Chunxiu GUO,Lide WANG ,Fugui HAN,Jianping MA,Fanglan HE,Shujuan LIU,Fanglin WANG,Yinghua ZHANG,linyuan WEI

State Key Laboratory of Desertification and Aeolian Sand Disaster Combating & Gansu Desert Control Research Institute,Wuwei Gansu 733000

Responsible editor:Lin FANG Responsible proofreader:Xiaoyan WU

The changes and differences of soil granularity and other physical properties can be used to determine the intensity of land degradation and development[1-2],which provides a reference for the management of abandoned farmland.Soil granularity,bulk density and other physical properties are its basic characteristics,especially the impact of land utilization type on soil physical properties[3-4],which was more reported at home and abroad.Ma xianghua et al.[3]studied the changes of soil moisture and physical and chemical properties of abandoned farmland in Loess Plateau.Zhang zhenguo et al.[5]studied the soil crust forming process of abandoned farmland and its physicochemical properties.Liu yong et al.[6]studied the changes of soil physical properties and plant communities of the Loess Plateau land which abandoned for 4 to 20 years,and revealed the changes of soil physical properties,plant root system and biomass in different abandonment time.Ma xianghua et al.[7]and Wang yibo et al.[8]studied the response of soil physical properties to the changing process of vegetation restoration and ecosystem,and indicated that the change of vegetation ecosystems caused strong changes in soil physicochemical property and soil degradation in high-cold environment.Harsh land environment in arid area,where was more wind and sand and serious desertification[9].Su yongzhong et al.[10],Wang baofang et al.[11],Hu yalin et al.[12]analyzed the physical characteristics,ecological impacts and the changes of vegetation and ecosystem of degraded land.The researches showed that soil bulk density increased,soil nutrient lost,grain composition roughened with the extension of abandonment year of abandoned land in prairie.Cultivation measure greatly affected the farmland of oasis ecosystems in arid region,which directly affected the stability of the entire oasis ecosystem after abandoning.Therefore,it was important to manage abandoned land reasonably,retard desertification and maintain local ecological stability.However,in the case of limited water resources in oasis region of desert,the studies about the change rules of soil physicochemical property and its drive mechanism of abandoned lands were relatively less.

Water resources in downstream of the Shiyang River were scarce,and the land was fertile,which was once lush vegetation and abundant species[13].Due to the expanding agriculture production scale,the demand of water resources increased by years,while the water of the Shiyang River that flowed into the downstream of oasis area reduced sharply,which resulted in ecological degradation[14]and a large area of abandoned lands[15].The ecological environment of Minqin County located in downstream of the Shiyang River was continues to deteriorate,which had been caused the public attention.The local government implemented"turn off the wells and cut down the fields" to restore degraded ecological environment.The farmland area reduced 2.64×104hm2by closing the PumPing wells and adjusting water-saving structure in"The plan to reduce farmland in Minqin County(2006-2010)".The protection and rational utilization of this part of farmland related to the development and stability of the entire oasis ecosystem.Therefore,the soil physical property of different years abandoned lands in the Minqin Oasis of downstream of the Shiyang River was investigated in order to provide a scientific basis for the management and use of abandoned lands in the downstream of the Shiyang River and its oasis construction.

Materials and Methods

Natural survey of testing areas

Abandoned lands of Huihuang village and Zhiyun village located in Minqin County of the downstream of the Shiyang River were selected as testing areas.The geographic locations was 103°36′02″-103°39′27″ E and 39°01′54″-39°03′28″N,the average elevation was 1 305 m,the annual average temperature was 7.5 ℃,the extreme maximum temperature was 38.1 ℃,the extreme minimum temperature was -28.8 ℃,the average annual precipitation was 115 mm,the rainfall in the 7,8,9 month occupied 72%of the average annual rainfall,the average annual evaporation capacity was 2 644 mm,the annual sunshine duration was 2 832.1 h,the annual average wind speed was 2.3 m/s,the frost-free periods were 175 days,the annual average gale days of more than 8 level winds were 37.7 days,the sandstorm days were 26.8 days[13].The climate was drought,more sand,scarce rainfall,strong evaporation,adequate light and large temperature difference between day and night,which belonged to typical arid desert climate.Zonal soil was gray desert,which was high salinity,and has difference in plant communities of different abandoned lands[14].Dominant plant species had Chenopodium album,Convolvulus arvensis,Sonchus oleraceus,Echinopilon divaricatum,Lepidium apetalum,Peganum nigellastrum,P.harmala,Acroptilon repens,Halogeton glomeratus,Lycium ruthenicum,Kalidium foliatum,Reaumuria soongoria and so on.

Research methods

In 2012,9 abandoned farmlands with abandonment time of 1 year,2 years,3 years,4 years,5 years,8 years,15 years,24 years and 31 years were selected as testing areas.The size was 100 m × 100 m,and each area equably divided into 5 quadrats with 10 m × 10 m by using five point method.The species,height,crown width,number of individuals,soil moisture,structure and texture of plants in every quadrat were surveyed.

Soil sampling and determination

Two soil samples sections were collected from each testing area by sampling along the diagonal.Stratified sampling in 0-10,10-20,20-30 and 30-40 cm layer of soil.Three soil samples were collected from different place on the same layer with 100 cm3of soil cutting ring,and the wet weight was weighted by aluminum box in testing ground,then put into ziplock bags and took back to the laboratory to detect the soil moisture content and bulk density by oven-drying method.Calculated the soil porosity.Meanwhile,1 kg of soil samples were collected into the soil sample bags from the same depth,and took back to the laboratory.Then the grain composition was measured by Malvern Mastersizer 2 000 laser particle size analyzer,the measurement range was 0.02-2 000 μm and the repeated measurement error was less than 2%.On the basis of international standards of soil texture classification system,the grain composition in testing areas was divided into clay (＜0.002 mm),silt (0.002-0.02 mm),fine sand (0.02-0.2 mm)and coarse sand (0.2-2.0 mm).And then analyzed the grain composition of 0-40 cm layer of abandoned farmlands with different abandonment years[16].Soil bulk density and porosity were calculated as follows:

Wρrefers to soil bulk density(g/cm3) and Wd refers to dry soil weight(g),Vt values 100 cm3。

Vfrefers to soil porosity (%),Wp refers to soil bulk density (g/cm3),ρrefers to soil specific gravity,soil specific gravity values 2.65 g/cm3.

Data statistics

Data were statistically analyzed by using Excel 2007.

Results and Analysis

Changes of soil bulk density and porosity

The results showed that the changes of soil bulk density of different soil layer in abandoned farmlands with different abandonment years were obvious(Table1).The maximum soil bulk density of abandoned farmlands was 1.59/cm3and the minimum was 1.07 g/cm3.The coefficient of variation of soil bulk density at layer of 0-20 cm was 2 times than 20-40 cm layer.In abandoned farmlands with abandonment time of 1 to 2 years,the change of soil bulk density at plough layer of 0-30 cm was larger,and the soil bulk density decreased with the increase of depth.The coefficient of variation of soil bulk density and porosity in vertical section became larger with the increase of abandonment time.On the contrary,the soil bulk density of abandoned farmlands with abandonment time of 2 to 31 years increased with the increase of depth.Except to the sample area with abandonment time of 15 years,the average soil bulk density of sample area with abandonment time of 1 to 2 years was 0.25 g and 0.27 g larger than abandonment time of 31 years respectively.0-20 cm layer was the maximum soil bulk density of abandoned farmlands with abandonment time of 1 to 2 years.Except to the abandonment time of 15 years,the soil bulk density of 0-30 cm layer with the rest abandonment time was larger.At the layer of 20-40 cm,the difference of average soil bulk density of sample areas with abandonment time of 1 to 2 years was less than 31 years.The coefficient of variation of average soil bulk density of different soil layer in sample area with abandonment time of 1 to 2 years was smaller than other abandonment time,and the soil bulk density of each soil layer was relatively uniform.Porosity and bulk density were associated indicators,and the change rule of these two indicators was opposite.The soil bulk density of 0-20 cm layer with abandonment time of 1 to 2 years was relatively large,and the porosity was relatively small,about 44.36% and 41.58% respectively.The soil porosity of abandoned farmland with abandonment time of 31 years reached to 51.71%.The coefficient of variation of average porosity of sample areas with abandonment time of 1 to 2 years was 5.0%smaller than 31 years.The soil bulk density of 0-40 cm layer decreased and the coefficient of variation of porosity increased relatively after the land abandoned for 2 years.

Changes of grain composition

Table1 Results of soil bulk density and porosity of abandoned farmlands with different abandonment years compared with abandonment time of 31 years

The proportion of fine sand in abandoned farmlands was largest,silt and clay followed,and the coarse sand was smallest.The content of fine sand was more than 50% and the maximum reached to 75.71%.The content of clay was less than 10%,and the soil type was sand.With the changes of abandoned years of abandoned farmlands,the proportion of each grain composition was slightly different(Fig.1),which was not continued to increase or decrease,but showed a certain change trends.At 0-10 cm layer of soil,the clay reduced with the increase of abandonment years after abandonment time of 3 years.The proportion of clay in abandoned farmland with abandonment time of 4 years decreased 24.75% compared with 2 years,and decreased 1.78% in 5 years,21.53% in 8 years,9.74% in 15 years,13.51% in 31 years and increased in the rest years.The soil silt relatively increased with the increase of abandonment time after the land abandoned for 1 year.The proportion of silt in abandoned farmland with abandonment time of 2 years increased 14.38%compared with 1 year,and increased 12.51%in 3 years,2.95%in 8 years,3.37% in 15 years,19.39% in 24 years,8.24% in 31 years and decreased in the rest years.The fine sand relatively reduced with the increase of abandoned years after the land abandoned for 1 year.The proportion of fine sand in abandoned farmland with abandonment time of 2 years decreased 5.41% compared with 1 year,and decreased 6.51%in 3 years,6.96% in 5 years,8.42% in 24 years,3.40% in 31 years and increased in the rest years.The change trends of grain composition showed larger increase or decrease after the land abandoned for 4 year,and then have some fluctuation.Granular variable coefficient of clay,silt,fine sand and coarse sand at the range of 0-40 cm of different abandoned land increased with the increase of abandoned years,which means that the variation of soil vertical section increased with the extension of abandonment years.

Conclusion and Discussion

With the extension of abandonment year,the soil bulk density of abandoned farmland of oasis in downstream of the Shiyang River decreased and the porosity increased.In abandoned farmland with abandonment time of 1 and 2 years,the soil bulk density of 0 -40 cm layer decreased with the increase of depth.It related to the plant roots of annual that grew largely in the earlier stage of abandonment year[15]improved soil structure and texture.Bulk density and grain composition of soil changed with the extension of abandonment year,but this change was not continuous and uniform.The change was more obvious in 3 to 4 years after abandonment,and soil bulk density and the reduced value of clay at the range of 0-10 cm of different abandoned land was relatively large.The soil porosity increased with the extension of abandonment year,which contrary to the phenomenon that soil bulk density increased with the extension of abandonment year in abandoned land of prairie[17].And similar to the change of soil physical characteristics of vegetative restoration in Loess Plateau[8]and the impact of afforestation in deteriorated grassland of semi-arid region on changes of sand soil[11].Soil bulk density and porosity were important indicators of soil porosity,which affected the permeability of soil moisture[3].With the extension of abandoned year,the differences of soil bulk density and soil porosity at the range of 0-40 cm of different abandoned land decreased and the permeability increased,which facilitated the infiltration of rainfall and also increased the chances of soil erosion.In the management process,annual growth should be controlled in order to ensure the differences between the layers of the soil and improved the effectiveness of anti-erosion.

In every sample area of abandoned farmlands,the proportion of fine sand was all more than 50%,the content of clay was all less than 10%,and the soil type was sandy.With the extension of abandonment year,the soil clay reduced and silt increased,and the maximum changes happened at layer of 0-10 cm.The farmland would happened nutrient loss and grain composition roughening after abandonment[3].The wind of downstream of the Shiyang River was big[14],which accelerated soil erosion and exacerbate the loss of clay.This phenomenon was similar to the change process of abandoned land in steppe[17].Grain composition was one of the basic physical properties of soil,which affected soil structure type.And it was the key factor that affected water and fertilizer conditions of soil[2-3].With the extension of abandonment year,richness and diversity indices of plant community presented fluctuations drop,the exposed area of soil increased,and the soil clay was easier to flow away.Therefore,the measures should be took to reduce soil desertification,maintain a certain amount of vegetation coverage in abandoned farmland,and improve the ecological stability of the region.The soil had high content of clay was easy to form water-stable aggregates and fissures.The soil had large proportion of fine sand or very fine sand was only formed unstable structure.The coarse sand could not be reunited[3-4].So the soil aggregates decreased and the quality declined with the extension of abandonment year.

There were more factors that influenced the change of soil physical properties,such as parent material,the original state of soil,the climate,the use patterns and the vegetation changes,which would all affected soil properties.Soil physical properties changed relatively moderate compared with chemical properties[3].But the soil in the abandoned farmland of sampling area changed obviously in 3 to 4 years after abandonment.The relative changes was faster,which was related to the aeolian sandy soil,big wind and sand,and arid climate.Due to the lack of long-term observation,the method that in this study used application space instead of time was only analyzed bulk density and grain composition of soil,so the relevant factors that affected its change needed further research.

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