Effects Of Continuous Straw Return On Soil Organic Carbon Sequestration And Saturation In A Wheat-Cotton Double Cropping System

The accumulation of soil organic carbon in farmland is of great significance to soil fertility,food security and ecological function changes.The results show that the direct return of crop straw can significantly improve the soil organic carbon sequestration and increase the soil carbon sequestration capacity.Since 2000,the direct return of crop straw has played a dominant role in soil carbon sequestration and is one of the important measures for agricultural sustainable development in China.However,in the multi-cropping area,a large amount of straw returning for a long period of time may have the phenomenon of soil organic carbon instability and carbon saturation,which may affect the potential of soil carbon sequestration and emission reduction,but the relevant characteristics are not clear yet.This study was conducted at the Experimental Station of Jiangsu Academy of Agricultural Sciences in Nanjing,Jiangsu Province and sandy loam soil in Dafeng District,Yancheng City,Jiangsu Province,China.Two wheat and cotton annual straw returning to the field for a long time（started from 2011）were fixed in the experimental field.Under the unified application of nitrogen and phosphorus fertilizer,three levels（W2,W1,W0）of wheat straw returning to the field,half returning to the field and no returning were set in the cotton season,and three levels（C2,C1,C0）of cotton straw returning to the field,half returning to the field and no returning were set in the wheat season,which combined to form nine annual straw returning treatments.K150,150 kg K₂O ha^-1and CK were used as the control.By investigating two soil types in different soil distribution and soil aggregate stability and physical properties such as bulk density,soil total organic carbon content,different aggregate graded and the active components of organic carbon content,and analyzing the distribution of soil organic carbon and aggregate,the relationship between the components of organic carbon,straw returned to soil organic carbon fixation and the influence of saturation,The aim is to provide scientific basis for reasonably and effectively popularizing straw returning technology and promoting agricultural sustainable development.The main results are as follows:1 Straw returning treatment significantly increased soil water content and soil porosity in 0-10 cm and 10-20 cm soil layers in Nanjing and Dafeng pilot areas,decreased soil bulk density,and then significantly affected the distribution and stability of soil aggregates.For surface soils（0-10 cm）in Nanjing and Dafeng,straw returning significantly increased the mass proportion of soil microaggregates and clay and silt granular aggregates,while decreased the mass proportion of large aggregates.On the contrary,in the 10-20 cm soil layer of Dafeng pilot,straw returning significantly reduced the specific gravity of soil micro aggregates and clay and silt granular aggregates,while increased the specific gravity of large aggregates.The Mean Weight Diameter（MWD）,Geometric Mean Diameter（GMD）and particle size>0.250 mm aggregates distribution parameters(R_0.25)of surface soil aggregates in the two experimental areas showed a decreasing trend with the increase of straw returning amount,indicating that the stability of surface soil aggregates decreased.Compared with the straw returning treatments,the effect of straw returning in two seasons of wheat and cotton was greater than that in a single season,the total amount of straw returning to the field was greater than half amount,and the effect of wheat straw returning to the field was greater than that of cotton straw.There was no significant difference between K150 and CK treatments.2 Straw returning significantly increased the contents of soil total organic carbon（TOC）,organic carbon of different aggregates and organic carbon of different active states in 0-10 cm and 10-20 cm soil for several consecutive years,and all of them increased with the increase of the amount of straw returning,and the effect of total straw returning in wheat and cotton seasons was the most significant.The increase of organic carbon content showed that the organic carbon content of large aggregates>and micro aggregates>and clay powder particles.Easily oxidized organic carbon>microbial biomass carbon and light organic carbon>soluble organic carbon and recombinant organic carbon.Potassium fertilizer treatment had significant effect on the contents of some organic carbon components,but had no effect on the total organic carbon.The correlation characteristics of soil total organic carbon,clay particulate organic carbon and large aggregate organic carbon in the two pilot areas were different,while the correlation characteristics of soil micro aggregate organic carbon were linear,and the rules of the two pilot areas were consistent.3 The treatment of straw returning significantly increased the soil total organic carbon density and soil carbon saturation in 0-10 cm and 10-20 cm soil layers in Nanjing and Dafeng for many years,but had no significant effect on the soil layer in 20-40 cm.The proportion of soil aggregate density to total organic carbon density in 0-10 cm soil layer showed that the proportion of large aggregate organic carbon density decreased with the increase of straw returning amount.Compared with CK,K application had no significant effect on the change of soil total organic carbon density,carbon saturation and carbon saturation rate in different soil layers,and the contribution rate of soil aggregates to organic carbon.In the 0-10 cm soil layer of the Nanjing pilot,the clay and silt organic carbon pool has reached saturation or even supersaturation state,while in the Dafeng pilot,this part of the soil organic carbon is close to carbon saturation state under the straw returning treatment.