Study On Carbon Budget Of Grassland Ecosystem By DNDC Model

The grassland ecosystem is an important part of the terrestrial ecosystem.By studying its carbon budget,the role of grassland in the global carbon cycle can be objectively understood,and it is of great significance to the formulation of grassland carbon sinks and other related policies.In order to explore the carbon budget of grassland ecosystems under different utilization methods,we selected degraded natural grassland,mixed grazed artificial grassland and high-yield mown artificial grassland in Ar Horqin Banner,Inner Mongolia,and conducted field experiments in 2019.Using the DNDC model to sort out the carbon source sink,carbon cycle and its dynamic process of grassland ecosystem under different utilization methods.Combined with the distribution map of natural grassland and artificial grassland in the study area obtained by remote sensing,the spatial pattern of carbon budget of grassland ecosystem in the study area was simulated.This research can provide a scientific basis at the mechanism level for the proper management and rational development and utilization of grassland by relevant departments of the grassland industry.The main research results are as follows:（1）Observational data of soil nutrients and greenhouse gas fluxes show that the establishment of artificial grassland including planting,irrigation and other human interventions have effectively improved soil quality and grassland growth,and the soil carbon and nitrogen holding capacity of grazed artificial grassland has increased.In addition,the establishment of artificial grassland promotes the CO₂ emission and CH₄ absorption of grassland to a certain extent,while the behavior of grazing reduces the function of grassland CH₄ sink to a certain extent.The N₂O emission of degraded natural grassland is the lowest in the whole growing season.（2）The parameters of the DNDC model were localized using measured data,and simulation and verification studies were carried out on a point scale.The results show that the simulated Nash–Sutcliffe model efficiency coefficient of soil surface temperature and water content are in the range of 0.37-0.60 and0.40-0.93,and the correlation coefficients are in the range of 0.77-0.90 and0.72-0.94,The Nash–Sutcliffe model efficiency coefficient of ground biomass simulation is between 0.49-0.92,and the correlation coefficient is between0.80-0.98.The correlation coefficient of CO₂ net exchange simulation is 0.91.Generally speaking,the DNDC model can simulate and express the water and heat conditions and carbon budget of grassland well.（3）Combining the measured data and the verified DNDC model,the ground biomass and net ecosystem carbon exchange of grassland under different utilization methods were simulated,and it was found that the ground biomass carbon of grasslands under different utilization methods during the entire growing season represented as follows:Mown artificial grassland>grazed artificial grassland>natural grassland.The net ecosystem carbon exchange of grassland under different utilization methods starts to change from carbon source to carbon sink around the 100th day,and maintains the function of carbon sink throughout the growing season.The annual total carbon sink of mown artificial grassland is the largest(6852.47 kg C?hm^-2?a^-1).It can be seen that the establishment of artificial grassland effectively enhances the carbon sink function of grassland.（4）A regional simulation was carried out based on the distribution of grassland utilization in the study area,and it was found that the grassland in the southern area of Ar Horqin Banner showed an increase in organic carbon,while the grassland in the northern region showed a decrease in organic carbon.The average soil organic carbon change of natural grassland showed a loss(-1.64kg C?hm^-2?a^-1),while artificial grassland shows accumulation(488.58 kg C?hm^-2?a^-1).Ar Horqin Banner has the highest soil carbon emissions in the artificial grassland planting areas in the south,and low-covering natural grasslands in the central and southern regions have lower soil carbon emissions.Although the average soil carbon emission of artificial grassland is much higher than that of natural grassland,the carbon sequestration effect of artificial grassland is also much higher than that of natural grassland.Therefore,the carbon exchange amount of the grassland net ecosystem in Ar Horqin Banner is divided by a small part of the high-coverage grassland in the north.In addition to carbon sources,the rest of the grasslands are carbon sinks.The average net ecosystem carbon exchange of natural grassland and artificial grassland are both expressed as carbon sinks,and the carbon sink function of artificial grassland is much higher than that of natural grassland.