study area had a drastic conversion,the saline-lkali land increased during 1970-1990,and decreased from 1990 to 2010.Both climate change and human activity are the main driving forces for land use change.Correlation analysis showed that change of climate factor,especially precipitation was the major factor infulencing the saline-alkali land,withR2=0.78 and passing the 0.01 significance test.The increase in rainfall improved the ecological condition of the vegetation,resulting in the reduction of salinealkali land.There was no obvious correlation between social economic indices and saline-alkali.The change of saline-alkali land had a weak relationship with the human activity intensity index,but the farmland correlated closely with the human activity intensity index.From Table 3,it shows that about 774.52 km2of saline-alkali land was converted to farmland,and only about 468.33 km2of saline-alkali land came from farmland during 1990-2010,indicating that human activities strengthened the decrease of saline-alkali land under sufficient rainfall.

4 Conclusions

Based on the land use classification data derived from the Landsat images in 1970,1990 and 2010,the land use conversions were investigated in soda salinealkali soil region of the central Northeast China during 1970--2010.The meteorological data and two land use indices were used as driving forces to explore the infulence of land use change.

(1) The land use types in soda saline-alkali soil region changed drastically during 1970-2010,especially on the farmland and grassland.The farmland had been increasing at the rate of 0.31% per year,whereas the grassland decreased at the rate of 0.35% per year.The area of saline-alkali land had experienced an increase during 1970-1990 and a decrease from 1990 to 2010.Statistics found that the increase in the salinealkali land was mainly came from grassland and the loss of saline-alkali land was converted to farmland and grassland.

(2) The rising annual average temperature was obvious in the study area.For the annual precipitation,there was a decreasing trend during 1970-1990,but in 1900-2010,the trend of annual precipitation slightly increase if precipitation in 1998 was eliminated.Analysis indicates that the saline-alkali land area is closely related to the precipitation,withR2=0.78 and passing the 0.01 significance test.

(3) Land use degree index and human activity index had no obvious correlation with saline-alkali land,and the changes of those indices only reflected the land use patterns.Under sufficient rainfall,human activities strengthened the decrease of saline-alkali land under sufficient rainfall.Climate was the major factor influencing the change of the saline-alkali land.