Temporal-spatial Evolution And Driving Factor Analysis Of Soil Organic Carbon In Hainan Island

The carbon peaking and carbon neutralization strategy of China involves in-depth socio-economic development and transformation,which helps to achieve low-carbon or even zero-carbon emissions.It is also a crucial opportunity for sustainable development.As a key component of the global carbon pool,the soil organic carbon has complex composition and various feedback mechanisms,which is strongly affected by human activities.The increment of soil organic carbon stability and its carbon sink is an important way to slow down the increase of atmospheric CO₂ concentration.So far,the sequestration characteristics of soil organic carbon at different regional scales has been widely reported.The existing researches mainly focus on quantifying the storage and distribution of organic carbon in regional ecosystems,while it remains insufficiently understood in the analysis of driving factors causing changes in soil carbon sequestration capacity,and excavation of the relative contribution rate of driving factors.This seriously hinders the accurate assessment of the spatiotemporal evolution characteristics of soil organic carbon under different soil types,vegetation coverage and land use patterns,and not conducive to exerting the function of soil carbon sink and improving the internal regulation function of the ecosystem.In addition,the global warming has aggravated the changes in soil organic carbon storage and the degree of dynamic fluctuations.Due to its unique geographic location and climatic conditions,tropical regions of high temperature,concentrated rainfall,large evaporation,and rapid decomposition of organic matter.The storage and turnover of soil organic carbon in tropical regions is more strongly affected by environmental conditions and human activities.Taking tropical regions as the research object,the present study aims to reveal the impact of rising temperature on the evolution of soil organic carbon and the contribution of different driving factors,thereby helping to assess and predict the spatiotemporal evolution of soil organic carbon under the background of global climate changes with rising temperature.Thus,this work focuses on Hainan Island,which is the largest tropical island in China.The distribution of soil organic carbon in Hainan Island from the profile level and spatial distribution was discussed.Besides,the spatial distribution of soil organic carbon storage and the main factors affecting its variation was explored.The correction analysis and random forest regression algorithm analyzed the contribution degree of each driving factor to the change of soil organic carbon storage in Hainan Island.The carbon sequestration potential of soil under different soil types and different land use patterns was also analysized.In addition,the carbon turnover under the grass growing measures of the mango orchard in Hainan Island is used as a case study.The regional organic carbon control to explore the main mechanism of mango quality improvement and soil organic carbon changes under the grass growing mode.This study explored ways to reduce soil carbon emissions and increase soil organic carbon storage in tropical areas,and provide a basis for formulating land use management policies in tropical areas.The main results are as follows:（1）The overall distribution characteristics of Hainan Island showed that soil organic carbon density is high in the middle area and low in the surroundings in Hainan Island.Among them,the central and eastern regions are relatively high（12.5～22.4 kg/m²）,while the northwest and coastal areas have relatively low organic carbon density（6.2～12.5kg/m²）.The organic carbon density decreases with the increase of soil depth.From 1980s to 2000s,the total soil organic carbon storage（0～100 cm）decreased from 325.75 Tg to289.44 Tg.The density of organic carbon in the topsoil of Hainan Island has decreased greatly,and the organic carbon storage of 0～5 cm soil layer has decreased by 5.61 Tg,and5～15 cm has decreased by 11.62 Tg.From 1980s to 2000s,the soil organic carbon storage of forest land in Hainan Island decreased from 172.58 Tg to 156.30 Tg,and the total organic carbon storage of cultivated land（rice fields and dry land）increased from 76.97Tg by 90.93 Tg.（2）The correlation between soil organic carbon storage in Hainan Island and various factors including climate,topographic,biological,soil parent material,soil physical and chemical properties,and anthropogenic effects has reached a significant level.Different driving factors can change the soil organic carbon,while the contributions of various driving factors are different.The iron content in the soil of Hainan Island has a high correlation coefficient with soil organic carbon storage,and the contribution rate（22.26%）of iron content to the change of soil organic carbon storage is relatively large.The physical and chemical properties of soil in the 1980s and 2000s were the key factors affecting the changes in soil organic carbon storage,and their relative contribution to the evolution of soil organic carbon storage in Hainan Island reached 60%.（3）There are differences in the carbon sequestration rate and carbon sequestration potential of different soil types or different land use types in Hainan Island.The overall organic carbon density of Hainan Island shows a downward trend.Under the current climatic conditions and human activities,the overall soil organic carbon density and reserves of Hainan Island will continue to decline.Among them,agricultural land（rice fields,dry land and garden land）has serious soil carbon storage losses,and soil organic carbon storage will further decline,while the soil carbon storage of forest land and grassland under natural conditions will gradually increase.The soil of cambisols,ferrisol and entisol have high carbon sequestration potential,and their carbon sequestration potential can increase to 29.59,54.54,and 29.28 Tg,respectively,by taking reasonable measures to regulate and control.The area of paddy fields,dry land,woodland and gardens are relatively large,which has high carbon sequestration potential.By taking reasonable measures,the overall carbon sequestration potential of the soil in Hainan Island can reach 424.60 Tg.（4）Grass planting plays an important role in improving the soil biodiversity,soil functions and services of mango orchards.Field experiments show that grass mulch has a significant impact on the soil microbial community in the mango orchard.Fungal diversity is more sensitive to grass cover than soil bacterial diversity.Brachiaria eruciformis has the strongest influence on soil fungal diversity.Grass mulching can effectively increase the diversity of soil fungi,but has little effect on the diversity of soil bacteria.Soil organic matter and total nitrogen are the most important predictors that affect the distribution of soil bacterial communities.The soil nutrients are the most important factors that mediate the structure of soil fungal communities.Hainan Island has a unique geographical location and tropical climate characteristics.Its active soil carbon pool not only affects the quality and nutrients of the soil,but also has an important impact on the regional carbon turnover and climate change.This study systematically studied the temporal and spatial distribution characteristics of soil organic carbon density and reserves in Hainan Island,and quantified the contribution rate of different driving factors to the evolution of soil organic carbon in Hainan Island.In combination with the case study of grass mulch in mango orchards,the effects of grass planting on soil organic carbon changes,microbial diversity and soil ecological service functions were explored.This research is helpful to reveal the changes of soil organic carbon storage in tropical areas and the corresponding driving factors,so as to explore the methods to reduce soil carbon emissions and increase soil organic carbon storage in tropical areas,and provide a basis for formulating land use management policies in tropical areas.