Microbial Carbon Utilization Research Articles

Microbial carbon utilization in a boreal lake under the combined pressures of brownification and eutrophication: insights from a field experiment

AbstractClimate and land use changes can increase terrestrial runoff to aquatic systems, leading to brownification and eutrophication in northern boreal lakes. Brownification may boost bacterial respiration and production, while eutrophication can enhance primary production and algal blooms. However, their combined effects on basal producers and bacterial carbon utilization are less understood. This study explores the combined impacts of the two stressors: brownification and eutrophication on microbial dynamics in Lake Bolmen. Utilizing a field mesocosm experimental design, treatments received different combinations of organic matter (OM) and inorganic nutrients to simulate predicted future scenarios. Results showed that OM additions significantly increased bacterial production and respiration, regardless of nutrient additions. Nutrient additions enhanced bacterial production but did not affect respiration. Both nutrients and OM stimulated bacterial growth efficiency. Labile carbon from DOM was the main driver of higher bacterial respiration and short-term production increases. Fluorescence data indicated that the combination of brownification and eutrophication led to higher terrestrial DOM utilization than each stressor alone. The study suggests that future boreal lakes may become more heterotrophic, thus increasing CO2 release. These findings highlight the complex interactions between DOM and nutrients and underscore the importance of considering multiple stressors in lake management and mitigation strategies.

Rhizosphere microbial carbon utilization stimulates soil phosphorus fraction transformation in response to maize (Zea mays L.) and soybean (Glycine max.) interspecific interactions

Rhizosphere microbial carbon utilization stimulates soil phosphorus fraction transformation in response to maize (Zea mays L.) and soybean (Glycine max.) interspecific interactions

Degradation-driven vegetation-soil-microbe interactions alter microbial carbon use efficiency in Moso bamboo forests

Microbial carbon utilization efficiency (CUE) is a crucial indicator for evaluating the efficiency of soil carbon sequestration and transformation, which is applied to quantify the proportion of soil carbon extracted by microbes for anabolism (growth) and catabolism (respiration). Previous studies have shown that the degradation of Moso bamboo forests (Phyllostachys edulis) destroyed the aboveground bamboo structure, reduced vegetation carbon storage, and weakened ecosystem carbon sequestration capacity. Interestingly, soil organic carbon stocks are gradually increasing. However, the mechanism by which degradation-induced changes in soil and vegetation characteristics affect microbial CUE and drive soil carbon sequestration remains unclear. Here we selected four stands with the same origin but different degradation years (intensive management, CK; 2 years' degradation, DM1; 6 years' degradation, DM2; and 10 years' degradation, DM3) based on the local management profiles. The principle of space-for-time substitution was used to investigate the changes in microbial CUE along a degradation time and to further identify the controlling biotic and abiotic factors. Our finding showed that microbial CUE increased by 12.27 %, 31.01 %, and 55.95 %, respectively, compared with CK; whereas microbial biomass turnover time decreased from 23.99 ± 1.11 to 17.16 ± 1.20 days. Promoting microbial growth was the main pathway to enhance microbial CUE. Massive inputs of vegetative carbon replenished soil carbon substrate content, and altered microbial communities and life history strategy, which in turn promoted microbial growth and increased microbial CUE. These findings provide theoretical support for the interactions between carbon dynamics and microbial physiology in degraded bamboo forests, and reinforce the importance of vegetation and microbial properties and soil carbon substrates in predicting microbial CUE.

Submerged macrophyte restoration enhanced microbial carbon utilization in shallow lakes

Submerged macrophytes are integral to the functioning of shallow lakes through their interaction with microorganisms. However, we have a limited understanding of how microbial communities in shallow lakes respond when macrophytes are restored after being historically extirpated. Here, we explored the interactions between prokaryotic communities and carbon utilization in two lakes where submerged macrophytes were restored. We found restoration reduced total carbon in sediment by 8.9 %–27.9 % and total organic carbon by 16.7 %–36.9 % relative to control treatment, but had no effects on carbon content in the overlying water. Sediment microbial communities were more sensitive to restoration than planktonic microbes and showed enhanced utilization of simple carbon substrates, such as Tween 40, after restoration. The increase in carbon utilization was attributed to declines in the relative abundance of some genera, such as Saccharicenans and Desertimonas, which were found weakly associated with the utilization of different carbon substrates. These genera likely competed with microbes with high carbon utilization in restored areas, such as Lubomirskia. Our findings highlight how restoring submerged macrophytes can enhance microbial carbon utilization and provide guidance to improve the carbon sequestration capacity of restored shallow lakes.

Recycled by leaf inhabitants: Terrestrial bacteria drive the mineralization of organic matter in lake water

AbstractTerrestrial organic matter subsidizes aquatic food webs and plays an important role in carbon cycling in lake ecosystems, where it is decomposed mainly by microbes. However, the contribution of terrestrial and aquatic microbiomes on terrestrial carbon cycling and their effects on the biochemical fate of carbon has remained understudied. Therefore, we explored the microbial carbon utilization of three chemically differing leaf species in lake water in microcosms and quantified the biochemical endpoints of leaf carbon in CO2, CH4, and microbial biomass. Additionally, we identified microbial taxa responsible for leaf carbon recycling and studied the role of epiphytic and endophytic leaf microbiomes in microbial community succession in lake water. Microbially utilized leaf carbon was mainly respired (82.7 ± 1.4%), whereas a small proportion (17.1 ± 1.4%) was assimilated into biomass. Carbon from nitrogen‐rich alder leaves was taken up at the fastest rate, whereas birch leaf addition produced the highest concentrations of CH4, suggesting that leaf chemistry affects the decomposition rate and biochemical fate of carbon. In particular, terrestrial bacteria shaped the succession of aquatic bacterial communities. The addition of leaves resulted in the equal contribution of epiphytic and endophytic bacteria in the lake water, whereas epiphytic fungi dominated the fungal community structure. Our results suggest that terrestrial bacteria originating from terrestrial leaves influence the microbiome succession in lake ecosystems and play a key role in linking terrestrial carbon to an aquatic food web and determining the quality of carbon emissions that are released into the atmosphere.

Field-aged rice hull biochar stimulated the methylation of mercury and altered the microbial community in a paddy soil under controlled redox condition changes

Mercury (Hg) contaminated paddy soils are hot spots for methylmercury (MeHg) which can enter the food chain via rice plants causing high risks for human health. Biochar can immobilize Hg and reduce plant uptake of MeHg. However, the effects of biochar on the microbial community and Hg (de)methylation under dynamic redox conditions in paddy soils are unclear. Therefore, we determined the microbial community in an Hg contaminated paddy soil non-treated and treated with rice hull biochar under controlled redox conditions (< 0 mV to 600 mV) using a biogeochemical microcosm system. Hg methylation exceeded demethylation in the biochar-treated soil. The aromatic hydrocarbon degraders Phenylobacterium and Novosphingobium provided electron donors stimulating Hg methylation. MeHg demethylation exceeded methylation in the non-treated soil and was associated with lower available organic matter. Actinobacteria were involved in MeHg demethylation and interlinked with nitrifying bacteria and nitrogen-fixing genus Hyphomicrobium. Microbial assemblages seem more important than single species in Hg transformation. For future directions, the demethylation potential of Hyphomicrobium assemblages and other nitrogen-fixing bacteria should be elucidated. Additionally, different organic matter inputs on paddy soils under constant and dynamic redox conditions could unravel the relationship between Hg (de)methylation, microbial carbon utilization and nitrogen cycling.

Nitrogen-fixing cyanobacteria enhance microbial carbon utilization by modulating the microbial community composition in paddy soils of the Mollisols region

Nitrogen-fixing cyanobacteria (NFC) are photosynthetic prokaryotic microorganisms capable of nitrogen fixation. They can be used as biofertilizers in paddy fields, thereby improving the rice tillering capacity and yield. To reveal the microbiological mechanisms by which nitrogen-fixing cyanobacteria alter soil carbon storage, we conducted a field experiment using NFC as a partial substitute for nitrogen fertilizer in paddy fields in the Sanjiang Plain of Northeast China's Mollisols region. Using metagenomic sequencing technology and Biolog Ecoplate™ carbon matrix metabolism measurements, we explored the changes in the soil microbial community structure and carbon utilization in paddy fields. The results indicated that the replacement of nitrogen fertilizer with NFC predisposed the soil microbial community to host a great number of copiotrophic bacterial taxa, and Proteobacteria and Actinobacteria were closely associated with the metabolism of soil carbon sources. Moreover, through co-occurrence network analysis, we found that copiotrophic bacteria clustered in modules that were positively correlated with the metabolic level of carbon sources. The addition of NFC promoted the growth of copiotrophic bacteria, which increased the carbon utilization level of soil microorganisms, improved the diversity of the microbial communities, and had a potential impact on the soil carbon stock. The findings of this study are helpful for assessing the impact of NFC on the ecological function of soil microbial communities in paddy fields in the black soil area of Northeast China, which is highly important for promoting sustainable agricultural development and providing scientific reference for promoting the use of algal-derived nitrogen fertilizers.

Effects of Organic Fertilizer of Kitchen Waste on Soil Microbial Activity and Function

Changes in soil microbial activity and ecological function can be used to assess the level of soil fertility and the stability of ecosystems. To assess the fertility and safety of organic fertilizer of kitchen waste (OFK), soils containing 0% (CK), 1%, 3%, and 5% OFK were cultured, and the physical, chemical, and microbial properties of the soils were measured dynamically with routine agrochemical analysis measures and amplicon sequencing. The results showed that compared with those in CK, the contents of organic matter, available phosphorus, available potassium, NH4+-N, and NO3--N in soils with OFK increased by 23.80%-35.13%, 13.29%-29.72%, 16.91%-39.37%, 164.7%-340.2%, and 28.56%-32.71%, respectively. The activities of hydrolases related to the cycle of carbon, nitrogen, and phosphorus (α-glucosidase, leucine aminopeptidase, acid phosphatase, etc.) were also significantly higher than those of the CK treatment. OFK stimulated the growth of soil microorganisms and increased the carbon content of the microbial biomass. The amplicon sequencing analysis found that the microbial community structures of different treatments were significantly different at both the class and genus levels. In addition, it was found that the abundance of beneficial microbes in the soils with OFK increased, whereas pathogenic microbes decreased. RDA results confirmed that soil properties (including soil pH, organic matter, available nutrients, and microbial biomass) had a significant impact on microbial community structure. The results of investing bacterial community based on PICRUSt and FAPROTAX revealed that the function of the soil bacterial community was similar in the four treatments, but OFK supply significantly improved the microbial carbon utilization and metabolic ability. Moreover, by using the FUNGuild software, we found that the application of OFK increased the proportion of saprotroph-symbiotroph and symbiotroph and stimulated the growth of ectomycorrhizal fungi-undefined saprophytic fungi but inhibited plant and animal pathogenic fungi in soil. These results implied that OFK could promote the establishment of symbiotic relationships and inhibit the growth of pathogenic fungi. In summary, OFK could improve soil fertility and hydrolase activity, stimulate the growth of beneficial microorganisms, and defend against pathogens, indicating a promising use as safe and efficient organic fertilizer.

Effects of long-term drainage and afforestation on carbon utilization function of soil microbial communities vary between the types of subtropical moss peatlands

Subtropical moss peatlands are valuable delicate ecosystems that play crucial roles in biodiversity preservation, carbon sequestration, and water retention. However, driven by economic interests, a large area of subtropical moss peatlands in the Yunnan-Guizhou Plateau of China has been used for drainage and afforestation of a Cryptomeria japonica forest, leading to changes in the ecological functions of the peatlands. The Biolog EcoPlate™ method was used to study the differences in the carbon utilization functions of the soil microbial communities between a typical natural Sphagnum fen (SF), a natural Polytrichum bog (PB) and a Cryptomeria swamp forest that were formed by long-term drainage and afforestation (more than 20 years) in the region. Combined with measurements of the soil physicochemical properties, the effects of drainage and afforestation on different types of moss peatlands and the underlying mechanisms were explored. The results showed that long-term drainage and afforestation significantly increased the soil microbial carbon utilization activity and functional diversity in the SF, but no significant effects were observed in the PB. Drainage and afforestation did not significantly alter the proportion structures of the soil microbial carbon source utilization in the SF or PB but did significantly affect soil microbial preferences for specific carbon source utilization in the SF. Carbohydrates, amino acids, carboxylic acids and polymers were the main carbon sources utilized by the microbial communities in the three vegetation types. The soil volumetric water content (SVW) and available phosphorus (AP) content were significant factors affecting the functional characteristics of soil microbial carbon utilization. There was a significant negative correlation between the SVW and the activity of soil microbial carbon source utilization and functional diversity, and there was a significant positive correlation between the AP and polymer utilization activity. Moreover, the SVW significantly and indirectly affected overall carbon source utilization by influencing the utilization of carbohydrates. In the restoration of subtropical moss peatlands affected by drainage and afforestation, in addition to emphasizing vegetation restoration, the SF restoration should be focused on the restoration of water conditions, while the PB restoration should be focused on the restoration of nutrients (such as AP).

Co-exposure of microplastics and sulfamethoxazole propagated antibiotic resistance genes in sediments by regulating the microbial carbon metabolism

The concerns on the carriers of microplastics (MPs) on co-existing pollutants in aquatic environments are sharply rising in recent years. However, little is known about their interactions on the colonization of microbiota, especially for the spread of pathogens and antibiotic resistance genes (ARGs). Therefore, this study aimed to investigate the influences on the propagation of ARGs in sediments by the co-exposure of different MPs and sulfamethoxazole (SMX). The results showed that the presence of MPs significantly enhanced the contents of total organic carbon, while having no effects on the removal of SMX in sediments. Exposure to SMX and MPs obviously activated the microbial carbon utilization capacities based on the BIOLOG method. The propagation of ARGs in sediments was activated by SMX, which was further promoted by the presence of polylactic acid (PLA) MPs, but significantly lowered by the co-exposed polyethylene (PE) MPs. This apparent difference may be attributed to the distinct influence on the antibiotic efflux pumps of two MPs. Moreover, the propagation of ARGs may be also dominated by microbial carbon metabolism in sediments, especially through regulating the carbon sources of carboxylic acids, carbohydrates, and amino acids. This study provides new insights into the carrier effects of MPs in sediments.

Alteration of microbial mediated carbon cycle and antibiotic resistance genes during plastisphere formation in coastal area

Microorganisms can attach on the surface of microplastics (MPs) through biological fouling process to form a diverse community called the "plastisphere", which has attracted extensive attention. Although the microbial structure and composition of biofilm have been studied, the knowledge of its microbial function and ecological risk is still limited. In this study, we investigated how the surface properties of MPs affect the biofilm communities and metabolic features under different environmental conditions, and explored the biofilm enrichment of antibiotic resistance genes (ARGs). The results showed that the incubation time, habitat and MPs aging state significantly influenced the structure and composition of biofilm microbial communities, and a small amount of pathogens have been found in the MPs-attached biofilm. The microbial carbon utilization capacity of the biofilm in different incubation habitats varies greatly with highest metabolism capacity appear in the river. The utilization efficiency of different carbon sources is polymer > carbohydrate > amino acid > carboxylic acids > amine/amide, which indicates that the biofilm communities have selectivity between different types of carbon sources. More importantly, ARGs were detected in all the MPs samples and showed a trend of estuary > river > marine. The aged MPs can accumulate more ARGs than the virgin items. In general, MPs in the aquatic environment may become a carrier for pathogens and ARGs to spread to other environment, which may enhance their potential risks to the ecosystem and human health.

Seasonal Regulation of Soil Microbial Carbon and Phosphorus Metabolisms in an Apple Orchard: Evidence from the Enzymatic Stoichiometry Method

Soil microbial carbon (C), nitrogen (N), and phosphorus (P) nutrient requirements and metabolic limitations are closely related to the availability of environmental nutrients. However, it is unclear how manure and chemical fertilization shift nutrient limitations for microbes in terms of the soil enzymatic stoichiometry in an apple orchard. Therefore, based on the long-term experiment located in an apple orchard established in 2008, this study applied the theory and method of soil enzyme stoichiometry to systematically investigate the effects of the combined application of manure and chemical fertilizers on soil C, N, and P turnover-related enzyme activities (β-1,4-glucosidase, BG; leucine aminopeptidase, NAG; β-1,4-N-acetylglucosaminidase, LAP; and acid or alkaline phosphatase, PHOS) and their stoichiometric characteristics and analyzed their relationships with environmental factors and microbial carbon use efficiency. The experiment was designed with four treatments, such as, no fertilization input as the control (CK), single application of chemical fertilizer (NPK), combined application of manure and chemical fertilizer (MNPK), and single application of manure (M). The results revealed that:① at different growth stages of fruit trees, the soil microbial biomass C (microC) content of manure fertilizer treatments (MNPK and M) was significantly higher than that of no manure fertilizer treatments (CK and NPK). The content of microbial biomass N (microN) in the NPK, MNPK, and M treatments increased by 89%, 269%, and 213%, respectively, compared with that in CK (P<0.05). ② Compared with those in the fertilization treatments, CK had higher leaf N and P contents (29.8 g·kg-1 and 2.17 g·kg-1) at the germination stage, and the leaf P content at the germination stage alone was significantly negatively correlated with soil available phosphorus (AP) content. ③ Soil enzyme stoichiometry analysis demonstrated that all data points in this study were above the 1:1 line, indicating that microbial communities had a strong phosphorus limitation. The range of vector length and angle was 0.56-0.79 and 59.3°-67.7°, respectively, in the growth period of fruit trees, and the vector angle was >45° in this study, which also reflected the strong phosphorus limitation of microorganisms. ④ RDA and random forest model analysis showed that organic carbon and available nitrogen (AN) were the main physical and chemical factors affecting vector length; AP, AN, and soil water content were the main physical and chemical factors affecting vector angle. Combined with SEM analysis, AN and dissolved organic carbon (DOC) directly affected microC and microN, AP directly affected microP and microN, DOC and AP directly affected vector length, and AP and microN directly affected vector angle. In addition, microbial carbon utilization was positively correlated with vector length and negatively correlated with vector angle. In summary, the combined application of manure and chemical fertilizers regulated microbial carbon and phosphorus metabolism by affecting soil carbon and phosphorus content at different growth stages of fruit trees, thereby affecting microbial carbon utilization. This study provides a scientific basis for manure and chemical fertilizers to improve soil quality and maintain soil health.

Carbon source shaped microbial ecology, metabolism and performance in denitrification systems.

The limited information on microbial interactions and metabolic patterns in denitrification systems, especially those fed with different carbon sources, has hindered the establishment of ecological linkages between microscale connections and macroscopic reactor performance. In this work, denitrification performance, metabolic patterns, and ecological structure were investigated in parallel well-controlled bioreactors with four representative carbon sources, i.e., methanol, glycerol, acetate, and glucose. After long-term acclimation, significant differences were observed among the four bioreactors in terms of denitrification rates, organic utilization, and heterotrophic bacterial yields. Different carbon sources induced the succession of denitrifying microbiota toward different ecological structures and exhibited distinct metabolic patterns. Methanol-fed reactors showed distinctive microbial carbon utilization pathways and a more intricate microbial interaction network, leading to significant variations in organic utilization and metabolite production compared to other carbon sources. Three keystone taxa belonging to the Verrucomicrobiota phylum, SJA-15 order and the Kineosphaera genus appeared as network hubs in the methanol, glycerol, and acetate-fed systems, playing essential roles in their ecological functions. Several highly connected species were also identified within the glucose-fed system. The close relationship between microbial metabolites, ecological structures, and system performances suggests that this complex network relationship may greatly contribute to the efficient operation of bioreactors.

Temperature-mediated microbial carbon utilization in China's lakes.

Microbes play an important role in aquatic carbon cycling but we have a limited understanding of their functional responses to changes in temperature across large geographic areas. Here, we explored how microbial communities utilized different carbon substrates and the underlying ecological mechanisms along a space-for-time substitution temperature gradient of future climate change. The gradient included 47 lakes from five major lake regions in China spanning a difference of nearly 15°C in mean annual temperatures (MAT). Our results indicated that lakes from warmer regions generally had lower values of variables related to carbon concentrations and greater carbon utilization than those from colder regions. The greater utilization of carbon substrates under higher temperatures could be attributed to changes in bacterial community composition, with a greater abundance of Cyanobacteria and Actinobacteriota and less Proteobacteria in warmer lake regions. We also found that the core species in microbial networks changed with increasing temperature, from Hydrogenophaga and Rhodobacteraceae, which inhibited the utilization of amino acids and carbohydrates, to the CL500-29-marine-group, which promoted the utilization of all almost carbon substrates. Overall, our findings suggest that temperature can mediate aquatic carbon utilization by changing the interactions between bacteria and individual carbon substrates, and the discovery of core species that affect carbon utilization provides insight into potential carbon sequestration within inland water bodies under future climate warming.

Changes in soil microbial metabolic activity following long‐term forest succession on the central Loess Plateau, China

AbstractForest succession can alter terrestrial ecosystem processes and potentially impact subsurface carbon dynamics. However, the effects of long‐term forest succession on soil microbial metabolic activity remain unclear. This study investigated a long‐term forest succession approximately ~160 years after farmland abandonment on the central Loess Plateau, China, to explore the changes in soil microbial metabolism following the forest succession. Among them, the soil basal respiration (BR), microbial quotient (qMB), and metabolic quotient (qCO2) were studied. The results showed that the BR and qCO2 were increased following forest succession, whereas the qMB showed the opposite trend. Forest succession also increased the activities of β‐1,4‐glucosidase (BG) and β‐D‐cellobiosidase (CBH). The increase of BR and qCO2 and the decrease of qMB occurred in the early stage (&lt;60 years). However, the increase of BG and CBH mainly occurred in the later period (&gt;100 years). Soil microbial metabolic activity was closely related to litter biomass, dissolved organic carbon, fungal, and bacterial community composition. In conclusion, long‐term forest succession changes soil microbial community structure and promotes soil organic carbon accumulation, while the increase of soil respiration and metabolism promotes the release of carbon element following forest succession, thus reducing soil microbial carbon utilization efficiency.

Effects of Nitrogen Addition on Microbial Carbon Use Efficiency of Soil Aggregates in Abandoned Grassland on the Loess Plateau of China

Soil microbial carbon use efficiency (CUE) plays a crucial role in terrestrial C cycling. However, how microbial CUE responds to nitrogen addition and its mechanisms in soil aggregates from abandoned grassland systems remains poorly understood. In this study, we designed a nitrogen (N) addition experiment (0 (N0), 10 (N1), 20 (N2), 40 (N3), 80 (N4) kg N ha−1yr−1) from abandoned grassland on the Loess Plateau of China. Subsequently, the enzymatic stoichiometry in soil aggregates was determined and modeled to investigate microbial carbon composition and carbon utilization. The vegetation and soil aggregate properties were also investigated. Our research indicated that soil microbial CUE changed from 0.35 to 0.53 with a mean value of 0.46 after N addition in all aggregates, and it significantly varied in differently sized aggregates. Specifically, the microbial CUE was higher and more sensitive in macro-aggregates after N addition than in medium and micro-aggregates. The increasing microbial CUE in macro-aggregates was accompanied by an increase in soil organic carbon and microbial biomass carbon, indicating that N addition promoted the growth of microorganisms in macro-aggregates. N addition significantly improved the relative availability of nitrogen in all aggregates and alleviated nutrient limitation in microorganisms, thus promoting microbial CUE. In conclusion, our study indicates that soil microbial CUE and its influencing factors differ among soil aggregates after N addition, which should be emphasized in future nutrient cycle assessment in the context of N deposition.

The ratio of ligninase to cellulase increased with the reduction of plant detritus input in a coniferous forest in subtropical China

Soil extracellular enzymes play a central role in mediating the decomposition of soil organic matter (SOM), and the activities of cellulase and ligninase therein quantify the preference of microbial carbon (C) utilization. However, the responses of cellulase and ligninase activities to plant detritus input change remain uncertain. Here, we investigated the activities of cellulase and ligninase after two years of detritus input manipulations (the detritus input and removal treatment (DIRT) include a control without litter manipulation, CK; double litter, DL; no litter, NL; no roots, NR; no litter and no roots, NRNL) in a coniferous forest in subtropical China. The litter removal treatments had negligible effect on cellulase activity, while the DL treatment significantly increased it by 55.7% compared to the CK treatment. The NL and NRNL treatments increased the activity of ligninase by 60.1% and 46.9%, respectively. However, the DL treatment did not significantly affect the activity of ligninase. Consequently, the ratio of ligninase to cellulase significantly increased under the litter removal treatments. Notably, the specific enzyme activity (the amount of enzyme produced per unit microbial biomass) increased under the litter removal treatment, but the DL treatment did not significantly affect it. The increased ratio of ligninase to cellulase under the litter removal treatments was primarily driven by the increased recalcitrance of substrates and higher proportion of fungal and gram-positive bacterial community. Moreover, the specific enzyme activity and ratio of ligninase to cellulase were positively correlated with microbial metabolic quotient (qCO2). Overall, our results provided an empirical evidence that microorganisms could shift substrate-using strategy by upregulating the production of ligninase with the reduction of plant detritus input. More importantly, the enhanced activity or proportion of ligninase resulted in higher qCO2, and thereby can accelerate soil C loss once plant detritus input is decreased caused by some global change drivers.

Variation of soil microbial carbon use efficiency (CUE) and its Influence mechanism in the context of global environmental change: a review.

Soil microbial carbon utilization efficiency (CUE) is the efficiency with which microorganisms convert absorbed carbon (C) into their own biomass C, also referred to as microorganism growth efficiency. Soil microbial CUE is a critical physiological and ecological parameter in the ecosystem’s C cycle, influencing the processes of C retention, turnover, soil mineralization, and greenhouse gas emission. Understanding the variation of soil microbial CUE and its influence mechanism in the context of global environmental change is critical for a better understanding of the ecosystem’s C cycle process and its response to global changes. In this review, the definition of CUE and its measurement methods are reviewed, and the research progress of soil microbial CUE variation and influencing factors is primarily reviewed and analyzed. Soil microbial CUE is usually expressed as the ratio of microbial growth and absorption, which is divided into methods based on the microbial growth rate, microbial biomass, substrate absorption rate, and substrate concentration change, and varies from 0.2 to 0.8. Thermodynamics, ecological environmental factors, substrate nutrient quality and availability, stoichiometric balance, and microbial community composition all influence this variation. In the future, soil microbial CUE research should focus on quantitative analysis of trace metabolic components, analysis of the regulation mechanism of biological-environmental interactions, and optimization of the carbon cycle model of microorganisms’ dynamic physiological response process.

Seasonal Variations in Litter Layers' Characteristics Control Microbial Respiration and Microbial Carbon Utilization Under Mature Pine, Cedar, and Beech Forest Stands in the Eastern Mediterranean Karstic Ecosystems.

The forest floor is hotspot of several functions integral to the stability of forest ecosystems. However, seasonal variations in litter decomposition rate contribute to biochemical and structural heterogeneity in the forest floor carbon (C) and nutrient cycling. We investigated the influence of seasonal variations in litter layers' micro-climate (temperature and moisture content) and chemical characteristics such as pH, electrical conductivity (EC), total organic C (TOC), total nitrogen (TN), and C/N ratio on microbial respiration, biomass, and C use efficiency under mature (> 80years stage age) pine, beech, and cedar forests in eastern Mediterranean Karstic ecosystems. In contrast to significantly higher microbial respiration in fall, winter, and spring under pine, beech, and cedar forests, the significantly lowest microbial biomass C (MBC) and microbial biomass N (MBN) were observed in winter under each forest. Microbial C use efficiency, measured as the metabolic quotient (qCO2 = CO2/MBC), varied strongly between forest stands and seasons but was generally higher in winter. The significant positive correlations between litter layer and microbial biomass C/N ratios, under beech and cedar forests, suggested strong CN stoichiometric coupling and microbial adaptation to substrate resource stoichiometry. qCO2 correlated significantly negatively with litter layers' temperature, positively with moisture content and EC. However, qCO2 had significant negative relationships with pH in pine and beech forests but significant positive under cedar forest. qCO2 showed significant positive relationships with C/N ratios under all forests but much stronger in beech and cedar forests suggesting higher C respired per unit MBC with an increase in C/N ratio. Despite variations between forest species, the highest MBC/TOC and MBN/TN ratios in fall indicated greater C and N incorporation into microbial biomass. Changes in MBC/MBN ratios under pine (9.62-10.6), beech (8.63-15.6), and cedar (7.32-16.2) forests indicated the shift in microbial communities as fungi have a higher C/N ratio than bacteria. Stepwise regression analysisfurther revealed that microbial respiration and biomass were controlled differentlyby litter layer characteristics in each forest. This study suggested that qCO2 independently or with other microbial indices can show litter layers' controls on organic matter turnover in Karst ecosystems and, taking into account the strong seasonal variations, can enhance the predictive potential of decomposition models.

Development of energetic and enzymatic limitations on microbial carbon cycling in soils

Soil organic carbon (SOC) constitutes an important reservoir in the global carbon cycle that is vulnerable to transformation and loss from land use and climate change. Anoxic conditions protect SOC from microbial degradation through limiting the energetics of respiration and inhibiting extracellular oxidative enzymes. Given growing evidence of prevalent anaerobic microsites in upland soils, we designed an experiment testing the development of dissolved organic carbon (DOC) signatures of energetic and enzymatic limitations on microbial carbon utilization across simulated soil aggregates or peds. Reactors comprised a soil column “aggregate” underlying an advective “macropore” channel. Soils received downward diffusive inputs of aerated porewater media with added nitrate, sulfate, or no amendment—where native ferrihydrite served as dominant anaerobic terminal electron acceptor (TEA). After 40 days, added nitrate resulted in highest bulk respiration and DOC production while sulfate did not differ from the control. Nominal oxidation state of carbon (NOSC) was higher (more favorable) with added TEAs at soil surfaces and decreased with depth, while NOSC in the non-amended soil remained lower and constant with depth. DOC generally increased with depth, which along with decreasing NOSC values indicates joint electron-donor and acceptor control over respiration energetics. Of all organic compound classes, only the relative abundance of phenolics increased between 0 and 0.5 cm depth, which aligns with the oxic-anoxic transition and suggests oxidative enzyme inhibition. Our results suggest that oxygen limitation within upland soil aggregates may preserve SOC via both energetic and enzymatic C protection mechanisms, which are vulnerable upon exposure to oxygen.