Study On The Spatial Variation Of Soil Organic Carbon And Its Driving Factors In The Inner Mongolian Steppes

Maintaining and enhancing the carbon stock in terrestrial ecosystems is one of the important pathways to achieve the goals of “carbon peak” and “carbon neutrality”,and to mitigate and adapt to global climate change.Soil organic carbon is the most important carbon pool in terrestrial ecosystems and an essential component of the global carbon cycle.Soil organic carbon can be divided into different fractions according to its physicochemical and biological properties,which have different responses and sensitivities to environmental changes.Microbial necromass carbon(MNC)contributes significantly to soil organic carbon sequestration and plays a key role in maintaining soil carbon stability.The natural grasslands are one of the most important ecosystems distributed in the world,accounting for about 30% of the world’s soil carbon stock.However,large-scale grassland degradation in recent decades has led to a decline in steppe productivity and soil carbon storage,posing a major challenge for global sustainable development.Clarifying the distribution patterns of soil organic carbon and its fractions and microbial necromass carbon content and their relationships with biotic and abiotic factors is the basis for understanding the mechanisms of soil organic carbon stability and change,and further developing management systems for restoration of grassland ecosystem health including carbon stocks.In the present study,we surveyed 58 grassland ecological sites in the steppe region in Inner Mongolia.We investigated the vegetation and soil properties with the aims to explore the variation in soil organic carbon content and its components,and analyzing their relations with biotic factors(plant diversity and productivity)and abiotic(climate and soil physicochemical properties)factors.The geographical detector was used to screen out the important environmental factors affecting plant aboveground biomass and soil organic carbon;the partial correlation analysis was used to determine the key environmental factors driving the changes of plant aboveground biomass,plant diversity,soil organic carbon(nitrogen)content.The random forest model was used to determine the important biotic and abiotic factors affecting soil organic carbon fractions and microbial necromass carbon,and then the structural equation model was fitted to analyze the effects of all the biotic and abiotic factors on organic carbon and its fractions,as well as microbial necromass carbon.The main results are as follows:(1)On a regional scale,climate humidity(i.e.,the combination of precipitation and temperature)was the main factor affecting plant community biomass and species richness in Inner Mongolia steppes,while soil texture(bulk density and sand content)and its determined soil water retention parameters(i.e.,field capacity and saturated water content)were the main factors regulating soil organic carbon and nitrogen content.(2)The main factors affecting soil organic carbon and its active fractions,particulate organic carbon(POC)and readily oxidized organic carbon(ROC),were soil total nitrogen(16.76%,19.23% and 17.45%),climate humidity(10.98%,8.91% and12.06%),soil p H value(9.52%,10.16% and 12.82%),and vegetation species richness(9.60%,7.76% and 10.40%),respectively.The main factors affecting mineralassociated organic carbon(MOC)were soil total nitrogen content(21.12%),climate humidity(12.57%),soil silt content(9.21%),soil p H value(6.59%),plant species richness(6.35%).Soil organic carbon content increased with climatic humidity and plant species richness,and decreased with soil sand content and soil p H value.The readily oxidized organic carbon(ROC)was the main contributor to soil organic carbon.(3)Soil total nitrogen(14.31%,14.58% and 15.51%)and organic carbon content(13.53%,15.10% and 14.49%),climate humidity(9.00%,7.85% and 9.56%)were the main factors affecting fungal necromass carbon,bacterial necromass carbon and microbial necromass carbon content.Soil p H value is significantly negatively correlated with fungal necromass carbon and microbial necromass carbon content,while high plant species richness was significantly positively associated with fungal necromass carbon content.Compared with fungal necromass carbon,bacterial necromass carbon was more sensitive to climate change.(4)The main source of soil organic carbon was microbial necromass carbon(>50%),among which fungal necromass carbon was the main contributor(>60%).The increase of climate humidity could significantly increase bacterial necromass carbon content,thereby increasing fungal necromass carbon content in soil.With the increase of climate humidity,plant species richness increased and soil p H value decreased,which indirectly affected fungal necromass carbon content.(5)Microbial necromass carbon is the main influencing factor of soil organic carbon fractions(>30%).Fungal necromass carbon was the most important factor affecting ROC and MOC,while bacterial necromass carbon was the most important factor affecting POC.In summary,the present study shows that climate humidity is undoubtedly one of the main factors affecting soil organic carbon in Inner Mongolia steppe ecosystems,but soil physical properties,such as soil texture and its related water retention parameters,has stronger explanatory power for soil organic carbon changes than climate humidity does.Meanwhile,plant diversity is also a key factor in regulating soil organic carbon fraction changes.These results provide new insights into the variation of soil organic carbon in the natural grassland,and provides the basis for the estimating regional soil organic carbon stocks,and for the development of management systems to achieve the goals of maintaining and raising grassland production and soil carbon stocks.