Microorganisms drive the formation of mineral-associated organic carbon in soils
Soil stores the largest amount of carbon in terrestrial ecosystem with～1500 Pg C at the lm soil depth which plays a crucial role in regulating greenhouse gas emission and global climate change.As the stable organic carbon pool in soils,mineral-associated organic carbon(MAOC)determines the composition,capacity and cycling of soil C.The sources and formation of MAOC,particularly from plants and microorganisms,have long been a subject of debate.Recently,it is widely believed that microbial-derived organic carbon may dominate the preservation of soil organic carbon(SOC)particularly the formation of MAOC.However,a number of evidences have also suggested the important contribution of plant derived organic carbon to MAOC.Plant derived biomolecules such as lipid,lignin and carbohydrate may account for a remarkable proportion of MAOC,making a vital contribution to the accumulation and stabilization of soil organic carbon.However,to date,there is a considerable uncertainty regarding the relative contributions and controlling factors of plant-and microbial-derived MAOC across different ecosystems which hinders the implementation of sustainable soil management strategies.In this study,we are the first to provide empirical evidences on the contribution of plant and microbial derived carbon to MAOC in soils of different ecosystems at a large scale.Surface soils(0-20 cm)were collected from uplands(n=24),paddy fields(n=19),and forest soils(n=21)in eastern China ranging from 18°N to 47°N,and the contributions and driving factors of microbial and plant-derived MAOC were scrutinized based on the analysis of amino sugars and lignin phenols.Correlation analysis,random forest(RF)analysis,variance partition analysis(VPA)and structural equation modeling(SEM)were employed to disentangle the environmental factors and soil attributes regulating the formation and accumulation of MAOC and the possible mechanisms.The major scientific hypothesis is that the contribution of plant-derived C to MAOC is greater than that of microbial-derived,especially for forest soils which have larger plant biomass input.Our results show that microbial necromass C accounts for 44.2％,40.4％,and 41.0％of MAOC in upland,paddy field,and forest soils,while plant-derived MAOC makes up 44.5％,50.1％,and 49.3％respectively.Plant-derived carbon was found to be the main source of MAOC in paddy and forest soils,while the contribution of microbial-derived C was insignificant among different ecosystems.Similar values are found for the fungal glucosamine/bacterial muramic acid ratios(10.6,10.0,and 11.5,P＞0.05)in MAOC.These observations suggest that microbial necromass C tends to be constant in MAOC for various ecosystems,and OC associated with minerals alters the composition of SOC,which is highly resistant to decomposition under varying land utilizations.We discovered that fungal necromass is the main factor affecting lignin phenol content in all ecosystems,indicating the vital roles of microbial anabolism in driving the accumulation of plant-derived carbon in MAOC.The higher proportions of plant-derived C in the stable C pool for paddy and forest soils are assigned to the environmental stress and larger input of plant residues in the two ecosystems,respectively.High clay and silt content and mean annual temperature(MAT)are conducive to the enrichment of plant-derived C in MAOC,while low soil pH and MAT avail the accumulation of microbial-derived C.The findings obtained from this work are of great significance for understanding the responses and stability of SOC to soil managements so as to increase soil C storage and reduce C emissions,and achieve C neutrality goals.Agricultural soils hold great promise in carbon sequestration and the formation of stable carbon pool through various management strategies to regulate soil microbial activity and increase the accumulation of microbial residues.
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