The Evolution Of Soil Organic Carbon Of Arable Mollisols And Its Relationship With Crop Productivity

Soil organic carbon is an important component for maintaining soil fertility.It not only affects crop production and soil functions but also plays a key role in regulating global carbon cycling and climate change.Agricultural soil carbon storage is the most dynamic carbon storage in global terrestrial ecosystems,and its soil organic carbon stock can change rapidly with human activities.During the centuries of cultivation,the soil organic carbon stock of arable Mollisols has decreased by half compared to pre-cultivation levels.As a result,soil fertility has declined severely,and some areas have become low-yielding fields.Moreover,soil organic carbon is still decreasing at an average rate of 5‰per year,seriously damaging food production and threatening national food security.Therefore,this study used the Roth C model and spatial analysis methods based on long-term experiment and large-scale spatial transects to clarify the evolution process of soil organic carbon in arable Mollisols and quantitatively evaluate the impact of climate change and agricultural management measures on the evolution of soil organic carbon.The study also constructed a relationship between soil organic carbon and crop productivity to provide a theoretical and scientific basis for national Mollisols conservation decisions.The main research results are as follows:（1）The soil organic carbon of arable Mollisols is not in equilibrium and is very sensitive to agricultural management measures and climate change.The Roth C model can accurately simulate the evolution characteristics of soil organic carbon of arable Mollisols by optimizing input parameters,with a mean error of 1.65 t ha^-1 and a root mean square error of 4.26%.The model accuracy is mainly affected by carbon input and initial soil organic carbon stock,and the root mean square error decreased with increasing carbon input and initial organic carbon stock.Without fertilization,the carbon input of arable Mollisols ranged from 1.08 to 1.61 t C ha^-1 yr^-1,and fertilization increased it by 35.9%-102.2%.However,the actual carbon input under current management measures cannot maintain the current soil organic carbon storage of arable Mollisols(>45 t ha^-1).The simulation results of the Roth C model showed that without fertilizer,soil organic carbon stock of arable Mollisols(30.33-103.59 t ha^-1)decreased at a rate of 0.14-1.11 t C ha^-1 yr^-1,and fertilization slowed down the rate of soil organic carbon loss.However,when the soil organic carbon stock is>45 t ha^-1,it still decreased at a rate of 0.31-0.86 t C ha^-1 yr^-1.Both short-term and long-term simulation results showed that 45 t ha^-1 can be regarded as the equilibrium point of arable Mollisols under current climate and agricultural management measures.Soil organic carbon stock above the quilibrium point will still decrease,while it will gradually increase below the quilibrium point.Meanwhile,climate warming accelerated the loss of soil organic carbon,while cold conditions are more conducive to soil organic carbon stock.The current carbon input under agricultural management measures is insufficient to cope with the impact of future climate change on soil organic carbon.Increasing carbon input can improve the carbon sequestration potential of arable Mollisols.Under no fertilizer conditions,for every 10%increase in carbon input,the relative carbon sequestration potential of the soil will increase by1.12-1.76 t ha^-1.When the soil organic carbon storage is less than 65 t ha^-1,fertilization combined with full straw return can increase the carbon sequestration potential of arable Mollisols(10.94-23.01 t ha^-1).However,when the soil organic carbon stock is greater than 65 t ha^-1,straw return alone cannot prevent the loss of soil organic carbon.（2）The soil organic carbon stock in typical Mollisol region showed obvious latitudinal zonality.By comparing the changes in soil organic carbon at different latitudes,it was found that at 43°N,the soil organic carbon increased by 0.38 g kg^-1,while it decreased by 1.91 g kg^-1 and 0.84 g kg^-1 at 49°N and 48°N,respectively.Climate factors are the main driving force for soil organic carbon,but their effects have decreased in the past 20 years.The current organic carbon stock in typical Mollisol region is 51.74 t ha^-1,which was relatively stable in the past twenty years.However,in terms of spatial distribution,the northeast is still decreasing rapidly,while the central region is increasing slowly.Temperature is the main influencing factor on soil organic carbon,and soil texture and agricultural production practices can also affect the changes in soil organic carbon.（3）Crop yields in typical Mollisol region increase from north to south,mainly driven by fertilizer application and temperature.As temperature increases 1°C,the yield of corn and soybean increases by 0.56 t ha^-1 and 0.36 t ha^-1,respectively.Therefore,it is difficult to quantify the relationship between soil organic carbon of arable Mollisols and crop productivity on a regional scale.Based on the results of long-term experiments excluding differences in climate and field management practices,it was found that fertilization and soil organic carbon have a significant impact on crop productivity,yield stability,and yield composition.Under no fertilizer treatment in cold climates（mean average temperature=2.8°C）,the grain yield is2.65-4.63 t ha^-1.Fertilization increased the yield by 92.17%-222.69%compared to no fertilizer.However,under warm conditions（mean average temperature=5.9°C）,fertilization only increased the yield by 35.25%-94.24%compared to no fertilizer.There is a relationship threshold between soil organic carbon and crop productivity arable Mollisols.The regression equations for Hailun and Dehui under no fertilizer treatment are y=1.52+1.02×x-0.11×x² and y=8.17+0.43×x-0.05×x²,respectively.When soil organic carbon is 5%,the theoretical yield of maize is the highest,and the maximum theoretical yield of Hailun and Dehui is 3.87 t ha^-1 and 6.47 t ha^-1,respectively.The regression equations for Hailun and Dehui under fertilization treatment are y=2.82+1.58×x-0.17×x² and y=6.20+1.52×x-0.181×x²,respectively.Based on the fitting results of the regression equations,the highest theoretical yield of maize is achieved when soil organic carbon is 4%,and the maximum yields for Hailun and Dehui are 9.09 t ha^-1 and 9.40 t ha^-1,respectively.This study simulated the evolution characteristics of soil organic carbon of arable Mollisols and proposed an equilibrium point for soil organic carbon.The spatiotemporal distribution characteristics of soil organic carbon in the typical Mollisol region were also evaluated.Overall,the arable Mollisols showed a trend of low-value increase and high-value decrease,both at the point and regional scales.In addition,the relationship threshold between soil organic carbon and crop productivity was proposed.This study can provide a basis for the protection and utilization of arable Mollisols.