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Short-term effects of labile organic C addition on soil microbial response to temperature in a temperate steppe
Authors: Chao Li,Chunwang Xiao,Bertrand Guenet,Mingxu Li,Li Xu,Nianpeng He

Abstract: It remains unclear how soil microbes respond to labile organic carbon (LOC) inputs and how temperature sensitivity (Q10) of soil organic matter (SOM) decomposition is affected by LOC inputs in a short-term. In this study, 13C-labeled glucose was added to a pristine grassland soil at four temperatures (10, 15, 20, and 25 °C), and the immediate utilization of LOC and native SOM by microbes was measured minutely in a short-term. We found that the LOC addition stimulated the native SOM decomposition, and elevated temperature enhanced the intensity of microbial response to LOC addition. The ratio between microbial respiration derived from LOC and native SOM increased with higher temperature, and more LOC for respiration. Additionally, LOC addition increased the Q10 of SOM decomposition, and the Q10 of LOC decomposition is higher than that of native SOM. Overall, these findings emphasize the important role of temperature and LOC inputs in soil C cycles. Original link
No increasing risk of a limnic eruption at Lake Kivu: Intercomparison study reveals gas concentrations close to steady state
Authors:Fabian Bärenbold ,Bertram Boehrer,Roberto Grilli,Ange Mugisha,Wolf von Tümpling,Augusta Umutoni,Martin Schmid

Abstract:Lake Kivu, East Africa, is well known for its huge reservoir of dissolved methane (CH4) and carbon dioxide (CO2) in the stratified deep waters (below 250 m). The methane concentrations of up to ~ 20 mmol/l are sufficiently high for commercial gas extraction and power production. In view of the projected extraction capacity of up to several hundred MW in the next decades, reliable and accurate gas measurement techniques are required to closely monitor the evolution of gas concentrations. For this purpose, an intercomparison campaign for dissolved gas measurements was planned and conducted in March 2018. The applied measurement techniques included on-site mass spectrometry of continuously pumped sample water, gas chromatography of in-situ filled gas bags, an in-situ membrane inlet laser spectrometer sensor and a prototype sensor for total dissolved gas pressure (TDGP). We present the results of three datasets for CH4, two for CO2 and one for TDGP. The resulting methane profiles show a good agreement within a range of around 5–10% in the deep water. We also observe that TDGP measurements in the deep waters are systematically around 5 to 10% lower than TDGP computed from gas concentrations. Part of this difference may be attributed to the non-trivial conversion of concentration to partial pressure in gas-rich Lake Kivu. When comparing our data to past measurements, we cannot verify the previously suggested increase in methane concentrations since 1974. We therefore conclude that the methane and carbon dioxide concentrations in Lake Kivu are currently close to a steady state. Original link
High inhaled oxygen concentration quadruples exhaled CO in healthy volunteers monitored by a highly sensitive laser spectrometer
Authors:Vivien Brenckmann, Irène Ventrillard, Daniele Romanini, Kévin Jaulin, Pascale Calabrèse & Raphaël Briot

Carbon monoxide (CO) monitoring in human breath is the focus of many investigations as CO could possibly be used as a marker of various diseases. Detecting CO in human breath remains a challenge because low concentrations (<ppm) must be selectively detected and short response time resolution is needed to detect the end expiratory values reflecting actual alveolar concentrations. A laser spectroscopy based instrument was developed (ProCeas) that fulfils these requirements. The aim of this study was to validate the use of a ProCeas for human breath analysis in order to measure the changes of endogenous exhaled CO (eCO) induced by different inspired fractions of oxygen (FiO2) ranging between 21% and 100%. This study was performed on healthy volunteers. 30 healthy awaked volunteers (including asymptomatic smokers) breathed spontaneously through a facial mask connected to the respiratory circuit of an anesthesiology station. FiO2 was fixed to 21%, 50% and 100% for periods of 5 minutes. CO concentrations were continuously monitored throughout the experiment with a ProCeas connected to the airway circuit. The respiratory cycles being resolved, eCO concentration is defined by the difference between the value at the end of the exhalation phase and the level during inhalation phase. Inhalation of 100% FiO2 increased eCO levels by a factor of four in every subjects (smokers and non smokers). eCO returned in a few minutes to the initial value when FiO2 was switched back to 21%. This magnification of eCO at 21% and 100% FiO2 is greater than those described in previous publications. We hypothesize that these results can be explained by the healthy status of our subjects (with low basal levels of eCO) and also by the better measurement precision of ProCeas. Original link
A new incubation and measurement approach to estimate the temperature response of soil organic matter decomposition.
Authors: Yuan Liu,Nianpeng He,Li Xu,Jing Tian,Yang Gao,Shuai Zheng,Qing Wang,Xuefa Wen,Xingliang Xu,Kuzyakov Yakov

Abstract: A reliable and precise estimate of the temperature sensitivity (Q10) of soil organic matter (SOM) decomposition is critical to predict feedbacks between the global carbon (C) cycle and climate change. In this study, we first summarize two commonly used approaches for estimating Q10 (Approach A: constant temperature incubation and discontinuous measurements, CDM model; Approach B: varying temperature incubation and discontinuous measurements, VDM model). We then introduced a newly developed approach (Approach C, VCM model) that combines rapidly varying temperature incubations and continuous measurements of SOM decomposition rates (Rs) that may be more realistic and suitable for Q10 estimation, especially for large scale estimation. Then, we conducted a 26-day incubation experiment using three different soils to compare the performance of these three approaches for estimating Q10 using R2 and P-values as indicators. Our results demonstrate that the fitting goodness of the exponential model was consistently higher for Approach C, with higher R2 values, lower confidence intervals, and lower P-values in almost all cases compared with Approaches A and B. Furthermore,results showed that Approaches A and B underestimated the Q10 value by 9.5–13% and 2.9–5.7%, respectively,in three different soils throughout the entire incubation period. Compared with traditional commonly used methods, the newly developed Approach C (VCM model) provides a more accurate and rapid estimation of the temperature response of SOM decomposition and can be used for large-scale estimation of Q10. Original link
Important interaction of chemicals, microbial biomass and dissolved substrates in the diel hysteresis loop of soil heterotrophic respiration
Authors: Qing Wang,Nianpeng He,Yuan Liu,Meiling Li,Li Xu,Xuhui Zhou 

Abstract: Background and aims Increasing the emission of carbon dioxide by heterotrophic respiration (Rh) might lead to global warming. However, issues remain on how Rh responds to changing temperatures, especially with respect to the hysteresis loop in the relationship between Rh and temperature at the daily scale, along with elucidating the underlying mechanisms.
Method We investigated hysteresis loop by measuring Rh in subtropical forest soil at the daily scale (12 h for warm-up (6–30 °C) and cool-down processes (30–6 °C), respectively) using continuous temperature variation and high resolution of measurements over a 56-day incubation period. The ratios of R20 and Q10 between warm-up and cooldown were calculated as the characteristics of diel hysteresis. We measured chemical (pH, conductivity,oxidation-reduction potential), microbial biomass and dissolved substrate (carbon and nitrogen) parameters to explain variation of diel hysteresis.
Results Rh was strongly dependent on temperature, with a clockwise hysteresis loop of Rh between the warm-up and cool-down daily processes. The average value of R20 [at a reference temperature of 20 °C] during the whole incubation period under the warm-up process was significantly higher (46.05 ± 0.96 μgC g−1 d−1) than that under the cool-down process (14.74 ± 0.03 μgC g−1 d−1). In comparison, the average value of Q10 under the cool-down process (5.27 ± 0.2) was significantly higher than that under the warm-up process (1.66 ± 0.02). Redundancy analysis showed that the interaction effects of soil chemical, microbial biomass, and dissolved substrate parameters explain most variation of diel hysteresis:98% variation in R20 and 93.5% variation in Q10.Compared with the weak effect of chemistry parameters on the diel hysteresis, the sole and interactive effects of microbial biomass and substrate were more important,especially their interaction.
Conclusions Interactions of chemical, microbial biomass,and dissolved substrate parameters dominated the variation in diel hysteresis of Rh with temperature,especially the interaction of microbial biomass and dissolved substrate. Of note, Q10 during the warm-up process might be overestimated when using the highly fitted temperature-dependent function of cool-down period.Furthermore, using a constant value of Q10 (Q10=2) in carbon cycle models might be an important source of uncertainty.
Original link
Widespread asymmetric response of soil heterotrophic respiration to warming and cooling
Authors: Liu Y, Wen XF, Zhang YH, Tian J, Gao Y, Ostle NJ, Niu SL, Chen SP, Sun XM, He NP. 

Abstract: Soil is the largest organic carbon (C) pool in terrestrial ecosystems. Periodic changes in environmental temperature occur diurnally and seasonally; yet, the response of soil organic matter (SOM) decomposition to varying temperatures remains unclear. In this study, we conducted a modified incubation experiment using soils from 16 forest ecosystems in China with periodically and continuously varying incubation temperature to investigate how heterotrophic respiration (Rh) responds to different temperature patterns (both warming and cooling temperature ranging between 5 and 30°C). Our results showed a pronounced asymmetric response of Rh to temperature warming and cooling among the soils of all forest ecosystems, with Rh increasing more rapidly during the warming phase compared to the cooling phase. This asymmetric response of Rh to warming and cooling temperatures was widespread in all soils. In addition, the amplitude of this asymmetric response differed among different forest ecosystems, with subtropical and warm-temperate forest ecosystems exhibiting greater asymmetric responses. Path analyses showed that soil pH and the microbial community explained most of the variation in this asymmetric response. Furthermore, the widespread asymmetric response of Rh to warming and cooling temperatures suggests that accumulated SOM decomposition might be overestimated on average by 20% for warming alone when compared with admix warming and cooling. These findings provide new insights on the responses of Rh to natural shifts in temperature, emphasizing the need to consider this widespread asymmetric response of Rh to warming and cooling phases to predict C-climate feedback with great accuracy, especially under future non-uniform warming scenarios. Original link
Soil microbial respiration rate and temperature sensitivity along a north-south forest transect in eastern China: Patterns and influencing factors
Authors: Wang Q, He NP, Yu GR, Gao Y, Wen XF, Wang RF, Koerner SE, Yu Q. 

Soil organic matter is one of the most important carbon (C) pools in terrestrial ecosystems, and future warming from climate change will likely alter soil C storage via temperature effects on microbial respiration. In this study, we collected forest soils from eight locations along a 3700km north-south transect in eastern China (NSTEC). For 8weeks these soils were incubated under a periodically changing temperature range of 6-30 degrees C while frequently measuring soil microbial respiration rate (Rs; each sample about every 20min). This experimental design allowed us to investigate Rs and the temperature sensitivity of Rs (Q(10)) along the NSTEC. Both Rs at 20 degrees C (R-20) and Q(10) significantly increased (logarithmically) with increasing latitude along the NSTEC suggesting that the sensitivity of soil microbial respiration to changing temperatures is higher in forest soils from locations with lower temperature. Our findings from an incubation experiment provide support for the hypothesis that temperature sensitivity of soil microbial respiration increases with biochemical recalcitrance (C quality-temperature hypothesis) across forest soils on a large spatial scale. Furthermore, microbial properties primarily controlled the observed patterns of R-20, whereas both substrate and microbial properties collectively controlled the observed patterns of Q(10). These findings advance our understanding of the driving factors (microbial versus substrate properties) of R-20 and Q(10) as well as the general relationships between temperature sensitivity of soil microbial respiration and environmental factors. Original link
Changes in the temperature sensitivity of SOM decomposition with grassland succession: implications for soil C sequestration
Authors:He Nianpeng, Wang Ruomeng, Gao Yang, Dai Jingzhong, Wen Xuefa, Yu Guirui

Abstract:Understanding the temperature sensitivity (Q 10) of soil organic matter (SOM) decomposition is important for predicting soil carbon (C) sequestration in terrestrial ecosystems under warming scenarios. Whether Q 10 varies predictably with ecosystem succession and the ways in which the stoichiometry of input SOM influences Q 10 remain largely unknown. We investigate these issues using a grassland succession series from free-grazing to 31-year grazing-exclusion grasslands in Inner Mongolia, and an incubation experiment performed at six temperatures (0, 5, 10, 15, 20, and 25°C) and with four substrates: control (CK), glucose (GLU), mixed grass leaf (GRA), and Medicago falcata leaf (MED). The results showed that basal soil respiration (20°C) and microbial biomass C (MBC) logarithmically decreased with grassland succession. Q 10 decreased logarithmically from 1.43 in free-grazing grasslands to 1.22 in 31-year grazing-exclusion grasslands. Q 10 increased significantly with the addition of substrates, and the Q 10 levels increased with increase in N:C ratios of substrate. Moreover, accumulated C mineralization was controlled by the N:C ratio of newly input SOM and by incubation temperature. Changes in Q 10 with grassland ecosystem succession are controlled by the stoichiometry of newly input SOM, MBC, and SOM quality, and the combined effects of which could partially explain the mechanisms underlying soil C sequestration in the long-term grazing-exclusion grasslands in Inner Mongolia, China. The findings highlight the effect of substrate stoichiometry on Q 10 which requires further study. Original link
Strong pulse effects of precipitation events on soil microbial respiration in temperate forests
Authors:Wang Qing,He Nianpeng,Liu Yuan,Li Meiling,Xu li

Abstract:Precipitation is a critical factor triggering soil biogeochemical processes in arid and semi-arid regions. In this study, we selected soils from two temperate forests—a mature natural forest and a degraded secondary forest—in a semi-arid region. We investigated the pulse effects of simulated precipitation (to reach 55% soil water-holding capacity) on the soil microbial respiration rate (RS). We performed high-intensity measurements (at 5-min intervals for 48 h) to determine the maximum value of RS (RS-max), the time to reach RS-max (TRS-max), and the duration of the pulse effect (from the start to the end of ½RS-max). The responses of RS to simulated precipitation were rapid and strong. RS-max was significantly higher in degraded secondary forest (18.69 µg C g soil–1 h–1) than in mature natural forest (7.9 4 µg C g soil–1 h–1). In contrast, the duration of the pulse effect and TRS-max were significantly lower in degraded secondary forest than in mature natural forest. Furthermore, the accumulative microbial respiration per gram of soil (ARS-soil) did not differ significantly between degraded secondary forest and mature natural forest, but the accumulative microbial respiration per gram of soil organic C (ARS-soc) was significantly higher in degraded secondary forest than in mature natural forest. Soil microbial biomass, soil nutrient, and litter nitrogen content were strongly correlated with the duration of the pulse effect and TRS-max. Soil physical structure, pH, and litter nitrogen content were strongly correlated with RS-max and ARS-soc. Our results indicate that the responses of soil microbial respiration to simulated precipitation are rapid and strong and that microbial respiration rate per gram C is able assess precisely the precipitation pulse of different soil samples as well as the effects of changing precipitation patterns on soil C content under various scenarios of global climate change. Original link
Regional variation in the temperature sensitivity of soil organic matter decomposition in China's forests and grasslands

Abstract:How to assess the temperature sensitivity (Q10 ) of soil organic matter (SOM) decomposition and its regional variation with high accuracy is one of the largest uncertainties in determining the intensity and direction of the global carbon (C) cycle in response to climate change. In this study, we collected a series of soils from 22 forest sites and 30 grassland sites across China to explore regional variation in Q10 and its underlying mechanisms. We conducted a novel incubation experiment with periodically changing temperature (5-30 °C), while continuously measuring soil microbial respiration rates. The results showed that Q10 varied significantly across different ecosystems, ranging from 1.16 to 3.19 (mean 1.63). Q10 was ordered as follows: alpine grasslands (2.01) > temperate grasslands (1.81) > tropical forests (1.59) > temperate forests (1.55) > subtropical forests (1.52). The Q10 of grasslands (1.90) was significantly higher than that of forests (1.54). Furthermore, Q10 significantly increased with increasing altitude and decreased with increasing longitude. Environmental variables and substrate properties together explained 52% of total variation in Q10 across all sites. Overall, pH and soil electrical conductivity primarily explained spatial variation in Q10 . The general negative relationships between Q10 and substrate quality among all ecosystem types supported the C quality temperature (CQT) hypothesis at a large scale, which indicated that soils with low quality should have higher temperature sensitivity. Furthermore, alpine grasslands, which had the highest Q10 , were predicted to be more sensitive to climate change under the scenario of global warming. Original link
The optimum temperature of soil microbial respiration: Patterns and controls
Authors: :Liu, Yuan   He, Nianpeng   Wen, Xuefa   Xu, Li   Sun, Xiaomin   Yu, Guirui   Liang, Liyin   Schipper, Louis A.

Abstract:The temperature response of soil microbial respiration (Rh) is of significance, with the optimum temperature ofRhbeing the key parameter for accurately modeling how it responds to temperature change under climatewarming scenarios. However, knowledge about Toptin natural ecosystems remains limited, especially at largescales, which increases the uncertainty of climate projections. Here, we collected 25 soils from tropical to cold-temperate forests in the northern hemisphere to quantify regional variation in Toptand the controls underlyingthis variation. Rhwas measured at high frequency using a novel system under the mode, with temperaturegradually increasing from 5 to 50 °C. The results showed that Toptranged from 38.5 to 46.0 °C (mean: 42.4 °C). Ofnote, this study is the first to demonstrate that Toptis far higher than the assumed value used in models (35 °C),varying greatly across different climatic zones and increasing with latitude from tropical to cold-temperate forestsoils. To some extent, our results supported the substrate supply hypothesis, and contrast with the climateadaption hypothesis. In addition, climate, nutrient, and soil microorganisms jointly regulate regional variation inTopt, together explaining 53% of variation in Topt. The higher Toptin northern regions indicated that theseregions have a greater potential to release more CO2from soil, which might lead to a positive feedback to globalwarming. In conclusion, process-based models should incorporate the high variability of Toptacross regions toimprove predictions of the carbon dynamics of terrestrial ecosystems under climate warming scenarios. Original link

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