Molecular-level Characteristics Of Organic Matter In Standard Reference Materials And Forest Surface Soils And The Influencing Factors

As the most active part of carbon pool,soil organic matter（SOM）plays an important role in global climate change and carbon cycle.Forest ecosystem carries the largest terrestrial soil carbon pool and plays an important role in regulating global carbon balance.However,the basic data of the composition characteristics and driving factors of forest SOM at a wide scale in China are still unclear.Therefore,it is of great significance to explore SOM characteristics of different geographical locations in China for further understanding the stability of soil carbon pool.The origin,composition and degradation degree of SOM may be affected by geographical environmental factors and soil physical and chemical properties,but the specific relationship between them is not clear at present.Therefore,this study selected 11 soil reference material samples and collected forest soil samples（0-5 cm and 5-10 cm deep）from 18 sites in five temperature zones in China.By means of targeted marker method,carbon isotope,X-ray fluorescence spectroscopy and solid ¹³C Nuclear Magnetic Resonance(¹³C NMR)spectroscopy,the differences in molecular level characteristics between Chinese standard substances and wild forest soils were explored.And the factors of geographical environment and physical and chemical properties that affect SOM characteristics.To sum up,this paper mainly draws the following conclusions:（1）With the increase of annual mean temperature and rainfall,the p H of soil standard matter and forest surface soil samples decreased,the content of iron and aluminum oxides increased,and the content of calcium,magnesium,potassium,sodium and other oxides decreased gradually.The degradation and accumulation of free lipids,conjugated lipids and lignin phenols in soil standard matter and forest topsoil were significantly correlated with the content changes of soil mineral elements such as iron and aluminum oxides and calcium oxide.（2）There was a negative correlation between the concentration of lignin markers in soil standard matter and the abundance of aromatic and phenolic C functional groups in ¹³C NMR.With the increase of weathering degree,the content of iron and aluminum oxides in soil standard material samples increased,while the concentration of lignin markers decreased gradually,which may be caused by the selective protection of lignin markers in soil standard material samples by iron and aluminum oxides.In addition,the concentration of cutin markers in soil standard matter increased with the increase of annual mean precipitation and temperature,which may be because high rain-heat conditions increased the primary productivity of plants and their input of cutin to soil.（3）The results of the 0-5 cm surface forest soil study showed that the alkyl C OM（e.g.carbohydrate）increased significantly with the increase of annual mean temperature and humidity.SOM content was negatively correlated with lignin marker concentration,indicating that lignin compounds did not play a major role in organic matter accumulation in this layer.5-10 cm subsurface soil had lower Ad/Al value and higher lignin marker concentration than 0-5 cm subsurface soil,which may be due to the low microbial activity in 5-10 cm subsurface soil.（4）The results of the 5-10 cm subsurface forest soil showed that the lower the p H value of the soil,the higher the concentration of cutin and suberin markers.With the increase of annual mean temperature and annual mean rainfall,the marker concentrations of cutin and suberin increased significantly.It is possible that the increase in primary productivity increased the input of vegetation litter into the soil.Aluminum oxide played a positive role in promoting the preservation of cutin and suberin markers in the environment,while iron oxide showed no correlation with cutin and suberin markers.Conversely,the higher the percentage of calcium oxide,the lower the concentration of cutin and suberin markers.Lignin markers in forest surface soil samples showed no significant correlation with mineral elements such as aluminum oxide and calcium oxide.In conclusion,regional climate（temperature and rainfall）and soil mineral characteristics are important factors that affect the chemical composition and characteristics of soil organic matter at a wide spatial scale in China.This paper provides basic data for further understanding the SOM content,chemical composition and environmental effects of forests in China,and provides a theoretical basis for forest soil management and ecological environmental protection.