Heterotrophic Respiration Research Articles

In situ simultaneous measuring method for the determination of key processes of soil organic carbon cycling: Soil microbial respiration using laser spectrometry.

Soil microbial heterotrophic C-CO2 respiration is important for C cycling. Soil CO2 differentiation and quantification are vital for understanding soil C cycling and CO2 emission mitigation. Presently, soil microbial respiration (SR) quantification models are based on native soil organic matter (SOM) and require consistent monitoring of δ13C and CO2. We present a new apparatus for achieving in situ soil static chamber incubation and simultaneous CO2 and δ13C monitoring by cavity ring-down spectroscopy (CRDS) coupled with a soil culture and gas introduction module (SCGIM) with multi-channel. After a meticulous five-point inter-calibration, the repeatability of CO2 and δ13C values by using CRDS-SCGIM were determined, and compared with those obtained using gas chromatography (GC) and isotope ratio mass spectrometry (IRMS), respectively. We examined the method regarding quantifying SR with various concentrations and enrichment of glucose and then applied it to investigate the responses of SR to the addition of different exogenous organic materials (glucose and rice residues) into paddy soils during a 21-day incubation. The CRDS-SCGIM CO2 and δ13C measurements were conducted with high precision (<1.0µmol/mol and 1‰, respectively). The optimal sampling interval and the amount added were not exceeded 4 h and 200mg C/100g dry soil in a 1 L incubation bottle, respectively; the 13C-enrichment of 3%-7% was appropriate. The total SR rates observed were 0.6-4.2µL/h/g and the exogenous organic materials induced -49%-28% of priming effects in native SOM mineralisation. Our results show that CRDS-SCGIM is a method suitable for the quantification of soil microbial CO2 respiration, requiring less extensive lab resources than GC/IRMS.

Tree diversity and arbuscular mycorrhizal trees increase soil carbon sequestration and stability in 1-m soils as regulated by microbial CAZymes-coworking in high-latitude Northern Hemisphere forests

Biodiversity loss and tree alternations often occurred in high-latitude Northern Hemisphere forests owing to climatic changes and human disturbance, while their effects on soil carbon sequestration remain unclear. In this paper, tree richness and arbuscular mycorrhizal (AM)-tree dominance experiments were settled in an experimental forest, and soil organic carbon (SOC) and glomalin-related soil protein carbon (Total, TG-C and easily extracted, EEG-C), stability (heterotrophic respiration, Rh, temperature sensitivity, Q10 and humidity sensitivity, Hs), in situ soil CO2, CH4, N2O fluxes, and CAZymes activities from the microbial metagenome technique were measured to identify their impacts and underlying mechanism. Both analysis of variance (ANOVA) and linear regression found tree richness and AM tree loss accompanied by soil carbon sequestration loss of total SOC storage, their mycorrhizal components (TG-C) and Rh. In 0–20 cm soils, high tree richness quadrats had 16–126.6 % higher SOC, TG-C, Rh, and in situ N2O flux, while AM-dominated stands had 34–39 % higher SOC and Rh than the EcM-dominated stands. In the 1 m soil profile, the effect size from tree richness and AM trees were more evident than the 0–20 cm soils. Compared with the richness gradient, much significant differences occurred between AM- and EcM-dominated stands. Structural equation modeling found that tree richness directly improved C stability, while the SOC accrual from the tree richness enrichment and AM-tree dominance were indirectly achieved by their decreasing in multiple CAZymes activities. Of CAZymes, glycoside hydrolases, polysaccharide lyases, carbohydrate esterases, glycosyl transferases, auxiliary activities, and carbohydrate-binding modules cooperated in their regulation of the soil C accrual. Our findings support carbon sequestration via biodiversity conservation and proper forest management related to tree mycorrhizal symbionts.

Anoxia begets anoxia: A positive feedback to the deoxygenation of temperate lakes.

Declining oxygen concentrations in the deep waters of lakes worldwide pose a pressing environmental and societal challenge. Existing theory suggests that low deep-water dissolved oxygen (DO) concentrations could trigger a positive feedback through which anoxia (i.e., very low DO) during a given summer begets increasingly severe occurrences of anoxia in following summers. Specifically, anoxic conditions can promote nutrient release from sediments, thereby stimulating phytoplankton growth, and subsequent phytoplankton decomposition can fuel heterotrophic respiration, resulting in increased spatial extent and duration of anoxia. However, while the individual relationships in this feedback are well established, to our knowledge, there has not been a systematic analysis within or across lakes that simultaneously demonstrates all of the mechanisms necessary to produce a positive feedback that reinforces anoxia. Here, we compiled data from 656 widespread temperate lakes and reservoirs to analyze the proposed anoxia begets anoxia feedback. Lakes in the dataset span a broad range of surface area (1-126,909 ha), maximum depth (6-370 m), and morphometry, with a median time-series duration of 30 years at each lake. Using linear mixed models, we found support for each of the positive feedback relationships between anoxia, phosphorus concentrations, chlorophyll a concentrations, and oxygen demand across the 656-lake dataset. Likewise, we found further support for these relationships by analyzing time-series data from individual lakes. Our results indicate that the strength of these feedback relationships may vary with lake-specific characteristics: For example, we found that surface phosphorus concentrations were more positively associated with chlorophyll a in high-phosphorus lakes, and oxygen demand had a stronger influence on the extent of anoxia in deep lakes. Taken together, these results support the existence of a positive feedback that could magnify the effects of climate change and other anthropogenic pressures driving the development of anoxia in lakes around the world.

Soil environment and annual rainfall co-regulate the response of soil respiration to different grazing intensities in saline-alkaline grassland

Grazing is a prevalent management strategy in grassland ecosystems that has profound effects on plants and soil microorganisms, consequently altering soil carbon (C) fluxes. However, the difference responses and regulatory mechanisms of soil autotrophic respiration (SRa) and heterotrophic respiration (SRh) to different grazing intensities remain unclear, especially in saline-alkaline grasslands. A six–year (2017–2022) field experiment involving four grazing intensities (no grazing, light grazing, moderate grazing, and heavy grazing) was conducted in a saline-alkaline grassland located in the agro-pastoral ecotone of northern China. Soil total respiration (SRtot) and its components (SRa and SRh), and the related biotic and abiotic factors (plants, soil microorganisms, and soil environment) were measured. The results showed that light grazing significantly decreased the mean SRh by 17.7% across six measuring years. The effects size of grazing on soil respiration fluctuated consistently with soil water content during the growing seasons (May–September), resulting in a neutral effect of grazing on the seasonal mean SRa. The cumulative soil C emissions of the current year varied with the seasonal precipitation in the previous year, whereas the changes in cumulative soil C emissions induced by grazing showed the same trends as annual precipitation. In addition, the key control factors for SRh and SRa differed and varied with grazing intensity and the soil environment played a critical role in regulating soil C fluxes. In conclusion, light grazing reduces soil microbial-driven C emissions and the soil environment and precipitation co-regulate the response of soil C fluxes to different grazing intensities in saline-alkaline grasslands.

Contrary responses of soil heterotrophic and autotrophic respiration to spring and summer drought in alfalfa on the Loess Plateau

Climate change has increased the intensity and frequency of seasonal droughts, which could fundamentally affect the terrestrial carbon (C) cycle. However, as the second-largest terrestrial C flux, soil respiration (Rs) and its heterotrophic (Rh) and autotrophic (Ra) components in response to seasonal drought are unclear. To investigate the effect of seasonal drought on the patterns and mechanisms of Rs and its components, a two-year continuous Rs measurement experiment was conducted in typical rain-fed managed alfalfa (Medicago sativa L.) fields on the Loess Plateau in China. The results showed that the ranges of Rs, Rh and Ra in the four seasons of managed alfalfa fields on the Loess Plateau were 0.45–3.70, 0.13–2.27, and 0.33–2.14 μmol m−2 s−1, respectively. There was no significant difference in the average annual Rs between the two years (P > 0.05). Both spring and summer droughts significantly decreased Rs; however, the seasonal Rh and Ra had different patterns under spring and summer droughts. Spring drought significantly reduced the seasonal Rh by 35.19 % but increased Ra by 20.35 %. In contrast, summer drought significantly increased the seasonal Rh by 45.51 % but decreased the Ra by 35.05 %. The differential response of Ra to seasonal drought was attributed to the opposite responses of the leaf area index (LAI) and photosynthetic rate to spring and summer drought, while Rh was related to the contrasting microorganism responses to spring drought and the pulse effect of rainfall following summer drought. Our study highlights the contrasting responses of Ra and Rh to seasonal drought. Spring drought significantly reduced the relative contribution of Rh to Rs, but summer drought increased the relative contribution of Rh to Rs. These results suggest that future increases in the frequency of summer droughts may accelerate the turnover of soil C and further affect the stability of soil C stocks in semi-arid regions.

Herbivore dung deposition increases soil respiration through elevated substrate availability and microbial biomass

Soil heterotrophic respiration (Rh) is the primary pathway for terrestrial ecosystem CO2 efflux. Herbivore grazing activity is known to affect grassland soil Rh, but the effects of grazing-related dung deposition on Rh and the mechanisms involved remain unknown. Here, we explored effects of dung on Rh via altered biotic and abiotic factors in a short-term experiment with yak dung application during a growing season in a Tibetan alpine meadow. Dung application significantly decreased soil pH, but increased soil Rh, concentrations of dissolved organic carbon (C), nitrogen (N), and phosphorus (P), microbial biomass, and activities of enzymes related to C-, N-, and P-degradation. Linear regression showed that Rh was positively correlated with labile substrates, microbial biomass, and enzyme activities, and negatively correlated with soil pH, suggesting that these factors are all potential drivers of Rh. Further, using piecewise structural equation modeling, we revealed that Rh was driven directly by altered microbial biomass and enzyme activities, and indirectly by altered pH and labile substrates. Particularly labile substrates and microbial biomass had more important roles in driving Rh than pH and enzyme activities. Taken together, our work revealed some mechanisms behind short-term dung deposition effects on Rh, which can be used to develop grassland management practices and forecast C feedback in grazed ecosystems.

Partitioning autotrophic and heterotrophic respiration in an ombrotrophic bog

Northern peatlands are globally significant carbon stores, but the sink strength varies from year to year due to changes in environmental conditions. Ecosystem respiration (ER) is composed of both autotrophic respiration (AR) that consists of respiration by plant parts, and heterotrophic respiration (HR) that consists of respiration by microbial bacteria in the soil, fungi, etc. Manual measurements only crudely partition AR and HR, which may lead to erroneous estimates if a change favours one form of respiration over another and may influence our interpretation in the magnitude of respiration. HR has also been thought to be more linked to vegetation dynamics, particularly in wetter, sedge-dominated ecosystems like fens. It is unknown whether such plant-soil-root interactions influence HR in peatlands dominated by woody shrubs whose water table is located further below the surface. The objectives of this study were to 1) determine the contributions of AR and HR at Mer Bleue, an ombrotrophic bog, 2) explore how environmental conditions influence ER and its components, 3) determine how different methodological approaches (e.g. directly measured respiration using automatic chambers vs. extrapolated calculations) can influence our interpretation in the magnitude of respiration, and 4) compare the respiration dynamics with those found in the literature for other peatland types. Our results revealed differences in AR and HR contributions to ER compared to other peatland types reported in the literature. The AR/HR ratio was 3.0 and AR contributions to ER were ∼75% at our study bog, which is generally higher than AR contributions from fens, but also decreased substantially during extended drier periods. HR contributions increased with rising temperature and water table depth. Directly measured ER was smaller than when ER was estimated using night-time relationships with temperature. The magnitude of ER changed depending on the plant biomass, which we believe to be a result of vegetation dynamics influencing HR. The results of this study improved our understanding of peatland carbon cycling as well as the conceptualization of HR.

Estimation of Heterotrophic Soil Respiration Response to the Summer Precipitation Regime and Different Depths of Snow Cover in a Temperate Continental Climate

Regime of precipitation and temperature conditions are key factors that regulate the rate of decomposition of soil organic matter in terrestrial ecosystems. The aim of this work was to assess the effect of the duration of dry periods in summer and different depths of snow cover in winter on heterotrophic soil respiration. The studies were carried out as part of a 2–year field manipulation experiment organized on gray soil (Haplic Luvisol) in the temperate continental climate conditions (southern Moscow region). Three variants were organized: (1) simulation of mild weather with uniform watering of the soil in summer and the absence of freezing in winter, (2) simulating two summer dry periods lasting 1–2 months with natural winter snow cover, (3) simulation of extreme weather with one long (~3 months) dry period in summer and complete removal of snow cover in winter. Heterotrophic soil respiration was measured by the closed chamber method on bare fallow during 2 years of continuous experiment and 1 more year after its completion. Medians of heterotrophic soil respiration for the entire period of the experiment in the three above–mentioned variants of the experiment were 38, 27 and 19 mg C/(m2 h), respectively. Two short dry periods led to an increase in heterotrophic soil respiration by 7–10%, which is associated both with the drying and rewetting cycles of the soil and with an increase in the average summer temperature of a 20–cm soil profile by 1.5°C. The prolonged dry period caused a decrease in heterotrophic soil respiration by 12–16% as a result of low soil moisture. Soil freezing led to a strong decrease in winter CO2 emission from soil, which reached 34–55% in the control variant and 57–72% when the snow cover was removed. The frost period (November–March) contributed from 25–34% without of soil freezing to 14–19% when its presence to the annual CO2 flux. We conclude that the change in the winter temperature regime of the soil due to manipulations with the snow depth led to a more significant change in the annual heterotrophic soil respiration than the lack of precipitations in the summer season.

Coordinated proteome change precedes cell lysis and death in a mat-forming cyanobacterium

Cyanobacteria form dense multicellular communities that experience transient conditions in terms of access to light and oxygen. These systems are productive but also undergo substantial biomass turnover through cell death, supplementing heightened heterotrophic respiration. Here we use metagenomics and metaproteomics to survey the molecular response of a mat-forming cyanobacterium undergoing mass cell lysis after exposure to dark and anoxic conditions. A lack of evidence for viral, bacterial, or eukaryotic antagonism contradicts commonly held beliefs on the causative agent for cyanobacterial death during dense growth. Instead, proteogenomics data indicated that lysis likely resulted from a genetically programmed response triggered by a failure to maintain osmotic pressure in the wake of severe energy limitation. Cyanobacterial DNA was rapidly degraded, yet cyanobacterial proteins remained abundant. A subset of proteins, including enzymes involved in amino acid metabolism, peptidases, toxin-antitoxin systems, and a potentially self-targeting CRISPR-Cas system, were upregulated upon lysis, indicating possible involvement in the programmed cell death response. We propose this natural form of cell death could provide new pathways for controlling harmful algal blooms and for sustainable bioproduct production.

Dynamics of carbon budget and meteorological factors of a typical maize ecosystem in Songnen Plain, China

Aim of study: Understanding the carbon budget and meteorological factor impacts of farmland ecosystems is helpful for scientific assessment of carbon budget and low-carbon agricultural production practices. Area of study: The Songnen Plain, NE China, in 2019. Material and methods: Based on eddy-related flux and soil heterotrophic respiration observations from a typical maize farmland ecosystem, using mathematical statistics and carbon balance equation methods, were analyzed. Main results: Soil respiration rate (Rs) and composition were influenced and controlled by the synergistic effect of surface soil temperature (Ts) and water content (Wcs). Ts played a leading role, while Wcs played an important role. Ts and Wcs had the greatest influence on the heterotrophic respiration rate (Rh), followed by Rs and autotrophic respiration rate (Ra). Daily variations of net ecosystem productivity were correlated with daily mean air temperature, latent heat flux, and sensible heat flux. Annual carbon revenue was 1139.67 g C m-2, annual carbon expenditure was 456.14 g C m-2, and annual carbon budget was -683.53 g C m-2 in 2019. While considering that maize grain yield (-353.44 g C m-2) was moved out of the field at harvest, the net ecosystem carbon balance was -330.09 g C m-2; then it was carbon sink in 2019. By fully utilizing climate resources and improving agricultural managements, carbon sink is increased in farmland ecosystems. Research highlights: Soil respiration rate and composition were influenced and controlled by the synergistic effect of soil temperature and water content; the maize farmland ecosystem is carbon sink.

To what extent are greenhouse-gas emissions offset by trees in a Sahelian silvopastoral system?

To assess the extent to which trees in a semi-arid silvopastoral system (SPS) can offset the greenhouse-gas (GHG) emissions of the system's livestock, this study used two process-based models (STEP-GENDEC-N2O and DynACof) to simulate 9 years of agricultural activity and resulting emissions in a SPS that has been operating in sahelian Senegal. STEP-GENDEC-N2O simulated soil N2O and CO2 fluxes, plus growth of the herbaceous layer, while DynACof focused on the tree layer. Outputs from the models included simulated time series of vegetative growth, water fluxes, and emissions. This output was validated through the use of published data, and measurements that were made at the SPS. Overall, the outputs from STEP-GENDEC-N2O agreed well with validation data for water fluxes, soil N, soil C, herbaceous biomass, and N2O emissions. Good agreement was also found between the measured fluxes of the SPS ecosystem, and the simulated values that were generated by combining STEP-GENDEC-N2O's simulations (of the herbaceous layer's heterotrophic respiration, autotrophic respiration, and gross primary productivity (GPP)) with DynACof's simulations of the tree layer's autotrophic respiration and GPP. Among the insights gained from the simulations was that in this SPS's sandy soils, nitrification was the dominant process that leads to N2O emissions. Our results show that the trees, at their current density (81 ha−1) offset 18 % to 41 % of the GHG emissions from livestock. With further development, the model set-up can be used for estimating the GHG offset at other tree densities, and will be useful for guiding future policies regarding climate-change adaptation and mitigation in the management of the Sahel's SPSs.

Combining thermal analyses and wet-chemical extractions to assess the stability of mixed-nature soil organic matter

In this study, we combined Rock-Eval® analysis, analytical pyrolysis, and wet-chemical extractions, assisted by Fourier transform infrared spectroscopy (FTIR), and measurements of soil heterotrophic respiration. Our objective was to assess the biological and thermal stability of mixed-nature soil organic matter (SOM) derived from grass litter and kerogen. We studied a Technosol constructed with Ca, Mg and kerogen-rich waste rock (black shales), which has been under pasture cultivation for 20 years. We compared the results with those from black shales and an adjacent natural soil under long-term pasture cultivation, both used as endmembers representing kerogen and plant-derived SOM, respectively. Analytical pyrolysis and FTIR analyses revealed a mixed composition of SOM in the Technosol. Predominantly, polysaccharides, lignin, lipids and N-compounds were originated from plant-derived SOM, while (poly)aromatic and most aliphatic compounds were traced back to kerogen. Rock-Eval analysis showed that 58% of SOM in Technosol was kerogen-derived, which was poorly accessible to soil microbiota, as evidenced by heterotrophic respiration. In addition, an important portion of plant-derived SOM (>70%) was only released during the Rock-Eval oxidation stage. The impact of chemical recalcitrance of kerogen compounds on short-term biological stability was remarkable and demonstrated a strong correlation with the thermal indices derived from the Rock-Eval pyrolysis stage. Conversely, the parameters from Rock-Eval oxidation stage showed a positive correlation with the amount of SOM involved in mineral-organic associations, particularly with Ca2+ and Mg2+ (i.e., cation bridging). Thus, to disentangle the contribution of chemical recalcitrance and mineral-organic associations to SOM stability, we recommend the assessment of all Rock-Eval thermograms (from pyrolysis and oxidation stages) in combination with chemical and biological assessments, especially when studying soils containing mixed-nature SOM.

N addition affects the heterotrophic respiration of bulk and rhizosphere soils through changes in root traits and C-degrading functional genes in semi-arid grassland

N addition affects the heterotrophic respiration of bulk and rhizosphere soils through changes in root traits and C-degrading functional genes in semi-arid grassland

CHANGES IN FOSSIL CO2 EMISSIONS IN MEXICO CITY DURING THE COVID-19 LOCKDOWN DEDUCED FROM ATMOSPHERIC RADIOCARBON CONCENTRATIONS

ABSTRACT We present atmospheric radiocarbon concentrations in CO2 integrated samples taken between January 2019 and December 2021 in the Mexico City Metropolitan Area (MCMA) and explain the variations in terms of changes in emission sources associated with the COVID-19 lockdown restrictions imposed from March 2020. Δ14C values for samples collected during 2019 range between –44.15‰ and –13.17‰, with lower values during months with higher fossil fuels consumption and air stagnation, whereas higher values were found for periods with high number of fires around MCMA or wet months with higher contribution of heterotrophic respiration. For samples collected during 2020, Δ14C values range between –17.7‰ and 2.25‰, with an increasing trend immediately after the initial lockdown and higher values obtained for samples collected during lockdown phases 2 and 3 and the period of extremely high epidemic risk. This agrees with the 38% and 52% decrease in gasoline and diesel sales. Once essential activities gradually opened from July 2020, Δ14C follow a decreasing trend as vehicle traffic started to increase again. Δ14C values for samples collected during 2021 range from –32.89‰ to –10.27‰, with the higher value obtained during a period of extremely high epidemic risk with a 30% reduction in gasoline and diesel consumption. Despite the complexity of emission sources in MCMA, from Δ14C variations it was possible to identify changes in fossil CO2 emissions resulting from the significant reduction in vehicle traffic due to the COVID-19 lockdown and the restrictions imposed to control transmission of the disease.

Carbon budget response to climate change varies with grassland type in Qilian Mountains, China

Carbon budget estimation is crucial for scientifically assessing the ecosystem’s carbon sequestration capacity. Grasslands in the Qilian Mountains are widely recognized as vulnerable and sensitive to ongoing climate change. However, it is still unclear how the carbon budget of different grassland types responds to climate change in this area. Here, we evaluated the net primary productivity (NPP), soil heterotrophic respiration (RH), and net ecosystem productivity (NEP) of grassland ecosystems in the Qilian Mountains. Correlation analysis, linear regression, and standardized coefficient methodologies were employed to explore the response of NPP, RH, and NEP in various grassland types to changes in temperature, precipitation, and solar radiation over the past two decades. The results showed that the alpine meadow has the highest NPP and NEP while the temperate typical steppe has the highest RH. Grassland NPP, RH, and NEP increased significantly from 2001 to 2020, with a distinct spatial pattern of being low in the northwest and high in the southeast of the Qilian Mountains. NPP and NEP correlated with precipitation and solar radiation more closely than with temperature. Precipitation and solar radiation, respectively, explained 43.7% and 52.9% of grassland NEP alterations, and there was a threshold effect on the response of NEP and NPP to climate factors. Specifically, the NPP and NEP increased at first and then decreased as the precipitation, temperature, and solar radiation increased. The highest NEP and NPP occurred when precipitation ranged from 415 to 455 mm, along with temperatures between − 0.1–1.5 ℃ and solar radiation reaching 7685–7905MJ/m2. The warming and humidification of climate is conducive to the increase of NPP and NEP in alpine meadows and alpine steppes, but when the precipitation exceeds 415 mm and the temperature is greater than 1.5 ℃, the NPP and NEP begin to decrease. Solar radiation dominated changes in NPP and NEP of alpine meadows, alpine steppes, and temperate desert steppes. With the increase of solar radiation, NPP and NEP of these three grasslands first increased and then decreased. However, the changes in NPP and NEP of the temperate typical steppe were most affected by precipitation, and NPP and NEP first increase and then decrease with the increase of precipitation The higher the temperature, the higher the NPP, RH, and NEP of the temperate typical steppe. This study helps better understand how climate change impacts the carbon budget components of different grassland types in the Qilian Mountains and provides more theoretical thoughts on managing local grasslands sustainably.

Estimating regional fossil fuel CO2 concentrations from 14CO2 observations: challenges and uncertainties.

The direct way to estimate the regional fossil fuel CO2 surplus (ΔffCO2) at a station is by measuring the Δ14CO2 depletion compared with a respective background. However, this approach has several challenges, which are (i) the choice of an appropriate Δ14CO2 background, (ii) potential contaminations through nuclear 14CO2 emissions and (iii) masking of ΔffCO2 by 14C-enriched biosphere respiration. Here we evaluate these challenges and estimate potential biases and typical uncertainties of 14C-based ΔffCO2 estimates in Europe. We show that Mace Head (MHD), Ireland, is a representative background station for the Integrated Carbon Observation System(ICOS) atmosphere station network. The mean ΔffCO2 representativeness bias when using the MHD Δ14CO2 background for the whole observation network is of order 0.1 ± 0.3 ppm. At ICOS sites, the median nuclear contamination leads to 25% low-biased ΔffCO2 estimates if not corrected for. The ΔffCO2 masking due to 14C-enriched heterotrophic CO2 respiration can lead to similar ΔffCO2 biases as the nuclear contaminations, especially in summer. Our evaluation of all components contributing to the uncertainty of ΔffCO2 estimates reveals that, due to the small ffCO2 signals at ICOS stations, almost half of the 14C-based ΔffCO2 estimates from integrated samples have an uncertainty that is larger than 50%. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Fire decreases soil respiration and its components in terrestrial ecosystems

Abstract The impact of fire on above‐ground biomass has significant consequences on soil carbon (C) dynamics, which is essential in predicting the global C budget during the Anthropocene. However, there is considerable spatiotemporal variability in the directions and magnitudes of fire effects on soil respiration, and the drivers associated with these effects are not well understood. Here, we conducted a global meta‐analysis of 1327 individual observations from 170 studies to determine the extent to which fire influenced soil total respiration (Rs), heterotrophic respiration (Rh) and autotrophic respiration (Ra). We found fires reduced Rs, Rh and Ra, with an average effect of −11.0%, −17.5% and −40.6%, compared with unburnt sites. Specifically, wildfires significantly reduced Rs and Rh (−20.4% and −25.0%, respectively), and prescribed fire significantly decreased Ra (−74.8%). The influences of fire on Rs and its components were moderated by fire severity, season, type, climate zones and biomes. After several years from the time of the fire, the negative effects of fire on Rs diminished and then recovered to a state not significantly different from unburnt sites; Rh exhibited a similar but decayed temporal response. Similarly, the negative effects on Ra disappeared after 3 years following the latest fire. The magnitude of the effect on Rs was strongly associated with soil temperature, cation exchange capacity, total nitrogen (N) content and N‐acquiring enzyme activity. In contrast, the magnitude of the effect on Rh significantly changed with pH, bulk density, texture, soil C and nutrient contents, and C‐acquiring enzyme activity. Our findings advance the understanding of the inhibition and associated mechanisms of fire on Rs and its components, highlighting the need for new research efforts to predict the spatial‐temporal shifts in underground C‐cycling induced by fire. Read the free Plain Language Summary for this article on the Journal blog.

Comparison of organic carbon properties in extracted soil solutions obtained underneath Cryptomeria japonica and Quercus acutissima and its implication on stream dissolved organic carbon

Dissolved organic carbon (DOC) in soils is released into streams, working as a main component of the carbon cycle. DOC in terrestrial and aquatic ecosystems participates in many biogeochemical reactions and processes such as heterotrophic respiration, sorption of metals, and transport of pollutants. In order to understand the connectivity of organic carbon among soil, soil water, and forest streams, we investigated the concentrations and dual carbon isotope ratios (δ13C and Δ14C) of soil organic carbon (SOC) and water-extractable organic carbon (WEOC) obtained from soils beneath two tree species stands, and compared these with stream Δ14C-DOC of the forest watershed. Soil samples were collected at different depths (0–10, 10–30, and 30–50 cm) beneath Japanese cedar (Cryptomeria japonica) and sawtooth oak (Quercus acutissima). Although the SOC concentration was not significantly different between the two tree species, the WEOC concentration ([WEOC]) of soil at 0–10 cm depth under Cryptomeria japonica was higher than that of Quercus acutissima, in general. The δ13C-SOC and δ13C-WEOC increased, while the Δ14C-SOC and Δ14C-WEOC decreased with increasing soil depth. The Δ14C-WEOC was higher than the Δ14C-SOC, indicating that WEOC could be primarily derived from the young, hydrophilic, and exchangeable fraction of SOC, rather than from SOC strongly bonded to mineral soils. However, the stream Δ14C-DOC was lower than Δ14C-WEOC in general, except during summer storms. The 14C-depleted DOC released from deep soils or groundwater might lower Δ14C-DOC of a stream, suggesting that relatively old DOC could be released into streams during baseflow. This is contrary to the results of previous studies that have reported positive stream Δ14C-DOC from temperate forests. The discrepancy warrants future research on forest stream Δ14C-DOC across entire seasons, particularly under Asian monsoon climates.

Long-term organic fertilisers application increase plant autotrophic, soil heterotrophic respiration and net ecosystem carbon budget in a hillslope agroecosystem

Long-term organic fertilisers application increase plant autotrophic, soil heterotrophic respiration and net ecosystem carbon budget in a hillslope agroecosystem

Interactions between nitrogen and phosphorus in modulating soil respiration: A meta-analysis

BackgroundEconomic and social development worldwide increases the input of nutrients, especially nitrogen (N) and phosphorus (P), to soils. These nutrients affect soil respiration (Rs) in terrestrial ecosystems. They may act independently or have interactive effects on Rs. The effect of N and P on Rs and its components (autotrophic respiration [Ra] and heterotrophic respiration [Rh]), however, either individually or together, is poorly understood. We performed a meta-analysis of 130 studies to examine the effects of different fertilization treatments on Rs and its components across terrestrial ecosystems. ResultsOur results showed that (1) The impact of fertilizer addition on Rs varies among different fertilizer types. N addition reduced Rs and Rh significantly but did not affect Ra; P addition had no significant effect on Rs, Rh, and Ra; NP addition increased Rs significantly but did not affect Rh and Ra. (2) Ecosystem type, duration of fertilization, fertilization rate, and fertilizer form influenced the response of Rs and its components to fertilizer application. (3) Based on our study, the annual average temperature may be a driving factor of Rs response to fertilizer addition, while soil total nitrogen may be an important predictor of Rs response to fertilizer addition. ConclusionOverall, our study highlights the complex and multifaceted nature of the response of soil Rs and its components to fertilizer application, underscoring the importance of considering multiple factors when predicting and modeling future Rs and its feedback to global change.