Effect Of Land-use Change On Soil Heay Metals In Karst Mountain Area

Guizhou Province as a typical karst ecologically fragile area,to understand the impact of land-use changes on soil heavy metals pollution,which is important to ensure the safety of agricultural products and ecological security.In this paper,three types of land-use change(forest land was converted into artificial grassland,natural grassland was converted into cultivated land,and cultivated land was converted into abandoned farmland,total five land use types)under different altitude gradients in Luodian,Huishui,and Panzhou of Guizhou Province were chosen as objects of study.Soil physical and chemical properties,and heavy metals(Cd,Cr,Pb,Ni,Hg and As)concentrations were analyzed.The results showed that:(1)The average content of heavy metal Cd in the soil of 0-30 cm layer of 5 land use types in study area were higher than the soil background value of Guizhou province.While the average content of Cr,Pb,Ni,Hg and As in each soil layer were lower than the soil background value.The pH of soil was 3.51-5.70,which was acidic soil.The average contents of cation exchange capacity of 0-10 cm,10-30 cm and 30-50 cm soil layers were 16.83 mmol/kg,15.22 mmol/kg and 15.26 mmol/kg,respectively.The average contents of soil organic matter in soil layers of 0-10 cm,10-30 cm and 30-50 cm were 55.17g/kg,43.20 g/kg and 32.90 g/kg,respectively.The organic matter content gradually decreased with the increase of soil depth.(2)In the vertical direction,the content of Cd in soil decreases with the increase of soil depth in different land use types,and it was mainly accumulated in the 0-30 cm soil layer.While the other five heavy metals were no obviously distribution regularly in soil profile in different land use patterns.(3)Comprehensive evaluation of different land use patterns,in the soil of 10-30 cm layer,forest land and artificial grassland were mildly polluted,natural grassland,cropland and abandoned farmland were moderately polluted;in the soil of 10-30 cm layer,forest land was not polluted,artificial grassland,natural grassland and cropland were lightly polluted,and abandoned farmland was moderately polluted;the soil of 30-50 cm layer,artificial grassland,natural grassland and abandoned farmland were lightly polluted,and cropland was not polluted.The soil contamination was mainly caused by Cd,and it was mainly accumulate in 0-30 cm layer.The other five elements have not caused pollution in different soil layers at different land use types.(4)The land use types of 0-30 cm soil layer were studied and analyzed.The results showed that the five land use types in low-altitude areas(Loudian)were not polluted by heavy metals,and the land use change types did not caused soil pollution to increase.The heavy metal pollution degree in middle-altitude areas(Huishui)was artificial grassland > cropland > natural grassland > abandoned farmland > forest land.The pollution was decreased after the forest land was converted into artificial grassland,and natural grassland into cropland,on the contrary,after the conversion of cultivated land into abandoned farmland the pollution was reduced.The heavy metal pollution degree in high-altitude areas(Panzhou)was abandoned farmland > artificial grassland > forest land > cropland > natural grassland.The contamination was increased after the land use change(conversion of forest land into artificial grassland,natural grassland to cultivated land and cropland to abandoned farmland).The element that caused soil heavy metal pollution and changes in different land use change types was Cd.(5)The heavy metals in the forest and natural grassland were mainly derived from the parent material and atmospheric deposition;The sources of heavy metals in artificial grassland,cropland and abandoned farmland were soil parent material,atmospheric deposition and agricultural activities.In summary,land-use change has a profound impact on soil heavy metals in the study areas.It is recommended to vigorously develop pollution-free agriculture in low-altitude areas,and more attention should be paid to Cd pollution in middle altitude and high-altitude areas.