Features And Mechanisms Of The Contribution Of Microbial Necromass To Soil Organic Carbon Formation In Different Ecosystems

There is growing evidence that microbial necromass are a major source of organic carbon and has an important role in the soil organic carbon(SOC)accumulation.However,since the soil microbial necromass formation and stabilization vary with ecosystem type,this will lead to different mechanisms of SOC formation and stabilization.The lack of quantitative information on the ecosystem differentiation characteristics of SOC formation contributed by microbial residues,the mechanisms and their regulatory factors greatly limit our understanding of SOC formation.The elucidation of the contribution of microbial necromass to SOC accumulation in different ecosystems,the formation/stabilization of microbial residues and their environmental factors is of great theoretical and practical importance for our understanding of the SOC formation regulated by microbial "carbon pumps".To address this central question,we selected global ecosystems(farmland,grassland and forest)and different ecosystems(forest,grassland,farmland and desert)of the Loess Plateau as the subjects of our study.By studying the compositional characteristics of fungal and bacterial necromass in different ecosystems,the contribution of microbial necromass to SOC accumulation,and the regulatory factors of microbial residue accumulation/decomposition.We attempted to answer the questions of where SOC comes from,the contribution of necromass to SOC and its stabilization mechanism in the context of vegetation restoration on the Loess Plateau from the global ecosystem scale to the regional scale of the Loess Plateau.We further elucidate the ecosystem differentiation mechanism of organic carbon formation by microbial necromass contribution and its fit and specificity with the global ecosystem.The results of the study indicate that:(1)The contribution of microbial necromass to SOC accumulation was 51%,47%and 35% for cropland,grassland and forest soils(0-20 cm surface layer)at global scale,respectively.Bacterial necromass accumulated gradually during litter decomposition in Loess Plateau forests(about 5-fold increase)while fungal residues remained stable,and these necromass accumulated at the transition between litter and mineral soil layers.This indicates that the SOC formation contributed by forest soil microbial necromass is stabilized by longer above-ground migration and then from the organic layer to the mineral soil layer.Thus,the contribution of microbial necromass to SOC accumulation in the layers of Ah1(0-5 cm),Ah2(5-15 cm)and AB(15-34 cm)of forest soils on the Loess Plateau was 40.7%,47.7% and 37.0%,respectively.The contribution of soil microbial necromass to SOC in 0-20 cm grassland on the Loess Plateau was about 40%,and this contribution reached a maximum(46%)at the 40 cm soil layer,and decreased significantly with increasing soil depth.Microbial residues increased with the increase of grassland root biomass,and the root biomass was important in determining the accumulation coefficient of microbial necromass and their contribution to SOC accumulation.Microbial necromass are the main source of SOC in cropland soils on the Loess Plateau,with a contribution of 50% in the surface 0-20 cm soil layer.And the contribution of microbial necromass to SOC in sandy soils of desert ecosystems was 12%-25% lower than in well-developed soils due to the insufficient clay content and microbial nitrogen limitation.(2)Fungal and bacterial necromass ratios increasing with ecosystem(i.e.,from cropland-desert-grassland-forest)and substrate complexity(i.e.,soil-litter carbon to nitrogen ratio),with higher fungal/bacterial necromass ratios in grassland and forest than in cropland and desert soils.This may be due to the fact that forest and grassland soils have a higher proportion of plant-derived components and inputs,and fungi are the main decomposers of not easily decomposed plant residues,this leads to a higher proportion of fungal communities which in turn promotes fungal residue accumulation.The fungal necromass of forest soils within the 0-100 cm soil layer were 7,4 and 11 times higher than those of cropland,grassland and desert ecosystems,respectively,while the bacterial necromass content of cropland was 4 and 5 times higher than those of grassland and desert,respectively.The results suggest that the compositional of microbial necromass(fungal and bacterial residues)on the Loess Plateau have distinct ecosystem characteristics,and this distribution pattern is regulated by external factors of specific ecosystems(e.g.,fertilization and tillage of agricultural soils).(3)Fungal and bacterial necromass increased with the increase of soluble nutrients and living microbial biomass in different ecosystems at global and Loess Plateau regional scales,indicating that high soluble nutrient content led to the increase of living biomass and thus contributed to the accumulation of microbial necromass.However,the microbial necromass content did not increase with the increase of living biomass and the decrease of microbial necromass N/ living biomass N during the litter decomposition,indicating that the deficiency of microbial nutrients during litter decomposition leads to the decomposition and recycling of residues.(4)Nutrient levels in the environment and microbial nutrient limitation regulate the dynamics between accumulation and decomposition of microbial necromass.Soluble and microbial biomass C/N are the most important factors affecting microbial necromass accumulation during litter decomposition in the Loess Plateau.Water as an important environmental factor regulates the changes of microbial homeostasis and necromass cycling during litter decomposition,and insufficient soluble nutrients leads to the recycling of microbial residues to maintain a stable microbial C/N homeostasis.Microbial demand for C and N determines soluble nutrient content and thus leads to changes in microbial necromass decomposition/accumulation in the litter-soil profile of a loess plateau forest.Fungal and bacterial necromass content decreased with increasing N-acquisition enzyme activity coefficients,suggesting that microbial nutrient limitation stimulates the mineralization and decomposition of necromass resulting in a lower than global contribution of microbial residues to SOC accumulation in Loess Plateau grasslands.The initial soil formation by biocrusts is characterized by microbial N limitation and low clay content,leading to high activities of enzymes,especially of the N cycle,poor protection and stabilization of microbial necromass,leading to intensive reutilization and thus a low contribution of microbial residues to SOC sequestration.(5)Microbial necromass are mainly stabilized by entering the mineral-associated organic carbon(MAOC)and particulate organic carbon(POC)pools.The increase of root biomass in loess grassland stimulates the accumulation of microbial necromass and then enters the POC and MAOC pools to contribute to SOC formation.The correlation coefficients were higher between the POC and the active carbon pool,while the correlation coefficients were higher between the MAOC and the slow and passive organic carbon pools.This suggests that the POC has a high carbon pool activity,while the MAOC is relatively slow and passive in terms of chemical activity.Microbial necromass was an important source of SOC,and was incorporated into the POC and MAOC.Because bacteria have smaller and thinner cell wall fragments as well as more proteins than fungi,bacterial necromass mainly contributed to the MAOC pool,while fungal residues remained more abundant in the POC.MAOC did not always increase with microbial necromass and POC more rapid accumulation than MAOC during initial soil formation suggests that the clay content was the limiting factor for stable C accumulation in this sandy soil.The necromass exceeding the MAOC stabilization level was stored in the labile POC pool.(6)The contribution of fungal and bacterial necromass mainly corresponds to the dominance of the living microbial groups and depends on land use,soil depth,mean annual precipitation,mean annual temperature,and soil p H.At the regional scale of the Loess Plateau,moisture content,soluble nutrient content,microbial nutrient limitation,protection of clay particles,changes in soil layers,and microbial stoichiometry homeostasis importantly determine microbial necromass accumulation.Microbial homeostasis in different ecosystems of Loess Plateau plays an important role in regulating the decomposition/accumulation of microbial residues.The regulation of microbial homeostasis under high moisture conditions leads to a high intensity of microbial necromass utilization and consequently to a low necromass accumulation.Microbial nutrient limitation and insufficient clay protection lead to an increase in carbon and nitrogen hydrolase enzymes activity or their coefficients to promote the microbial necromass decomposition to meet their nutrient requirements.This study clarified that microbial necromass are an important source of SOC in different ecosystems on the Loess Plateau,and their contribution to SOC accumulation is consistent with global ecosystems.Microbial necromass in cropland and grassland soils contributing more to SOC accumulation than forest soils.However,the influence of regional-scale moisture conditions,microbial nutrient limitation,and soil clay content on the Loess Plateau importantly determine the accumulation and stabilization of microbial necromass,which makes it unique in some ways compared to global ecosystems.This is reflected in the microbial nutrient limitation in Loess Plateau grasslands,which facilitates the recycling of microbial necromass and thus leads to a slightly lower contribution to SOC than global levels.Microbial nitrogen limitation and insufficient clay protection in desert ecosystems leads to the rapid recycling of residue by microorganisms,which results in a significantly lower contribution of necromass to SOC than other ecosystems.Thus,the SOC formation contributed by microbial necromass is importantly influenced by ecosystem type and regulated by its specific environmental conditions.These unique characteristics shape the ecosystem pattern of SOC formation by fungal and bacterial necromass reflect the richness and complexity of SOC formation in different ecosystems on the Loess Plateau.