Response Of Heterotrophic Respiration Research Articles

Changes in soil oxidase activity induced by microbial life history strategies mediate the soil heterotrophic respiration response to drought and nitrogen enrichment.

Drought and nitrogen deposition are two major climate challenges, which can change the soil microbial community composition and ecological strategy and affect soil heterotrophic respiration (Rh). However, the combined effects of microbial community composition, microbial life strategies, and extracellular enzymes on the dynamics of Rh under drought and nitrogen deposition conditions remain unclear. Here, we experimented with an alpine swamp meadow to simulate drought (50% reduction in precipitation) and multilevel addition of nitrogen to determine the interactive effects of microbial community composition, microbial life strategy, and extracellular enzymes on Rh. The results showed that drought significantly reduced the seasonal mean Rh by 40.07%, and increased the Rh to soil respiration ratio by 22.04%. Drought significantly altered microbial community composition. The ratio of K- to r-selected bacteria (BK:r) and fungi (FK:r) increased by 20 and 91.43%, respectively. Drought increased hydrolase activities but decreased oxidase activities. However, adding N had no significant effect on microbial community composition, BK:r, FK:r, extracellular enzymes, or Rh. A structural equation model showed that the effects of drought and adding nitrogen via microbial community composition, microbial life strategy, and extracellular enzymes explained 84% of the variation in Rh. Oxidase activities decreased with BK:r, but increased with FK:r. Our findings show that drought decreased Rh primarily by inhibiting oxidase activities, which is induced by bacterial shifts from the r-strategy to the K-strategy. Our results highlight that the indirect regulation of drought on the carbon cycle through the dynamic of bacterial and fungal life history strategy should be considered for a better understanding of how terrestrial ecosystems respond to future climate change.

Impacts of fire on soil respiration and its components: A global meta-analysis

As a general disturbance in terrestrial ecosystems, fire can have far-reaching consequences on the carbon (C) cycle. Although soil respiration (SR) is important in regulating atmospheric CO2 concentrations, a general pattern of the response of SR to fire in terrestrial ecosystems remains unclear. In this study, a meta-analysis of 91 studies on fires across 116 global sites was conducted to investigate the effects of fire on SR and its components. The results revealed that the responses of SR and heterotrophic respiration (HR) to fire were dependent on fire severity (low-, moderate-, and high-severity) and types (wildfire and prescribed fire). Specifically, the high- and low-severity fires reduced SR and HR, whereas moderate-severity fires had negligible effects on these parameters. On average, wildfires significantly decreased SR and HR by 13.9% and 18.6%, respectively, across all ecosystems, while prescribed fire reduced SR by 8.4% but did not change HR. In addition, the responses of SR and HR varied with the ecosystem types. Fire decreased SR in all three types of forests (boreal forests: -14.8%, temperate forests: -13.8%, and tropical forests: -28.9%), whereas decreased HR only in boreal and temperate forests (-23.8% and 20.7%, respectively) but not in tropical forests. Moreover, SR decreased directly followed by an increase, whereas HR declined with time after low- and high-severity fires. At a global scale, the responses of SR and HR to fire largely resulted from the changes in soil organic C, dissolved C, microbial biomass C, and belowground biomass, as indicated by the correlation analysis. In addition, the response of SR to fire was negatively affected by the mean annual precipitation in the forests. Overall, our study revealed the mechanisms driving the effects of fire and provided a framework for understanding soil C emissions under intensified fire regimes.

Response of heterotrophic respiration to vegetation restoration in a karst area of SW China

AbstractThe Grain‐for‐Green Program in China aims to restore sloping karst cropland severely degraded by intensive agriculture to secondary forest. Heterotrophic respiration (Rh) is a major process of carbon release from soils and is associated with the sequestration of soil organic carbon (SOC). However, we still do not have a comprehensive understanding of Rh and what drives it along soil profile horizons during the natural vegetative recovery process in typical karst soils. We investigated the responses of Rh (C release per gram of soil) and specific Rh (C release per gram of SOC) in soil horizons from the soil surface to bedrock at the different vegetation recovery stages in a karst region. Coincident soil microbial properties (e.g., bacterial and fungal abundance, total microbial biomass, potential enzyme activities) and physicochemical properties were quantified. Vegetation restoration after cropland abandonment significantly increased Rh rates due to increased soil nutrients and microbial biomass, bacterial abundances, and hydrolase activities (all, p < 0.05). The rates of Rh enhanced in sloping cropland (SC) from the soil surface to bedrock but reduced in the recovering stages (ASC: abandoned sloping cropland and SF: secondary forest) and primary forest (PF). The specific Rh between SF (1.34 mg CO2‐C g−1 SOC d−1) and PF (1.32 mg CO2‐C g−1 SOC d−1) were not significantly different but both of them were greater than those in SC (0.79 mg CO2‐C g−1 SOC d−1) and ASC (0.8 mg CO2‐C g−1 SOC d−1). Soil physicochemical properties and microbial properties explained approximately 48% and 22% of the variations in Rh along vegetation recovery, respectively. Nitrogen to phosphorus stoichiometry exerted the most direct and positive effects on Rh, suggesting the importance of managing soil nutrient status to regulate carbon decomposition during vegetation recovery in karst soils. The increased microbial biomass was the most important microbial factor regulating Rh in later vegetation recovery phases. Our results provide scientific insight into the impact of vegetation restoration on Rh in degraded ecosystems, which is important for reducing carbon loss in karst soils.

Mapping global nitrogen deposition impacts on soil respiration

Soil respiration (Rs) is a key indicator of belowground biological activities of terrestrial ecosystems. Despite ongoing atmospheric nitrogen (N) deposition due to anthropogenic activities, it remains uncertain how Rs responds to globally varied atmospheric N deposition. Based on a meta-analysis of 340 simulated experimental nitrogen addition studies, we aimed to identify the key factors altering the responses of Rs to N deposition and extrapolate these results to the global mapping of Rs changes under N deposition. We found the overall experimental N addition effect on Rs was insignificant, but the responses of Rs significantly shifted from positive to negative with increasing accumulated N addition amount and lower soil pH, and the negative responses to increasing N amounts were significantly intensified in acid soils. Also, the response of heterotrophic respiration to N addition significantly increased with a lower N amount, and both responses of heterotrophic and autotrophic respiration were significantly more negative in soils with lower pH. Our mapping efforts showed that global Rs overall increased by 2.8 % in response to the accumulated N deposition from 2000 to 2020. Regions with combined characteristics of high accumulated N deposition amounts and low soil pH, including Eastern U.S., Europe, and Eastern Asia, were hotspots of Rs declines under current and future atmospheric N deposition. Our findings challenge the long-held notion that N deposition has universal negative impacts on Rs, and suggest the spatial heterogeneity in the impacts of N deposition on belowground activities and carbon release across the globe.

Attributing Past Carbon Fluxes to CO2 and Climate Change: Respiration Response to CO2 Fertilization Shifts Regional Distribution of the Carbon Sink

AbstractOver the past century, increased atmospheric CO2 concentrations have enhanced photosynthesis through CO2 fertilization across the globe. However, the increased growth has also led to greater respiration rates—both from vegetation (autotrophic respiration) and through the breakdown of plant litter and soil organic matter (heterotrophic respiration). The resulting change in carbon flux—and its spatial distribution—that can be attributed to increasing CO2 and climate change remains unknown. We used the Carbon Data Model Framework, a model‐data fusion system that assimilates global observations from satellites and other sources to create an ensemble of observationally constrained carbon cycle representations, to determine the photosynthesis and respiration fluxes that can be attributed to increased atmospheric CO2 and associated climate change from 1920 to 2015. Across the globe, the response of photosynthesis and respiration to atmospheric CO2 dominates their response to climate alone. The regional distribution of the carbon sink attributable to climate change and CO2 is strongly influenced by the 'loss ratio of carbon gained'—the fraction of enhanced photosynthesis that is lost to respiration. While the wet tropics' attributable photosynthesis flux is 1.4 times larger than that of the temperate region, the attributable flux of net carbon uptake is actually 1.25 larger in the temperate region, due to the wet tropics' greater heterotrophic respiration response to enhanced plant growth. At the global scale, the loss ratio of carbon gained is 83 ± 0.6%. Our results highlight the importance of the respiration responses to enhanced plant growth in regulating the land carbon sink.

Significant effects of precipitation frequency on soil respiration and its components-A global synthesis.

Global warming intensifies the hydrological cycle, which results in changes in precipitation regime (frequency and amount), and will likely have significant impacts on soil respiration (Rs ). Although the responses of Rs to changes in precipitation amount have been extensively studied, there is little consensus on how Rs will be affected by changes in precipitation frequency (PF) across the globe. Here, we synthesized the field observations from 296 published papers to quantify the effects of PF on Rs and its components using meta-analysis. Our results indicated that the effects of PF on Rs decreased with an increase in background mean annual precipitation. When the data were grouped by climate conditions, increased PF showed positive effects on Rs under the arid condition but not under the semi-humid or humid conditions, whereas decreased PF suppressed Rs across all the climate conditions. The positive effects of increased PF mainly resulted from the positive response of heterotrophic respiration under the arid condition while the negative effects of decreased PF were mainly attributed to the reductions in root biomass and respiration. Overall, our global synthesis provided for the first time a comprehensive analysis of the divergent effects of PF on Rs and its components across climate regions. This study also provided a framework for understanding and modeling responses of ecosystem carbon cycling to global precipitation change.

Carbon Addition Modified the Response of Heterotrophic Respiration to Soil Sieving in Ectomycorrhizal-Dominated Forests

Soil heterotrophic respiration (Rh) is an important pathway of carbon (C) dioxide release from terrestrial soils to the atmosphere. It is often measured using sieved soil in a laboratory, but the uncertainty of how it is influenced by soil sieving persists, which limits the accuracy of predicting soil organic C dynamics in C models. To address how soil sieving during laboratory incubation affects Rh and its response to increased carbon availability, we investigated Rh in sieved and intact soil cores and its response to 13C-glucose addition. This was conducted through a 27-day laboratory incubation in four forests, including two ectomycorrhizal-dominated (ECM) forests and two arbuscular mycorrhizal-dominated forests. The significant influence of soil sieving on Rh in all forests was not observed during incubation when glucose was not added. After adding glucose, the Rh in the sieved soils on the 5th day of incubation was averaged 27.2% lower than that in intact soils in ECM forests. On the 27th day it was 22.1% lower in the Pinus massoniana forest, but 78.0% higher in the Castanea mollissima forest. Strong relationships were detected between Rh in sieved and intact soils (r2 = 0.888), and in soils both with and without the addition of glucose (r2 = 0.827). The measured soil variables explained 74.7% and 49.7% of the variation in Rh on the 5th and 27th day of incubation, and the role of soil nutrients and microbial PLFA groups in regulating Rh varied temporally. Our findings suggest that plant mycorrhizal types influenced the role of increased C availability to microbes in regulating the response of Rh to sieving in forest ecosystems.

Nonlinear response of ecosystem respiration to gradient warming in paddy field in Northeast China

Ecosystem respiration (ER) is greatly affected by global warming and responds nonlinearly to different warming levels. However, the mechanisms of the response of ER to gradient warming in paddy field remain obscure. Studying the effects and mechanisms of different warming levels on ER and its components in paddy field ecosystems will provide a theoretical basis for predicting future carbon budgets under climate change scenarios. In this study, 2-year gradient warming experiments with 5 warming treatments (+0 °C [control, CK], +0.3 °C [500 W warming, T1], +0.6 °C [1000 W warming, T2], +1.0 °C [1500 W warming, T3], +2.3 °C [3000 W warming, T4]) were performed by using a free air temperature increasing (FATI) system in paddy field in Northeast China. ER and its two components, autotrophic respiration (AR) and heterotrophic respiration (HR), were measured to reveal the mechanisms of the response of ER to gradient warming. The results showed that ER, AR and HR increased significantly under warming treatments and exhibited nonlinear responses to different warming levels. The maximum increases in ER, AR and HR appeared in the 1500 W warming treatment. The nonlinear response of AR was primarily related to total plant biomass (TB), while the nonlinear response of HR was closely related to soil microbial biomass carbon (MBC). Furthermore, warming nonlinearly enhanced the AR/ER ratios but reduced the HR/ER ratios. The AR/ER ratios were much greater than the HR/ER ratios. ER was closely correlated with its two components, and its nonlinear response might mainly derive from that of AR. The results highlight that the nonlinear response patterns of ER and its components to warming should be taken into global carbon cycling models to predict future carbon-climate feedbacks.

Soil respiration in an irrigated oasis agroecosystem: linking environmental controls with plant activities on hourly, daily and monthly timescales

To investigate the responses of different components of soil respiration to environmental factors at different timescales in a vineyard ecosystem. The trenching method was used to separate total soil respiration (TSR) into autotrophic respiration (AR) and heterotrophic respiration (HR). Soil respiration rates were measured by an LI-8100 automated flux system. On the hourly scale, there were contrasting responses in TSR, HR and AR to soil temperature at 5 cm (ST5), with clockwise hysteresis loop responses of TSR and HR to ST5 but a counterclockwise hysteresis loop between AR and ST5. The daily TSR didn’t exponentially response to ST5 during the growing season. On the monthly scale, the relationship between TSR and ST5 showed a counterclockwise hysteresis loop. Meanwhile, the diel respiration peak lagged the peak of gross primary productivity (GPP), but the monthly peak of TSR preceded the monthly peak of GPP. The daily TSR and the daily soil water content at 5 cm (SWC5) in different months showed a quadratic relationship, but there was an exponential correlation between the monthly TSR and the monthly SWC5. The relationship between soil respiration and environmental factors derived for a specific timescale cannot be directly applied to other timescales.

Investigating drivers of microbial activity and respiration in a forested bog

Northern peatlands store nearly one-third of terrestrial carbon (C) stocks while covering only 3% of the global landmass; nevertheless, the drivers of C cycling in these often-waterlogged ecosystems are different from those that control C dynamics in upland forested soils. To explore how multiple abiotic and biotic characteristics of bogs interact to shape microbial activity in a northern, forested bog, we added a labile C tracer (13C-labeled starch) to in situ peat mesocosms and correlated heterotrophic respiration with natural variation in several microbial predictor variables, such as enzyme activity and microbial biomass, as well as with a suite of abiotic variables and proximity to vascular plants aboveground. We found that peat moisture content was positively correlated with respiration and microbial activity, even when moisture levels exceeded total saturation, suggesting that access to organic matter substrates in drier environments may be limiting for microbial activity. Proximity to black spruce trees decreased total and labile heterotrophic respiration. This negative relationship may reflect the influence of tree evapotranspiration and peat shading effects; i.e., microbial activity may decline as peat dries and cools near trees. Here, we isolated the response of heterotrophic respiration to explore the variation in, and interactions among, multiple abiotic and biotic drivers that influence microbial activity. This approach allowed us to reveal the relative influence of individual drivers on C respiration in these globally important C sinks.

Soil respiration negatively correlated with volume gains by a young Pinus radiata clone over five months

It is assumed that greater plant productivity enhances autotrophic respiration (Ra). In a glasshouse trial using a one year old Pinus radiata D. Don clone selected for rapid growth, we examined the relationship between soil respiration, its components and tree growth rates over approximately five months. Soil respiration and Ra were significantly influenced by temperature; the response of heterotrophic respiration (Rh) approached significance. Soil moisture content had no effect on respiration. The productivity of individual clones, measured as volume increment, was not consistently positively correlated with soil respiration across the measurement intervals. Indeed, over the life of the trial it was found that individual tree volume increments were significantly negatively correlated with soil respiration. These results indicate that the most productive trees were associated with lower rates of autotrophic respiration, contradicting past studies. Further work is recommended to determine if this relationship holds true for these, and other, P. radiata clones in forest settings.

Linking tree physiological constraints with predictions of carbon and water fluxes at an old‐growth coniferous forest

AbstractOld‐growth coniferous forests of the Pacific Northwest are among the most productive temperate ecosystems and have the capacity to store large amounts of carbon for multiple centuries. To date, there are considerable gaps in modeling ecosystem fluxes and their responses to physiological constraints in these old‐growth forests. These model shortcomings limit our ability to understand and project how the old‐growth forests of the Pacific Northwest will respond to global climate change. This study applies the cohort‐based Ecosystem Demography Model 2 (ED2) to the Wind River Experimental Forest (Washington, USA), a well‐studied old‐growth Douglas‐fir–western hemlock ecosystem. ED2 is calibrated and validated using an extensive suite of forest inventory, eddy covariance, and biophysical observations. ED2 is able to reproduce observed forest composition and canopy structure, and carbon, water, and energy fluxes at the site. In the simulations, the effect of limited water supply on ecosystem carbon fluxes is mediated primarily by the forest's gross primary productivity (GPP) response, rather than its heterotrophic respiration response. The simulation indicates that stomatal conductance is mainly determined by soil moisture during periods of low vapor pressure deficit (VPD). However, when VPD is high, stomatal conductance is greatly reduced regardless of soil moisture status. During summer droughts, reduced soil moisture and increased VPD result in considerable stomatal closure and GPP reduction, which in turn decreases net carbon uptake. Cohort‐based scheme integrates all canopy layers (species) that have distinct sensitivity to microclimate and respond distinctly to drought. This study is an initial first step to explore the potential importance of cohort‐based model in simulating forest with complex structure, and to lay the foundation for applying cohort‐based model at regional scales across the Pacific Northwest.

Divergent responses of ecosystem respiration components to livestock exclusion on the Qinghai Tibetan Plateau

AbstractGrazing exclusion (GE) is an effective method for protecting degraded grasslands, and it can profoundly affect ecosystem carbon (C) cycles. Ecosystem respiration (ER), which includes both autotrophic and heterotrophic respiration (HR), accounts for the largest land‐to‐atmosphere C fluxes. How ER responds to GE is still unclear, however, and to investigate this, a controlled GE experiment was conducted at a meadow grassland near Qinghai Lake, China. Animal exclusion enhanced ER and aboveground plant respiration (Ragb) by 10.5% and 40.1%, respectively, but it suppressed soil respiration by 12.4% and HR by 17.6%. Positive responses of ER and Ragb were linked to increased aboveground biomass, particularly graminoids biomass. Negative responses of soil respiration and HR were associated with GE‐induced changes in microbial biomass C and nitrogen. These results show that grassland responded in complex ways to GE and that ER and its components were regulated by both abiotic and biotic factors. Moreover, the divergent responses of respiration components have important implications for models of terrestrial C cycles and climate under enhanced human activities and changes in land use.

Reducing the uncertainty of parameters controlling seasonal carbon and water fluxes in Chinese forests and its implication for simulated climate sensitivities

AbstractReducing parameter uncertainty of process‐based terrestrial ecosystem models (TEMs) is one of the primary targets for accurately estimating carbon budgets and predicting ecosystem responses to climate change. However, parameters in TEMs are rarely constrained by observations from Chinese forest ecosystems, which are important carbon sink over the northern hemispheric land. In this study, eddy covariance data from six forest sites in China are used to optimize parameters of the ORganizing Carbon and Hydrology In Dynamics EcosystEms TEM. The model‐data assimilation through parameter optimization largely reduces the prior model errors and improves the simulated seasonal cycle and summer diurnal cycle of net ecosystem exchange, latent heat fluxes, and gross primary production and ecosystem respiration. Climate change experiments based on the optimized model are deployed to indicate that forest net primary production (NPP) is suppressed in response to warming in the southern China but stimulated in the northeastern China. Altered precipitation has an asymmetric impact on forest NPP at sites in water‐limited regions, with the optimization‐induced reduction in response of NPP to precipitation decline being as large as 61% at a deciduous broadleaf forest site. We find that seasonal optimization alters forest carbon cycle responses to environmental change, with the parameter optimization consistently reducing the simulated positive response of heterotrophic respiration to warming. Evaluations from independent observations suggest that improving model structure still matters most for long‐term carbon stock and its changes, in particular, nutrient‐ and age‐related changes of photosynthetic rates, carbon allocation, and tree mortality.

Interannual variability in Australia's terrestrial carbon cycle constrained by multiple observation types

Abstract. Recent studies have shown that semi-arid ecosystems in Australia may be responsible for a significant part of the interannual variability in the global concentration of atmospheric carbon dioxide. Here we use a multiple constraints approach to calibrate a land surface model of Australian terrestrial carbon and water cycles, with a focus on interannual variability. We use observations of carbon and water fluxes at 14 OzFlux sites, as well as data on carbon pools, litterfall and streamflow. We include calibration of the function describing the response of heterotrophic respiration to soil moisture. We also explore the effect on modelled interannual variability of parameter equifinality, whereby multiple combinations of parameters can give an equally acceptable fit to the calibration data. We estimate interannual variability of Australian net ecosystem production (NEP) of 0.12–0.21 PgC yr−1 (1σ) over 1982–2013, with a high anomaly of 0.43–0.67 PgC yr−1 in 2011 relative to this period associated with exceptionally wet conditions following a prolonged drought. The ranges are due to the effect on calculated NEP anomaly of parameter equifinality, with similar contributions from equifinality in parameters associated with net primary production (NPP) and heterotrophic respiration. Our range of results due to parameter equifinality demonstrates how errors can be underestimated when a single parameter set is used.

Pore-scale investigation on the response of heterotrophic respiration to moisture conditions in heterogeneous soils

The relationship between microbial respiration rate and soil moisture content is an important property for understanding and predicting soil organic carbon degradation, CO2 production and emission, and their subsequent effects on climate change. This paper reports a pore-scale modeling study to investigate the response of heterotrophic respiration to moisture conditions in soils and to evaluate various factors that affect this response. X-ray computed tomography was used to derive soil pore structures, which were then used for pore-scale model investigation. The pore-scale results were then averaged to calculate the effective respiration rates as a function of water content in soils. The calculated effective respiration rate first increases and then decreases with increasing soil water content, showing a maximum respiration rate at water saturation degree of 0.75, which is consistent with field and laboratory observations. The relationship between the respiration rate and moisture content is affected by various factors, including pore-scale organic carbon bioavailability, the rate of oxygen delivery, soil pore structure and physical heterogeneity, soil clay content, and microbial drought resistivity. Overall, this study provides mechanistic insights into the soil respiration response to the change in moisture conditions, and reveals a complex relationship between heterotrophic microbial respiration rate and moisture content in soils that is affected by various hydrological, geophysical, and biochemical factors.

The impact of sieving on heterotrophic respiration response to water content in loamy and sandy topsoils

Understanding the relation between soil heterotrophic respiration and water content is important for the prediction of climate change effects on soil CO2 emissions. In order to quantify the influence of air-drying and sieving with 2mm meshes on the heterotrophic respiration response to soil water content we incubated intact cores and sieved samples of two loamy and two sandy soils for six levels of effective soil water saturation. We further determined soil textural properties and the soil water retention curves of the soils with the intent to identify links between soil physical characteristics and moisture sensitivity functions of heterotrophic respiration. The incubation of sieved and intact soils revealed distinct differences in the response of heterotrophic respiration to soil water content. The sieved soils exposed a threshold-type behaviour, whereas the undisturbed soils exposed a quadratic increase of heterotrophic respiration with increasing effective soil water saturation. Further, we detected significant correlations between the moisture response functions of the undisturbed soils and soil texture. From the comparison of sieved and intact soil incubations we conclude that the destruction of soil structure by sieving hampers the transferability of measured soil moisture response of heterotrophic respiration to real-world conditions. For modelling purposes we suggest the use of a quadratic function between relative respiration and effective saturation for soils with a clay fraction<20%.

Addition of nitrogen fertiliser increases net ecosystem carbon dioxide uptake and the loss of soil organic carbon in grassland growing in mesocosms

Maintaining and increasing soil organic carbon stocks in grasslands is essential for sustainable productivity and to offset anthropogenic carbon emissions. Direct measurements of net ecosystem carbon dioxide exchange, FN, can be used to detect whether an ecosystem is a net sink or a source of carbon. However partitioning heterotrophic respiration, RH, from ecosystem respiration, RE, is needed to determine the impacts of land-use and global change on soil organic carbon stocks. We extracted intact soil cores from an intensively grazed dairy farm to establish mesocosms growing in controlled conditions and we subjected them to low and high nitrogen treatments (100 and 400kgNha1y−1, respectively). After concurrent clipping and addition of nitrogen, we measured the timing for the recovery of FN and its components, photosynthesis, A, and ecosystem respiration, RE, by measuring them daily. Subsequently, we measured RH from the same mesocosms seven days after the treatments were applied, using a non-disruptive, natural abundance carbon isotope technique. To test the significance of the presence of living roots when measuring RH, we compared the results obtained from the isotopic approach to those obtained from a root exclusion technique, which involved removing the roots from the mesocosms. As the plants grew after clipping, FN decreased (increasing net CO2 uptake) exponentially to mean (±standard error) steady-state values of 1.11±0.26μmolm−2s−1 (net source) and −0.19±0.33μmolm−2s−1 (near neutral) for the low and high nitrogen treatments, respectively. When measured using the isotopic approach, RH increased by 60%, from 1.26±0.29μmolm−2s−1 in the low, to 2.06±0.55μmolm−2s−1 in the high nitrogen treatment. Thus, addition of the high nitrogen resulted in an increase in soil organic carbon loss concurrently with an increase in net uptake of carbon by the ecosystem in the high nitrogen treatment compared with the low nitrogen treatment. When measured in the absence of living roots using the root exclusion technique, RH was overall much higher than the value obtained with the isotopic technique (4.34±0.13μmolm−2s−1), indicating an apparent negative rhizosphere priming effect. Furthermore, when using the root exclusion technique, no difference was found between the nitrogen treatments, suggesting that the presence of roots mediated the response of heterotrophic respiration to addition of nitrogen. These results highlight the need to include measurements of changes in RH alongside measurements of FN in non-disturbed ecosystems to interpret the processes regulating the effects of management practices on long-term changes in soil organic carbon stocks.

Sensitivity of decomposition rates of soil organic matter with respect to simultaneous changes in temperature and moisture

AbstractThe sensitivity of soil organic matter decomposition to global environmental change is a topic of prominent relevance for the global carbon cycle. Decomposition depends on multiple factors that are being altered simultaneously as a result of global environmental change; therefore, it is important to study the sensitivity of the rates of soil organic matter decomposition with respect to multiple and interacting drivers. In this manuscript, we present an analysis of the potential response of decomposition rates to simultaneous changes in temperature and moisture. To address this problem, we first present a theoretical framework to study the sensitivity of soil organic matter decomposition when multiple driving factors change simultaneously. We then apply this framework to models and data at different levels of abstraction: (1) to a mechanistic model that addresses the limitation of enzyme activity by simultaneous effects of temperature and soil water content, the latter controlling substrate supply and oxygen concentration for microbial activity; (2) to different mathematical functions used to represent temperature and moisture effects on decomposition in biogeochemical models. To contrast model predictions at these two levels of organization, we compiled different data sets of observed responses in field and laboratory studies. Then we applied our conceptual framework to: (3) observations of heterotrophic respiration at the ecosystem level; (4) laboratory experiments looking at the response of heterotrophic respiration to independent changes in moisture and temperature; and (5) ecosystem‐level experiments manipulating soil temperature and water content simultaneously.

Variations in atmospheric CO2growth rates coupled with tropical temperature

Previous studies have highlighted the occurrence and intensity of El Niño-Southern Oscillation as important drivers of the interannual variability of the atmospheric CO2 growth rate, but the underlying biogeophysical mechanisms governing such connections remain unclear. Here we show a strong and persistent coupling (r(2) ≈ 0.50) between interannual variations of the CO2 growth rate and tropical land-surface air temperature during 1959 to 2011, with a 1 °C tropical temperature anomaly leading to a 3.5 ± 0.6 Petagrams of carbon per year (PgC/y) CO2 growth-rate anomaly on average. Analysis of simulation results from Dynamic Global Vegetation Models suggests that this temperature-CO2 coupling is contributed mainly by the additive responses of heterotrophic respiration (Rh) and net primary production (NPP) to temperature variations in tropical ecosystems. However, we find a weaker and less consistent (r(2) ≈ 0.25) interannual coupling between CO2 growth rate and tropical land precipitation than diagnosed from the Dynamic Global Vegetation Models, likely resulting from the subtractive responses of tropical Rh and NPP to precipitation anomalies that partly offset each other in the net ecosystem exchange (i.e., net ecosystem exchange ≈ Rh - NPP). Variations in other climate variables (e.g., large-scale cloudiness) and natural disturbances (e.g., volcanic eruptions) may induce transient reductions in the temperature-CO2 coupling, but the relationship is robust during the past 50 y and shows full recovery within a few years after any such major variability event. Therefore, it provides an important diagnostic tool for improved understanding of the contemporary and future global carbon cycle.