Effects Of Fertilization And Straw Returning On Soil Organic Carbon Content In Cropland Of China Based On Meta-analysis And DNDC Modeling Study

The problem of global climate change has been becoming increasingly serious,and increasing soil organic carbon content in cropland of China is important to solve the problem.Fertilizer application and straw returning are important measures to enhance cropland carbon sequestration,but there is no uniform conclusion on the effects.Therefore,it is necessary to systematically assess the effects of fertilizer application and straw returning on cropland soil organic carbon content,aiming to provide a scientific basis for making fertilizer application and straw returning measures and enhancing the function of cropland soil carbon sink.In this study,the Meta-analysis method was used to study the overall effects of fertilization and straw returning on soil organic carbon content in cropland of China and the effects under different production conditions.And partial correlation analysis was used to find the main influencing factors of promoting corn field carbon sequestration under the condition of returning straw.At the same time,the Denitrification-Decomposition Model（DNDC）was calibrated using field trial data.Sensitivity analysis was performed on the factors affecting cropland soil carbon sequestration,and then the results were compared with the results of the partial correlation analysis to further clarify the effects of fertilizer application and straw returning on the soil organic carbon content in cropland of China.Finally,based on the future meteorological data（up to 2100）under the two representative concentration paths of RCP8.5 and RCP4.5,the DNDC model was used to simulate the trend of changes of soil organic carbon content to assess the effect of the program on soil carbon sequestration.The findings are as follows.（1）27 validated papers collected from fertilizer application trials,which were mainly located in Northeast,North,Central,East and Northwest of China,with 102 total samples.The fertilizer application treatments were divided into three types:chemical fertilization treatment only,the combination of organic and chemical fertilizer treatment and organic fertilization treatment only.The results showed that compared with no-fertilizer treatment,fertilization overall significantly increased soil organic carbon by 0.23 g·kg^-1·a^-1,and the chemical fertilization treatment only,the combination of organic and chemical fertilizer treatment and organic fertilization treatment only significantly increased soil organic carbon content by 0.08,0.36 and 0.32 g·kg^-1·a^-1,respectively.In addition,the effect was more significant under low mean annual temperature（<9.0℃）,applying organic fertilizer had a higher soil carbon sequestration rate than applying chemical fertilizer only under crop rotation or initial soil carbon to nitrogen lower than 13.00.（2）42 valid papers were collected from straw returning trials,which were mainly located in Northwest,North,Northeast and East of China,with 194 total samples.In this study,the straw treatments were divided into no-straw returning treatment and straw returning treatment.The results showed that straw returning treatment significantly increased soil organic carbon contents,but the effect decreased with increasing soil depth,compared with the treatment of no straw returned to field.In addition,the increasing soil organic change rate reached 0.58,0.19and 0.09 g·kg^-1·a^-1when straw returning duration were 3-5,6-9 and more than 10 years,respectively,showing that the effect reduced along with returning duration.The results also showed that soil sequestration performed well in top soil（0-20 cm）with proper N fertilizer input（100～400 kg/hm²）,high annual mean temperature（≥13.0℃）,high annual mean precipitation（≥800 mm）and high soil initial C/N,but soil below 20 cm depth was not significantly susceptible to these factors.The results of the partial correlation analysis showed that straw returning duration,the amount of nitrogen applied and the initial organic carbon content were the main factors affecting corn field soil carbon sequestration under the condition of returning straw to field.（3）The results of the calibration of the DNDC model using the actual observations of soil organic carbon content at the test sites of compound fertilizer with straw returning to the fields during 2016 to 2019 showed that the relative root mean square error（RMSE）was 12.65%,the normalized root mean square error（N-RMSE）was 9.72%,and the consistency index（d）was0.70,indicating that the DNDC model can better simulate changes of soil organic carbon content.Considering the results of the previous Meta-analysis,the sensitivity analysis of the parameters was conducted by selecting different natural and anthropogenic factors such as nitrogen application amount,straw returning rate,mean annual temperature,mean annual precipitation and initial organic carbon.The results showed that soil organic carbon content was positively correlated with straw returning rate and was highly sensitive to the factors of initial soil organic carbon content,straw returning rate and nitrogen application amount,indicating that this result were highly consistent with the results of the partial correlation analysis.The results also showed that the changes of soil organic carbon content in the two RCPs would increase year by year,but the increasing rate of organic carbon content decreased year by year.The soil carbon pool reached equilibrium at 77_th year.The soil carbon sequestration rate affected by the current management practices could respectively reach 0.25 and 0.23 g·kg^-1·a^-1 at RCP4.5 and RCP8.5,indicating that the management would have a bad effect of soil carbon sequestration in future.In summary,both fertilizer application and straw returning measures can significantly increase the cropland soil organic carbon content,but the effects of the two measures on soil carbon sequestration are different under different natural climatic conditions,soil physical and chemical properties,as well as anthropogenic management.In addition,in the future production practice,straw should return to field within the appropriate range,and measures should be taken to make the initial physical and chemical properties of cropland soil conducive to cropland soil carbon sequestration in order to obtain higher potential of soil carbon sequestration in Beijing area.