Modelling Greenhouse Gas Emissions Of Maize Farmland Under Climate Change Using DayCent

Elevated atmospheric CO₂ concentration,rising temperature and precipitation are the main driving forces of global climate change,which directly and indirectly affect greenhouse gas emissions in the cropland.Therefore,under the background of carbon peak and carbon neutrality,understanding the response of greenhouse gas emissions to climate change is of significance for the carbon emission reduction target from farmland.In order to evaluate the effect of climate change on greenhouse gas emissions from maize farmland,we employed DayCent model to simulate the long-term greenhouse gas emissions based on our field experiment.In the present study,we parameterized and calibrated the DayCent model using two-year field experimental data,including N₂O,CH₄ and CO₂fluxes from maize farmland under natural atmospheric CO₂ concentration of 400 ppm and elevated CO₂concentration of 700 ppm.Then the calibrated DayCent was applied to simulate the responses of N₂O,CH₄ and CO₂emissions to the changes of temperature,precipitation and atmospheric CO₂ concentration.Thereafter,we explored greenhouse gas emissions from maize farmland during 2021～2060 using DayCent,under the low and medium developing scenarios of SSP126 and SSP245 according to Coupled Model Intercomparison Project phase 6.The main conclusions were as follows.（1）The DayCent was parameterized using the weather and soil data from a typical semi-arid region,and verified according to the observed greenhouse gas emissions from maize farmland.The results showed that simulated N₂O,CH₄ and CO₂ fluxes were highly consistent with observed data under different CO₂ concentrations,and the model efficiencies（EF）were 0.58～0.87,0.45～0.65 and 0.25～0.62,respectively,root mean square error（RMSE）were 0.80～1.33,0.67～0.82 and 0.58～0.80,respectively,and the coefficients of determination（R²）ranged 0.80～0.91,0.53～0.80 and 0.53～0.85,respectively.Therefore,the DayCent model can simulate N₂O,CH₄ and CO₂ emissions from maize cropping system well.（2）The Climate scalar and CO₂ system module in DayCent model was used to set the climate factors to continuously change for 40 years,including temperature increases（0.0,+0.5,+1.0,+1.5,+2.0,+2.5,+3.0℃）,precipitation changes（-30%,-20%,-10%,0%,+10%,+20%,+30%）and elevated CO₂ concentration（20%,40%,60%,80%）.The simulated results showed that CH₄ oxidation,N₂O and CO₂ emissions from maize farmland ecosystem were enhanced with the increasing of temperature.In addition,the increased annual precipitation stimulated N₂O and CO₂ emission,while inhibited CH₄ oxidation;the declining of precipitation inhibited N₂O and CO₂ emission,and stimulated CH₄ oxidation.（3）The effects of elevated CO₂ on greenhouse gas emissions were correlated with the conditions of temperature and precipitation.Under the conditions of increased temperature combined with the declining of precipitation,the effects of elevated CO₂ on soil CH₄ uptake,N₂O and CO₂ emissions were significantly higher than that under the conditions of increased temperature and annual precipitation:With the conditions of increased temperature combined with the declining of precipitation,the effects of elevated CO₂ on soil CH₄ uptake decreased by 4.25%～7.41%,N₂O and CO₂ emissions increased by8.93%～21.93%,1.26%～3.32%respectively.With the conditions of increased temperature and annual precipitation,the effects of elevated CO₂ on soil CH₄ uptake decreased by3.32%～3.94%,N₂O and CO₂ emissions increased by 3.35%～8.03%,0.10%～0.96%.（4）According to the DayCent model,compared to the annual greenhouse gas emissions during 2001～2020,the average annual emission rates of greenhouse gas from2021 to 2060 will be increased under the SSP126 and SSP245 climate scenarios:the mean annual rates of N₂O emission increased by 22.8%and 24.9%,the mean annual emissions of CO₂ increasing by 6.7%and 8.0%,and the uptake of CH₄ in maize farmland decreased by13.6%and 13.4%.（5）According to the DayCent model,under the future climate scenarios of SSP126 and SSP245,there was a significant positive correlation between soil greenhouse gas emissions and air temperature,also a positive correlation was found between the emissions of N₂O and CO₂ and precipitation,while there was a significant negative correlation between soil CH₄ uptake and precipitation.In view of the balance between grain yields and environmental impacts,the recommended N application rate was 225 kg·hm^-2 for maize farmland under the future climate scenarios of SSP126 and SSP245.In clonclusion,DayCent model can simulate soil N₂O,CH₄ and CO₂ emissions well.The combined effects of global warming,increased precipitation,and elevated atmospheric CO₂ concentration increased greenhouse gas emissions from maize farmland.Maize farmland acted as a source of greenhouse gas emissions under the future climate scenarios of SSP126 and SSP245.Greenhouse gas emissions were closely related to air temperature and precipitation,and the recommended N application rate was 225 kg·hm^-2 according to the tradeoff between yield and greenhouse gas emissions.The simulation results can provide basic data support for sustainable agricultural development under climate change.