Carbon Use Efficiency Research Articles

Multiple Factors Jointly Lead to the Lower Soil Microbial Carbon Use Efficiency of Abies fanjingshanensis in a Typical Subtropical Forest in Southwest China

Abies fanjingshanensis trees are the only remaining Abies species in a type of subtropical forest of southwest China and are in imminent danger. Previous studies suggested that the massive death of Abies was caused by the unbalanced chemometrics and nutrients in the soil. To the best of our knowledge, for the first time, we evaluated the microbial carbon use efficiency (CUE) in the rhizospheric topsoil and subsoil of A. fanjingshanensis, at high elevation, middle elevation, and low elevation as well as investigated their physicochemical indices, soil enzyme activities, bacteria, fungi, and microbial biomass. The results showed that the physicochemical parameters (TP, SOC, AK, AP, MC, TN, NO3-N, NH4-N and cation exchange capacity) of the topsoil were higher than those of the subsoil. Acidobacteria, Proteobacteria, Planctomycetes, and Actinobacteria were the dominant phyla in the two soil layers. Candidatus_Koribacter was the main indicator species in the rhizospheric topsoil and subsoil. The positive correlation in the bacterial co-occurrence networks implied that cooperation was dominant between the bacteria in four soil types, and the same phenomenon was found in the co-occurrence networks of fungi. A structural equation model confirmed that pH was the most important factor affecting microbial CUE in the topsoil and subsoil. We inferred that the microorganisms in the acidic soil environment were forced to consume more energy to maintain cellular pH, while less energy was used for growth. The increased solubility of some toxic metals in the acidic soil affected the microbes, resulting in a lower microbial CUE in the A. fanjingshanensis rhizospheric soil. Our results highlight that pH values in soil mainly affected microbial CUE, and a lower microbial CUE may be another important factor in the death of large numbers of A. fanjingshanensis. Several measures must be carried out to improve the microbial CUE in the rhizospheric soil of A. fanjingshanensis by the department of forest management, such as adding the appropriate biochar and nitrogenous fertilizer.

Potential bioavailability of representative pyrogenic organic matter compounds in comparison to natural dissolved organic matter pools

Abstract. Pyrogenic organic matter (PyOM) from wildfires impacts river corridors globally and is widely regarded as resistant to biological degradation. Though recent work suggests PyOM may be more bioavailable than historically perceived, estimating bioavailability across its chemical spectrum remains elusive. To address this knowledge gap, we assessed potential bioavailability of representative PyOM compounds relative to ubiquitous dissolved organic matter (DOM) with a substrate-explicit model. The range of potential bioavailability of PyOM was greater than natural DOM; however, the predicted thermodynamics, metabolic rates, and carbon use efficiencies (CUEs) overlapped significantly between all OM pools. Compound type (e.g., natural versus PyOM) had approximately 6-fold less impact on predicted respiration rates than simulated carbon and oxygen limitations. Within PyOM, the metabolism of specific chemistries differed strongly between unlimited and oxygen-limited conditions – degradations of anhydrosugars, phenols, and polycyclic aromatic hydrocarbons (PAHs) were more favorable under oxygen limitation than other molecules. Notably, amino sugar-like, protein-like, and lignin-like PyOM had lower carbon use efficiencies relative to natural DOM of the same classes, indicating potential impacts in process-based model representations. Overall, our work illustrates how similar PyOM bioavailability may be to that of natural DOM in the river corridor, furthering our understanding of how PyOM may influence riverine biogeochemical cycling.

Whole-soil-profile warming does not change microbial carbon use efficiency in surface and deep soils

The paucity of investigations of carbon (C) dynamics through the soil profile with warming makes it challenging to evaluate the terrestrial C feedback to climate change. Soil microbes are important engines driving terrestrial biogeochemical cycles; their carbon use efficiency (CUE), defined as the proportion of metabolized organic C allocated to microbial biomass, is a key regulator controlling the fate of soil C. It has been theorized that microbial CUE should decline with warming; however, empirical evidence for this response is scarce, and data from deeper soils are particularly scarce. Here, based on soil samples from a whole-soil-profile warming experiment (0 to 1 m, +4 °C) and 18O tracing approach, we examined the vertical variation of microbial CUE and its response to ~3.3-y experimental warming in an alpine grassland on the Qinghai-Tibetan Plateau. Microbial CUE decreased with soil depth, a trend that was primarily controlled by soil C availability. However, warming had limited effects on microbial CUE regardless of soil depth. Similarly, warming had no significant effect on soil C availability, as characterized by extractable organic C, enzyme-based lignocellulose index, and lignin phenol-based ratios of vanillyls, syringyls, and cinnamyls. Collectively, our work suggests that short-term warming does not alter microbial CUE in either surface or deep soils, and emphasizes the regulatory role of soil C availability on microbial CUE.

Long-term integrated crop-livestock grazing stimulates soil ecosystem carbon flux, increasing subsoil carbon storage in California perennial agroecosystems

The strategic use of ruminant grazing in perennial cropland is steadily increasing throughout Mediterranean perennial agroecosystems. Integrated sheep-vineyard (ISV) management, where small ruminant livestock graze on understory vegetation, is viewed by some practitioners as a feasible transition opportunity to facilitate less petrochemically intensive vineyard understory management. However, our knowledge of soil carbon dynamics associated with grazing in perennial integrated crop-livestock (ICL) agroecosystems is notably limited, especially within Mediterranean climate contexts. Here, we use a series of on-farm paired surveys to assess soil ecosystem habitat and resource conditions related to SOC flux and storage in vineyards utilizing sheep-integration (ISV) and conventional understory management techniques (CONV). Our results show that long-term grazing increased the quantity of active, labile, and soluble carbon (C) within ISV soils, with much higher quantities of microbial biomass carbon (MBC). Vineyard soils with sheep grazing also showed increases in phospholipid fatty acid (PLFA) biomarkers, particularly amongst core functional groups related to decomposition. Soil microbial communities under ISV had higher C mineralization rates as well as higher carbon use-efficiency, as indicated by less CO2-C respired relative to the size of the MBC pool. Whereas inorganic soil nitrogen (N) and phosphorous (P) were also higher under ISV, microbial communities showed distinct metabolic investment strategies related to nutrient acquisition, with lower P-cycling enzyme activity and higher N-cycling enzyme activity. Additionally, ISV resulted in an increase in subsoil SOC storage, including higher quantities of physicochemical stabilization in the mineral-associated organic carbon (MAOC) pool of the deepest measured subsoil layer (30–45 cm). We observed no differences in soil structure indicators between treatments nor differences in the carbon fractions associated with four distinct aggregate size categories. We propose a framework to explain observed shifts in SOC dynamics of perennial ICL systems that include i) deposition of C and nutrient inputs with higher lability and solubility; ii) ruminant-induced decoupling of C from N and P, resulting in increased nutrient bioavailability; and iii) altered soil microbial metabolic strategies with more efficient biomass accumulation. These findings show strong potential of strategically applied ICL grazing to enhance soil functioning and increase SOC storage in Mediterranean perennial agroecosystems.

Microbial carbon use efficiency in different ecosystems: A meta-analysis based on a biogeochemical equilibrium model.

Soil microbial carbon use efficiency (CUE) is a crucial parameter that can be used to evaluate the partitioning of soil carbon (C) between microbial growth and respiration. However, general patterns of microbial CUE among terrestrial ecosystems (e.g., farmland, grassland, and forest) remain controversial. To address this knowledge gap, data from 41 study sites (n = 197 soil samples) including 58 farmlands, 95 forests, and 44 grasslands were collected and analyzed to estimate microbial CUEs using a biogeochemical equilibrium model. We also evaluated the metabolic limitations of microbial growth using an enzyme vector model and the drivers of CUE across different ecosystems. The CUEs obtained from soils of farmland, forest, and grassland ecosystems were significantly different with means of 0.39, 0.33, and 0.42, respectively, illustrating that grassland soils exhibited higher microbial C sequestration potentials (p < .05). Microbial metabolic limitations were also distinct in these ecosystems, and carbon limitation was dominant exhibiting strong negative effects on CUE. Exoenzyme stoichiometry played a greater role in impacting CUE values than soil elemental stoichiometry within each ecosystem. Specifically, soil exoenzymatic ratios of C:phosphorus (P) acquisition activities (EEAC:P ) and the exoenzymatic ratio of C:nitrogen (N) acquisition activities (EEAC:N ) imparted strong negative effects on soil microbial CUE in grassland and forest ecosystems, respectively. But in farmland soils, EEAC:P exhibited greater positive effects, showing that resource constraints could regulate microbial resource allocation with discriminating patterns across terrestrial ecosystems. Furthermore, mean annual temperature (MAT) rather than mean annual precipitation (MAP) was a critical climate factor affecting CUE, and soil pH as a major factor remained positive to drive the changes in microbial CUE within ecosystems. This research illustrates a conceptual framework of microbial CUEs in terrestrial ecosystems and provides the theoretical evidence to improve soil microbial C sequestration capacity in response to global change.

BESSv2.0: A satellite-based and coupled-process model for quantifying long-term global land–atmosphere fluxes

Recent remote-sensing-based global carbon, water and energy budgets over land still include considerable uncertainties. Most existing flux products of terrestrial carbon, water and energy components were developed individually, despite the inherently coupled processes among them. In this study, we present a new set of global daily surface downwelling shortwave radiation (SW), net radiation (Rnet), evapotranspiration (ET), gross primary productivity (GPP), terrestrial ecosystem respiration (TER) and net ecosystem exchange (NEE) datasets at 0.05° resolutions from 1982 to 2019, by improving a satellite-based and coupled-process model—the Breathing Earth System Simulator (BESS). The new version of BESS (v2.0) integrated a newly developed ecosystem respiration module, an optimality-based maximum carboxylation rate (Vcmax) model, and extended the temporal coverage of flux datasets from 1982 to 2019. We evaluated BESS products against the FLUXNET2015 dataset at the site scale, and against several remote sensing and/or machine learning products on a global scale. At the site scale, BESS products agreed well with FLUXNET measurements, capturing 84%, 53%, 65%, 51% and 31% of daily variation in Rnet, ET, GPP, TER and NEE, respectively. Interannual variation in BESS NEE showed relatively low consistency with the FLUXNET measurements, while the rest fluxes explained approximately half of the interannual variation. On a global scale, we found marked discrepancies in spatio-temporal patterns between BESS and several benchmark products. Over the period 1982–2019, BESSv2.0 estimated the mean annual global Rnet, ET, GPP, TER and NEE to be 340.97 ± 5.22 ZJ yr−1 (mean ± 1 SD), 67.67 ± 0.71 103 km3 yr−1, 125.74 ± 5.95 Pg C yr−1, 109.30 ± 3.16 Pg C yr−1, and − 16.28 ± 2.95 Pg C yr−1, respectively, with significant annual linear trends (P < 0.01) by −0.05 103 km3 yr−2 for ET, by 0.52 Pg C yr−2 for GPP, by 0.28 Pg C yr−2 for TER, and by −0.25 Pg C yr−2 for NEE. We further evaluated various coupled processes derived by BESS in terms of functional properties (i.e., Budyko relation, carbon-use efficiency, water-use efficiency, and light-use efficiency), which agreed well with FLUXNET observations, unlike the benchmark products. Overall, BESSv2.0 can serve as a set of reliable and independent products from other global satellite products, facilitating studies related to global carbon, water and energy budgets in a coupled and comprehensive manner.

Revealing the globally multiscale controls of environmental factors on carbon use efficiency

The carbon use efficiency (CUE), which is the ratio of net primary production to gross primary production, is an essential element for detecting the terrestrial carbon cycle and ecosystem function. The spatial variation of CUE is controlled by environmental factors independently or interactively with different intensity. However, previous studies have mainly focused on the effect of climate on the local CUE at the sampling scale, while neglecting the effects of topography or soil on the global CUE, and even its spatially predictive model. In the study, the relative contributions of potentially influencing factors (i.e., climatic, topographic, and edaphic factors), and their interactions on the global CUE were analyzed using the combined methods of curvelet transform and geographical detector model, and the spatial model of CUE were established based on its relationships with influencing factors. The results showed that CUE values at the sampling scale were generally greater in the mid- and high-latitude regions than those in the low-latitude region, which was characterized by its spatial pattern at the large scale. Climate had the greater effects on CUE variations at the large scale, while topography was the main factor controlling CUE at the small or medium scale. However, the explanatory power of the interaction among factors on CUE was greater than any single factor, among which the interaction between climatic and topographical factors was the strongest at all scales. The CUE predication based on scale-dependent effects was more accurate than that based on the sampling scale especially in the high-latitude, and temperature and elevation was the main predictors. Based on the model, the spatial patterns of CUE under future scenarios with any climatic changes could be extracted. This study can further advance our understanding on spatial variation of CUE, and provide a unique insight for CUE prediction responding to climate changes.

Diverging climate response of corn yield and carbon use efficiency across the U.S.

In this paper, we developed an open-source package to analyze the overall trend and responses of both carbon use efficiency (CUE) and corn yield to climate factors for the contiguous United States. Our algorithm enables automatic retrieval of remote sensing data through the Google Earth Engine (GEE) and U.S. Department of Agriculture (USDA) agricultural production data at the county level through application programming interface (API). Firstly, we integrated satellite products of net primary productivity and gross primary productivity based on the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor, and climatic variables from the European Centre for Medium-Range Weather Forecasts. Secondly, we calculated CUE and commonly used climate metrics. Thirdly, we investigated the spatial heterogeneity of these variables. We applied a random forest algorithm to identify the key climate drivers of CUE and crop yield, and estimated the responses of CUE and yield to climate variability using the spatial moving window regression across the U.S. Our results show that growing degree days (GDD) has the highest predictive power for both CUE and yield, while extreme degree days (EDD) is the least important explanatory variable. Moreover, we observed that in most areas of the U.S., yield increases or stays the same with higher GDD and precipitation. However, CUE decreases with higher GDD in the north and shows more mixed and fragmented interactions in the south. Notably, there are some exceptions where yield is negatively correlated with precipitation in the Missouri and Mississippi River Valleys. As global warming continues, we anticipate a decrease in CUE throughout the vast northern part of the country, despite the possibility of yield remaining stable or increasing.

Microbial carbon use efficiency promotes global soil carbon storage

Soils store more carbon than other terrestrial ecosystems1,2. How soil organic carbon (SOC) forms and persists remains uncertain1,3, which makes it challenging to understand how it will respond to climatic change3,4. It has been suggested that soil microorganisms play an important role in SOC formation, preservation and loss5–7. Although microorganisms affect the accumulation and loss of soil organic matter through many pathways4,6,8–11, microbial carbon use efficiency (CUE) is an integrative metric that can capture the balance of these processes12,13. Although CUE has the potential to act as a predictor of variation in SOC storage, the role of CUE in SOC persistence remains unresolved7,14,15. Here we examine the relationship between CUE and the preservation of SOC, and interactions with climate, vegetation and edaphic properties, using a combination of global-scale datasets, a microbial-process explicit model, data assimilation, deep learning and meta-analysis. We find that CUE is at least four times as important as other evaluated factors, such as carbon input, decomposition or vertical transport, in determining SOC storage and its spatial variation across the globe. In addition, CUE shows a positive correlation with SOC content. Our findings point to microbial CUE as a major determinant of global SOC storage. Understanding the microbial processes underlying CUE and their environmental dependence may help the prediction of SOC feedback to a changing climate.

Promoting Urban Farming for Creating Sustainable Cities in Nepal

This paper responds to the research question, “can urban farming in Nepal help create sustainable cities?” Especially after the COVID-19 pandemic, urban residents have begun to realize that food transported from long distances is not always reliable. Urban farming can help produce fresh food locally and help urban residents become self-reliant by engaging in healthy eating habits and practicing sustainable agricultural techniques in food-desert areas, while creating a positive impact on the environment through regenerative agricultural methods. In doing so, urban farms can help the growers save on food expenditures and even earn some additional income, while also improving air quality and minimizing the effects of urban heat islands. This practice also helps reduce greenhouse gases through plant carbon use efficiency (CUE), as vegetation carbon dynamics (VCD) can be adjusted while supporting the circular economy. As urban lands command higher prices than agricultural land, urban farming usually happens on residential yards, roofs, balconies, community gardens, and dedicated areas in public parks. Rainwater harvesting and redirecting can help irrigate urban farms, which can be part of rain gardens. The national census of 2021 identified that 66% of Nepal’s population lives in urban areas. However, the World Bank (2018) showed that only 21 of Nepal’s population was projected to live in urban areas in 2021. It is not debatable that the urbanization process in Nepal is on the rise. Thus, urban agriculture can play an important role in supplementing residents’ food needs. Many cities in Nepal have already successfully adapted to urban farming wherein residents grow food on their building sites, balconies, and rooftop, often growing plants in pots, vases, and other types of containers. The UN-Habitat, with the support of the European Union and local agencies, published a rooftop farming training manual (2014), showing the feasibility of urban farming in Nepal. This paper discusses how public-private partnership (PPP) can promote urban agriculture and make the process more effective and attractive to urban-farming households. It also analyzes how a PPP approach also facilitates the use of better technology, advisory support, and use of research extension activities. This paper draws on a literature review, uses remote-sensing imagery data and data from National Census Nepal 2021, and the authors’ professional experiences related to best practices in the areas to analyze the benefits and challenges related to urban farming both in Nepal and Arizona, USA. The paper provides recommendations for Nepali cities to maximize the benefit provided by urban farming. It is expected to be useful to Nepali policymakers, government agencies, and nonprofit organizations which promote sustainability, and organic farming with a sustainable supply chain.

Long-term successive rotation affects soil microbial resource limitation and carbon use efficiency in Chinese fir (Cunninghamia lanceolata) monoculture plantations

Long-term successive rotation has resulted in negative effects on stand productivity, soil nutrient pool and microbial diversity and function in monoculture tree plantations. However, how successive rotation influences soil microbial nutrient limitation and carbon use efficiency (CUE) and their linkage with resource carbon (C): nitrogen (N): phosphorus (P) stoichiometry remain largely unknown. To solve this puzzle, we compared the C:N:P stoichiometries of leaf litter, fine root and soil, as well as microbial biomass and enzyme activities among three Chinese fir (Cunninghamia lanceolata) plantations with first (FRP), second (SRP) and third (TRP) generations and a natural broad-leaved forest (BL). We found that long-term successive rotation of Chinese fir decreased litter biomass by 64.7% ∼ 80.9% and enhanced litter C:N ratio by 2.7% ∼ 36.0% compared to the BL. Successive rotation decreased the contents of dissolved organic C, total N, and soil C:P and N:P ratios, and resulted in an increased C:N imbalance between soil microorganisms and nutrients in the TRP generation. Long-term successive rotation increased microbial C limitation by 1.5% ∼ 11.1% but decreased N and P limitations by 127.3% ∼ 160.9% and 15.3% ∼ 37.5%, compared with the BL, and resulted in significant increases in microbial CUE and substrate-induced respiration rate of C to N sources in the TRP generation. Structural equation modeling suggested that successive rotation indirectly affected microbial CUE and SIR by changing litter C:N, enzyme C:N and soil C:N imbalance. Collectively, these findings indicated that long-term successive rotation aggravated soil microbial C (energy) limitation and affected microbial CUE and capacity due to the increased stoichiometric imbalance between their resources. Future management practices that could increase litter and soil quality could be helpful to decrease microbial C limitation and maintain microbial functionality for sustainable monoculture plantations.

Components explain, but do eddy fluxes constrain? Carbon budget of a nitrogen-fertilized boreal Scots pine forest.

Nitrogen (N) fertilization increases biomass and soil organic carbon (SOC) accumulation in boreal pine forests, but the underlying mechanisms remain uncertain. At two Scots pine sites, one undergoing annual N fertilization and the other a reference, we sought to explain these responses. We measured component fluxes, including biomass production, SOC accumulation, and respiration, and summed them into carbon budgets. We compared the resulting summations to ecosystem fluxes measured by eddy covariance. N fertilization increased most component fluxes (P < 0.05), especially SOC accumulation (20×). Only fine-root, mycorrhiza, and exudate production decreased, by 237 (SD = 28) g C m-2 yr-1 . Stemwood production increases were ascribed to this partitioning shift, gross primary production (GPP), and carbon-use efficiency, in that order. The methods agreed in their estimates of GPP in both stands (P > 0.05), but the components detected an increase in net ecosystem production (NEP) (190 (54) g C m-2 yr-1 ; P < 0.01) that eddy covariance did not (19 (62) g C m-2 yr-1 ; ns). The pairing of plots, the simplicity of the sites, and the strength of response provide a compelling description of N effects on the C budget. However, the disagreement between methods calls for further paired tests of N fertilization effects in simple forest ecosystems.

Contextualized response of carbon-use efficiency to warming at the plant and ecosystem levels

Carbon-use efficiency (CUE) has been widely used as a constant value in many earth system models to simulate how assimilated C is partitioned in ecosystems, to estimate ecosystem C budgets, and investigate C feedbacks to climate warming. Although correlative relationships from previous studies indicated that CUE could vary with temperature, and relying on a fixed CUE value could cause large uncertainty in model projections, however, due to the lack of manipulative experiment, it remains unclear how CUE at the plant (CUEp) and ecosystem (CUEe) levels respond to warming. Based on a 7-year manipulative warming experiment in an alpine meadow ecosystem on the Qinghai-Tibet Plateau, we quantitatively distinguished various C flux components of CUE, including gross ecosystem productivity, net primary productivity, net ecosystem productivity, ecosystem respiration, plant autotrophic respiration, and microbial heterotrophic respiration and explored how CUE at different levels responded to climate warming. We found large variations in both CUEp (0.60 to 0.77) and CUEe (from 0.38 to 0.59). The warming effect on CUEp was positively correlated with ambient soil water content (SWC) and the warming effect on CUEe was negatively correlated with ambient soil temperature (ST), but was positively correlated with warming-induced changes in ST. We also found that the direction and magnitude of the warming effects on different CUE components scaled differently with changes in the background environment, which explained the variation in CUE's warming response under environmental changes. Our new insights have important implications for reducing modelling uncertainty of ecosystem C budgets and improving our ability to predict ecosystem C-climate feedbacks under climate warming.

Spatio-Temporal Dynamic Characteristics of Carbon Use Efficiency in a Virgin Forest Area of Southeast Tibet

The sequestration of carbon in forests plays a crucial role in mitigating global climate change and achieving carbon neutrality goals. Carbon use efficiency (CUE) is an essential metric used to evaluate the carbon sequestration capacity and efficiency of Vegetation. Previous studies have emphasized the importance of assessing CUE at specific regions and times to better understand its spatiotemporal variations. The southeastern region of Tibet in the Qinghai-Tibet Plateau is recognized as one of the most biodiverse areas in China and globally, characterized by diverse vegetation types ranging from subtropical to temperate. In this study, we focused on Nyingchi, which is the largest virgin forest area in southeast Tibet, to explore the spatial-temporal dynamic characteristics of regional CUE based on MODIS remote sensing products. The following results were obtained: (1) On a monthly scale, regional CUE exhibits significant seasonal variations, with varying patterns among different vegetation types. Specifically, the fluctuation of CUE is the lowest in high-altitude forest areas and the greatest in grasslands and barrens. On an annual scale, forests exhibit higher fluctuations than areas with sparse vegetation and the overall volatility of CUE increased over the past 11 years. (2) There are regional differences in the trend of CUE changes, with a substantial downward trend in the Himalayan region and a significant upward trend in the residual branches of the Gangdise Mountains. More than 75% of the regions exhibit no persistent trend in CUE changes. (3) Vegetation type is the main determinant of the range and characteristics of vegetation CUE changes, while the geographical location and climatic conditions affect the variation pattern. CUE in the southern and northern regions of Nyingchi at 28.5°N exhibits different responses to temperature and precipitation changes, with temperature having a more significant impact on CUE.

Does liming improve microbial carbon use efficiency after maize litter addition in a tropical acidic soil?

Soil pH is one of the main drivers of soil microbial functions, including carbon use efficiency (CUE), the efficiency of microorganisms in converting substrate C into biomass, a key parameter for C sequestration. We evaluated liming effects after maize-litter addition on total CUE (including microbial residues), CUE of microbial biomass (CUEMB), and fungal biomass on an acidic Acrisol with a low C. We established a 6-week incubation experiment to compare limed and unlimed Acrisol treatments and a reference soil, a neighboring Nitisol with optimal pH. Fungal biomass (ergosterol) increased ~ 10 times after litter addition compared with soils without litter, and the final amount was greater in the limed Acrisol than the Nitisol. Litter addition induced a positive priming effect that increased with increasing pH. The increases in soil pH also led to increases in litter-derived CO2C and decreases in particulate organic matter (POM)C. Thus, in spite of increasing microbial biomass C, CUE decreased with increasing pH and CUEMB was similar across the three soils. CUEMB was positively associated with saprotrophic fungi, implying that fungi are more efficient in incorporating litter-derived C into microbial, especially fungal biomass after 42 days. By including undecomposed maize litter and microbial residues, CUE provided a more comprehensive interpretation of pH and liming effects than CUEMB. Nevertheless, longer-term studies may provide further information on substrate-C turnover and the persistence of liming and pH effects.

Comprehensive Effects of Atmosphere and Soil Drying on Stomatal Behavior of Different Plant Types

The soil water supply and atmospheric humidity conditions are crucial in controlling plants’ stomatal behavior and water use efficiency. When there is water stress caused by an increase in saturated water vapor pressure (VPD) and a decrease in soil water content (SWC), plants tend to close stomata to reduce water loss. This affects the gross primary productivity (GPP) and evapotranspiration (ET), subsequently leading to changes in water use efficiency (WUE) and carbon use efficiency (CUE) in plants. However, land–atmosphere interactions mean that water vapor in the atmosphere and soil moisture content causing water stress for plants are closely related. This study aims to compare and estimate the effects of VPD and SWC on the carbon cycle and water cycle for different plant functional types. Based on the fluxnet2015 dataset from around the world, the WUE and CUE of five plant functional types (PFTs) were estimated under varying levels of VPD and SWC. The results showed that high VPD and low SWC limit the stomatal conductance (Gs) and gross primary productivity (GPP) of plants. However, certain types of vegetation (crops, broad-leaved forests) could partially offset the negative effects of high VPD with higher SWC. Notably, higher SWC could even alleviate limitations and partially promote the increase in GPP and net primary production (NPP) with increasing VPD. WUE and CUE were directly affected by Gs and productivity. In general, the increase in VPD in the five PFTs was the dominant factor in changing WUE and CUE. The impact of SWC limitations on CUE was minimal, with an overall impact of only −0.05μmol/μmol on the four PFTs. However, the CUE of savanna plants changed differently from the other four PFTs. The rise in VPD dominated the changes in CUE, and there was an upward trend as SWC declined, indicating that the increase in VPD and decrease in SWC promote the increase in the CUE of savanna plants to some extent.

Mechanisms underlying the responses of microbial carbon and nitrogen use efficiencies to nitrogen addition are mediated by topography in a subtropical forest

Microbial carbon use efficiency (CUE) and nitrogen use efficiency (NUE) are key parameters determining the fate of C and N in soils. Atmospheric N deposition has been found to heavily impact multiple soil C and N transformations, but we lack understanding of the responses of CUE and NUE to N deposition, and it remains uncertain whether responses may be mediated by topography. Here, a N addition experiment with three treatment levels (0, 50 and 100 kg N ha−1 yr−1) was conducted in the valley and on the slope of a subtropical karst forest. Nitrogen addition increased microbial CUE and NUE at both topographic positions, but the underlying mechanisms differed. In the valley, the increase in CUE was associated with an increase in soil fungal richness:biomass and lower litter C:N, whereas on the slope, the response was linked with a reduced ratio of dissolved soil organic C (DOC) to available phosphorus (AVP) which reduced respiration, and increased root N:P stoichiometry. In the valley, the increase in NUE was explained by stimulated microbial N growth relative to gross N mineralization, which was associated with increased ratios of soil total dissolved N:AVP and fungal richness:biomass. In contrast, on the slope, the increase in NUE was attributed to reduced gross N mineralization, linked to increased DOC:AVP. Overall, our results highlight how topography-driven soil substrate availability and microbial properties can regulate microbial CUE and NUE.

Clipping increases ecosystem carbon use efficiency by decreasing the dominance of grasses

Ecosystem carbon use efficiency (CUE) is a critical variable in quantifying the efficiency and capacity of an ecosystem to sequestrate carbon (C) from the atmosphere. However, how CUE responds to land-use change and climate warming remains elusive and lacks empirical evidence. We implemented a field manipulative experiment to examine the effects of clipping and warming on CUE in an alpine meadow on the Qinghai-Tibetan Plateau over 7 years. The results showed that clipping significantly increased CUE by 7.1%, which was mainly attributed to the stronger positive effects of clipping on gross ecosystem productivity (GEP, 13.0%) than on ecosystem respiration (ER, 5.3%). In contrast, warming had a minor effect on CUE due to the synchronized increase in GEP (8.8%) and ER (7.3%). There was no significant interactive effect of clipping and warming on CUE. Moreover, the dominance of grasses played a pivotal role in controlling the responses of CUE to clipping and warming by increasing ecosystem C uptakes but decreasing microbial decomposition. Soil temperature and above-ground net primary productivity also contributed to the changes in CUE under clipping and warming. These results suggest that clipping once a year at the end of the growing season could increase the efficiency of this alpine meadow for C sequestration, while climate warming could not change ecosystem C use efficiency. We highlight that the dominance of grasses is an emerging property in determining CUE under land-use change and climate warming in grasslands.

Nitrogen addition decreases the soil cumulative priming effect and favours soil net carbon gains in Robinia pseudoacacia plantation soil

Carbon (C) and nutrient inputs regulate the soil carbon balance by affecting microbial growth and the priming effect (PE). However, we still lack a comprehensive understanding of the microbial mechanism of the soil carbon balance. To examine the regulatory effect of C or nutrient deficiency on microbial C turnover in forest systems of different ages, we collected soil from two Robinia pseudoacacia stands of different ages and conducted an incubation experiment using 13C-labelled glucose combined with nitrogen and/or phosphorus (N and/or P) addition. We determined 13C partitioning in CO2 and phospholipid fatty acids (PLFAs). Glucose and nutrient addition caused both positive and negative PEs during the 30-day incubation. The PE of the 15-year-old (15Y) stand changed from positive (0–15 days) to negative (15–30 days), while the PE of the 45-year-old (45Y) stand ranged from negative (0.5 days) to positive (after 1 day). Different microbial mechanisms play diverse roles during various decomposition stages. The 45Y stand led to a higher cumulative positive PE than the 15Y stand. For both stands, fungi assimilated a greater proportion of 13C-glucose in the glucose plus nitrogen (CN addition) treatment than in the glucose addition alone treatment, which alleviated the fungal N requirements and decreased the cumulative PE. However, the glucose plus phosphorous treatment (CP addition) had no significant influence on the cumulative PE. CN addition favoured high-yielding microbial strategists (Y-strategists) and glucose-derived SOC accumulation. This phenomenon was supported by the relatively high microbial carbon use efficiency (CUE), which increased the soil net C balance. In contrast, CP addition was conducive to resource-acquisition strategists (A-strategists), which increased soil organic carbon mineralization and decreased the soil net C balance. This was evidenced by higher 13C incorporation into the PLFAs of gram-positive bacteria and the positive PE for C and nutrient acquisition under CP addition. Consequently, CN addition decreased the cumulative PE and increased 13C-fungi and CUE, which favoured the soil net C gain. Our study highlights the importance of N fertilization for soil C sequestration in Robinia pseudoacacia plantations.

Tradeoffs of microbial life history strategies drive the turnover of microbial-derived organic carbon in coastal saline soils.

Stable soil organic carbon (SOC) formation in coastal saline soils is important to improve arable land quality and mitigate greenhouse gas emissions. However, how microbial life-history strategies and metabolic traits regulate SOC turnover in coastal saline soils remains unknown. Here, we investigated the effects of microbial life history strategy tradeoffs on microbial carbon use efficiency (CUE) and microbial-derived SOC formation using metagenomic sequencing technology in different salinity soils. The results showed that high-salinity is detrimental to microbial CUE and microbial-derived SOC formation. Moreover, the regulation of nutrients stoichiometry could not mitigate adverse effects of salt stress on microbial CUE, which indicated that microbial-derived SOC formation is independent of stoichiometry in high-salinity soil. Low-salinity soil is dominated by a high growth yield (Y) strategy, such as higher microbial biomass carbon and metabolic traits which are related to amino acid metabolism, carbohydrate metabolism, and cell processes. However, high-salinity soil is dominated by stress tolerance (S) (e.g., higher metabolic functions of homologous recombination, base excision repair, biofilm formation, extracellular polysaccharide biosynthesis, and osmolytes production) and resource acquisition (A) strategies (e.g., higher alkaline phosphatase activity, transporters, and flagellar assembly). These trade-offs of strategies implied that resource reallocation took place. The high-salinity soil microbes diverted investments away from growth yield to microbial survival and resource capture, thereby decreasing biomass turnover efficiency and impeding microbial-derived SOC formation. Moreover, altering the stoichiometry in low-salinity soil caused more investment in the A-strategy, such as the production of more β-glucosidase and β-N-acetyl-glucosaminidase, and increasing bacterial chemotaxis, which thereby reduced microbial-derived SOC formation. Our research reveals that shift the microbial community from S- and A- strategies to the Y-strategy is important to increase the microbial CUE, and thus enhance SOC turnover in coastal saline soils.