Study On The Characteristics Of Soil Organic Carbon Content And Its Influencing Factors In The Ploughing Layer Of Farmland At The Southern Foot Of Qilian Mountains ——Taking Menyuan County As An Example

Farmland soil carbon is the most active part of the soil carbon pool,and its changes profoundly affect the global carbon cycle.The study of farmland soil organic carbon is not only an important way to understand the carbon cycle of farmland ecosystems,but also an important link to explore soil fertility.Menyuan County is located at the southern foot of the Qilian Mountains in the northeastern Qinghai-Tibet Plateau.It is a sensitive area with fragile alpine ecology in my country.The area is rich in farmland resources and is the main grain and oil base in the province.In this paper,taking the soil in the cultivated layer of farmland at an altitude of 2400～3200 m in Menyuan County,Qinghai Province as the research object,through field sampling and indoor experiments,the soil physicochemical properties and microbial community diversity characteristics were analyzed,and the content of soil organic carbon in the cultivated layer of farmland at different altitudes was discussed.Variation characteristics and their influencing factors.This paper analyzes the effects of soil physical and chemical properties and microbial diversity on soil organic carbon content,in order to provide basic data reference for the healthy and sustainable development of local farmland soil.It provides certain basic data and scientific support for the study of soil carbon pools in alpine farmland and carbon sequestration and emission reduction under the background of climate change.The main conclusions are as follows:（1）The average content of soil organic carbon in the ploughing layer of farmland at 2400～3200 m altitude in Menyuan County was 28.18 g·kg^-1,the average organic carbon density was 5.57 kg·m^-2,and the average value of soil organic matter was about 48.58 g·kg^-1.According to the nutrient classification standard of the second national soil census,its organic matter content is at a high level,and the farmland soil in the study area is relatively fertile.（2）With the increase of altitude,soil organic carbon content and organic carbon density generally decreased first and then increased.The average values of soil organic carbon content in the tillage layer of the four main soil types,leaching chernozem soil,calcareous lime-cinnamon soil,cold calcium soil and calcareous meadow soil,were 38.33 g·kg^-1,28.54 g·kg^-1,and 29.4,respectively.g·kg^-1 and 20.7g·kg^-1.The average value of organic carbon density from large to small is as follows:leaching chernozem soil>cold calcium soil>calcareous cinnamon soil>calcareous meadow soil.（3）Linear analysis showed that soil clay,water content,total nitrogen,and alkaline hydrolyzed nitrogen had a positive correlation with soil organic carbon;soil bulk density had a negative correlation with soil organic carbon content.The path analysis of the above soil physical and chemical factors and soil organic carbon content shows that soil total nitrogen has the greatest impact on soil organic carbon content,which is a direct positive effect;soil bulk density has the smallest effect on soil organic carbon content,which is an indirect negative effect.Soil alkali-hydrolyzed nitrogen,water content and clay had indirect positive effects on soil organic carbon content mainly through soil total nitrogen.（4）The composition of soil fungal community in the cultivated layer of Menyuan County contains a large number of unclassified fungal phyla;the soil bacterial community phyla is dominated by Proteobacteria and Acidobacteriota,and the sum of the two accounts for 60%of the soil bacterial community.Correlation analysis showed that the ace index,chao1 index and shannon index of soil bacterial community had a significant positive correlation（P<0.01）and a significant positive correlation（P<0.05）with soil organic carbon content,respectively.Soil fungal community ace index and chao1 index were significantly negatively correlated with soil organic carbon content（P<0.01）.The results indicated that the increase in the richness of soil bacterial community diversity in Menyuan County had a positive effect on the increase of soil organic carbon content,while the abundance of soil fungal community OTU numbers had a negative effect on soil organic carbon content.