Effect Of Land Use Change On Soil Organic Carbon Storage In A Typical County Of Loess Plateau And Its Simulation

Land use change is the second most important human activity that leads to climate warming,and the carbon cycle of terrestrial ecosystem involved in it has become the focus of dealing with future climate change.Since the implementation of the policy of Grain for Green Project in the Loess Plateau,the types and directions of regional land use have been greatly changed.This paper took Ansai District of Yan’an City,which is located in a typical loess hilly and gully region,as the research object,analyzed the quantity,dynamics and transfer characteristics of land use changes before and after returning farmland to forest from 1990 to 2020 with the help of remote sensing and process model,and estimated the density and reserves of organic carbon in the soil surface layer（0～20 cm）under different land use types.The main conclusions are as follows:（1）Affected by human activities such as policy-driven and socio-economic development,the area of various land use types in Ansai District changed significantly from 1990 to 2020,and the area of forestland increased from 226.10 km² to 1378.97 km²,and gradually expanded from south to north in space,mainly from grassland and farmland.Since 1990,a large amount of farmland in central and northern Ansai District had been restored to grassland and forestland,and the farmland area had decreased by 79.46%from 1990 to 2020.Compared with farmland,the change of construction land area was more drastic,with an annual change rate of 12.69%from 1990 to 2005.（2）The average density and storage of soil organic carbon in Ansai District changed greatly before and after returning farmland to forest.In the past 30 years,the overall trend of carbon sequestration effect of surface soil had remained unchanged:forestland>grassland>farmland.The soil organic carbon density（SOCD）of forestland increased from1.15 kg/m² to 1.41 kg/m².And the superposition advantage of the rapid expansion of forestland area and the obvious increase of average density significantly increased the soil organic carbon storage（SOCS）of it by nearly 7 times.The SOCD of grassland first decreased and then increased,but its SOCS first increased and then decreased.The SOCD of farmland decreased continuously by 0.08 kg/m^2,and SOCS decreased from 1.22 t to 0.23t.The size of SOCS in Ansai District was largely determined by the area occupied by various land use types,while SOCD was greatly influenced by land use types（p<0.01）,mainly due to the drastic changes in vegetation coverage of forestland and grassland,different human disturbance degrees and frequent transformation among various land use types after the implementation of ecological restoration projects.（3）Taking 2007～2010 and 2015～2020 as the research periods,the changes of SOC content under the dynamic changes of point-scale farmland were simulated and evaluated,and it was found that the model had a good overall fitting result for farmland and forestland.It was found that the conversion of farmland into forestland was helpful to increase carbon storage and move towards carbon sink.From 2007 to 2010,the content,density and storage of topsoil organic carbon increased from 3.63 g/kg,0.50 kg/m² and 1543.58 kg to 4.05 g/kg,0.55 kg/m² and 1698.68 kg,but there was no significant difference among different years（p>0.05）.After the transformation,the SOC content,SOCD and SOCS ranged from 4.27 to 5.53 g/kg,0.58 to 0.74 kg/m² and 1776.22～2283.30 kg from 2015 to 2020,respectively,and the lowest value appeared in 2016,and was significantly smaller than that in other years（p<0.05）.SOC content,SOCD and SOCS increased significantly in the process of land use type transformation.Under the dynamic change of land use type,the surface SOCD was significantly correlated with soil bulk density,atmospheric CO₂ concentration,fertilizer types and land use types（p<0.01）.This paper analyzed the influence of land use change on soil surface organic carbon before and after returning farmland to forest in Ansai District,and simulated and evaluated the transformation of soil surface organic carbon based on DNDC model,which could provide scientific basis for vegetation restoration and land carbon cycle estimation in Loess Plateau.