Decreased Temperature Sensitivity Research Articles

Decreased temperature sensitivity of soil respiration induced by warming slowed topsoil carbon turnover in a semi-arid grassland

The carbon feedback between soil and climate is partly determined by topsoil organic carbon turnover time (τ). However, the τ under future warmer climate still remains unclear. Herein, a manipulative experiment was conducted in a semi-arid grassland on the Loess Plateau of China to investigate the underlying mechanisms of the response of topsoil τ to warming. The results showed that warming significantly increased topsoil τ, which was directly regulated by the decreased temperature sensitivity of soil microbial respiration and indirectly due to the decreased pH and the increased aboveground biomass, soil inorganic nitrogen and soil carbon substrate quality. These results implied that short-term warming could potentially promote soil carbon sequestration in the semi-arid grassland.

A comparison of thermal sensitivities of wing muscle contractile properties from a temperate and tropical bat species.

Endotherms experience temperature variation among body regions, or regional heterothermy, despite maintaining high core body temperatures. Bat forelimbs are elongated to function as wings, which makes them vulnerable to heat loss and exaggerates regional heterothermy. A tropical bat species, Carollia perspicillata, flies with distal wing muscles that are substantially (>10°C) cooler than proximal wing muscles and significantly less temperature sensitive. We hypothesized that the difference between proximal and distal wing muscles would be even more extreme in a temperate bat species that is capable of flight at variable environmental temperatures. We measured the contractile properties of the proximal pectoralis muscle and distal extensor carpi radialis muscle at a range of temperatures in the big brown bat, Eptesicus fuscus, and compared their thermal dependence with that of the same muscles in C. perspicillata. We found that, overall, temperature sensitivities between species were remarkably similar. The sole exception was the shortening velocity of the pectoralis muscle in E. fuscus, which was less temperature sensitive than in C. perspicillata. This decreased temperature sensitivity in a proximal muscle runs counter to our prediction. We suggest that the relative lability of body temperature in E. fuscus may make better pectoralis function at low temperatures advantageous.

Shrubification decreases soil organic carbon mineralization and its temperature sensitivity in alpine meadow soils

Shrubification is widespread in global grasslands. However, whether shrubification promotes the mineralization of soil organic carbon (SOC) remains uncertain. We compared sizes and compositions of soil non-cellulosic and amino sugar pools, soil stoichiometry, SOC decomposition and its temperature sensitivity between herbaceous and shrubby patches in alpine meadows on the Qinghai-Tibetan Plateau. The SOC content and soil stoichiometric C/P and N/P ratios in shrubby patches markedly increased with increasing plant size compared with herbaceous areas, indicating the increased P limitation to microorganisms relative to C and N with SOC accumulation under shrubs. The mass ratio of galactose plus mannose to arabinose plus xylose in the non-cellulosic carbohydrate pool of plant input was higher under shrubs than under grasses but was generally similar in soils between herbaceous and shrubby patches. This suggests the retarded microbial transformation of plant-derived carbohydrates under shrubs. Shrubs decreased accumulation of microbial necromass in SOC but increased the proportion of fungal origin in the microbial necromass. Lower total CO2 efflux per unit mass of SOC at either 15 °C or 25 °C from shrubby than from herbaceous patches over the 76-day incubation was mainly associated with the increased soil C/P or N/P ratio under shrubs. Shrubs decreased temperature sensitivity of SOC decomposition compared with grasses only at 20–40 cm soil depth, where microbial-synthesized substances had greater dominance over organic matter than at 0–20 cm depth and were less abundant under shrubs relative to grasses. Our findings show that increased P limitation to microorganisms in shrubification resulted in the decreased SOC decomposability and indicate that microbial-synthesized substances determined the temperature sensitivity of SOC mineralization. Therefore, shrubification mitigates CO2 emissions in grasslands by decreasing SOC mineralization and its temperature sensitivity in the context of global warming.

A resonant high-pressure sensor based on dual cavities

High-pressure sensors enable expansive demands in ocean sciences, industrial controls, and oil explorations. Successful sensor realized in piezoresistive high-pressure sensors which suffer from the key issue of compromised accuracies due to serious temperature drifts. Herein, this paper presents a high accuracy resonant high-pressure sensor with the pressure range of 70 MPa. Different from conventional resonant high-pressure sensor, the developed sensor utilized a dual-resonator-cavity design to minimize temperature disturbances and improve the pressure sensitivities. Besides, four circle cavities were used to maintain a high vacuum level for resonators after anodic bonding process. In details, Dual resonators, which is parallelly placed in the tensile and compressive stresses areas of a rectangular pressure sensitive diaphragm, are separated vacuum-packaged in the parallel dual cavities. Thus, pressure under measurement bends the pressure sensitive diaphragm, producing an increased pressure sensitivity and a decreased temperature sensitivity by the differential outputs of the dual resonators. Parameterized mathematical models of the sensor were established and the parameters of the models were optimized to adjust the pressure sensitivities and the temperature sensitivities of the sensor. Simplified deep reactive ion etching was used to form the sensing structure of the sensor and only once anodic bonding was used to form vacuum packaging for the dual resonators. Experimental results confirmed that the Q values of the resonators were higher than 32 000. Besides, the temperature sensitivity of the sensor was reduced from 44 Hz °C−1 (494 ppm °C−1) to 1 Hz °C−1 (11 ppm °C−1) by the differential outputs of the dual resonators in the temperature range of −10 °C–60 °C under the pressure of 1000 kPa. In addition, the accuracy of the sensor was better than 0.02% FS within the pressure range of 110–6500 kPa and the temperature range of −10 °C–60 °C by using a polynomial algorithm.

Temperature, soil moisture, and microbial controls on CO2 and CH4 emissions from a permafrost peatland

AbstractPeatlands are significant carbon dioxide (CO2) sinks and methane (CH4) sources. In this study, we investigated changes in CO2 and CH4 emissions from topsoil (0–20 cm) to subsoil (20–40 cm) in a permafrost peatland, and the related soil microbial abundance in response to increasing temperature and soil water content by using an incubation experiment. Our results indicated that CO2 and CH4 emissions from the permafrost peatland are highly sensitive to temperature and soil water content. CO2 emissions from topsoil and subsoil at 15°C were 3.36 and 2.74 times larger, respectively, compared to those at 5°C under the field moisture condition, and were 1.70 times larger under the waterlogged treatment in both topsoil and subsoil. CH4 emissions from 0–20 to 20–40 cm soils at 15°C were 34 and 83 times larger, respectively, than those at 5°C under the original state and 17 and 32 times larger under the waterlogged treatment. These results indicated that CH4 emissions are more sensitive than CO2 emissions, and waterlogged conditions could decrease temperature sensitivity of CO2 and CH4 emissions. Microbial analyses showed that the cumulative emissions amount of CO2 positively correlated with bacterial, fungal, and methanotroph abundances. Positive relationships were observed between CH4 emissions and abundances of bacteria, fungi, and archaea. These findings suggested that changes in temperature and water content alter CO2 and CH4 emissions from permafrost peatlands through controlling abundances of soil bacteria, fungi, archaea, and methanotrophs. These variables bear importance in accurately estimating C emissions from permafrost peatlands.

Improved plaque assay for human coronaviruses 229E and OC43.

In light of the COVID-19 pandemic, studies that work to understand SARS-CoV-2 are urgently needed. In turn, the less severe human coronaviruses such as HCoV-229E and OC43 are drawing newfound attention. These less severe coronaviruses can be used as a model to facilitate our understanding of the host immune response to coronavirus infection. SARS-CoV-2 must be handled under biosafety level 3 (BSL-3) conditions. Therefore, HCoV-229E and OC43, which can be handled at BSL-2 provide an alternative to SARS-CoV-2 for preclinical screening and designing of antivirals. However, to date, there is no published effective and efficient method to titrate HCoVs other than expensive indirect immunostaining. Here we present an improved approach using an agarose-based conventional plaque assay to titrate HCoV 229E and OC43 with mink lung epithelial cells, Mv1Lu. Our results indicate that titration of HCoV 229E and OC43 with Mv1Lu is consistent and reproducible. The titers produced are also comparable to those produced using human rhabdomyosarcoma (RD) cells. More importantly, Mv1Lu cells display a higher tolerance for cell-cell contact stress, decreased temperature sensitivity, and a faster growth rate. We believe that our improved low-cost plaque assay can serve as an easy tool for researchers conducting HCoV research.

Effect of Biochar on Changes of the Temperature Sensitivity of Soil Respiration and Bacterial Community Structure

To explore the effects of biochar on the temperature sensitivity of soil respiration and microbial community structure, the soil was subjected to an indoor culture test under two major treatments which were control (CK) and the addition of 3% (mass ratio) biochar (BC). Each major treatment contained four temperature conditions. We analyzed the changes in soil CO2 emission, the content of different soil organic carbon, and the characteristics of the soil bacterial community. The results showed that:①The input of biochar to soil respiration under various temperature conditions was promoted in the early stage and inhibited in the later stage, and after 14 days of cultivation, the input of biochar significantly reduced the temperature sensitivity Q10 values of soil respiration. ② For soil organic carbon fractions, the decline rate of the content of soil inert organic carbon under the CK treatment increased with increasing temperature, and was sensitive to the changes in temperature. However, the input of biochar significantly reduced the temperature sensitivity of the soil inert organic carbon, and narrowed its decline rate under each temperature condition. The decline rate did not increase with increasing temperature. ③The 16S rDNA high-throughput sequencing results showed that at the end of the cultivation, the relative abundance of Massilia in the soil of the CK treatment decreased with increasing temperature, and the input of biochar significantly increased its temperature sensitivity, making the decline rate increase. In contrast to Massilia, the relative abundance of Haliangium in soil of the CK treatment increased significantly with increasing temperature, and the input of biochar significantly reduced its temperature sensitivity, rendering the difference of relative abundance between each temperature condition not significant. This study shows that the input of biochar can significantly reduce the temperature sensitivity of soil respiration, which is related to decreased temperature sensitivity of soil inert organic carbon, and the change in the relative abundance of Massilia and Haliangium after the input of biochar.

Stand ages adjust fluctuating patterns of soil respiration and decrease temperature sensitivity after revegetation

AbstractRevegetation plays an essential role in controlling soil erosion and influencing soil respiration (Rs) by changing the vegetation coverage and soil biochemical characteristics. Soil respiration and its temperature sensitivity (Q10) have been studied comprehensively. However, the effect of stand age after revegetation on Rs and Q10 remains unclear. We selected four stands (i.e., eroded hillslopes [EBs] and 0‐ to 5‐yr, 5‐ to 10‐yr, and 10‐yr apple orchards) to investigate the changes in Rs and Q10 values. A soil CO2 flux system (Li‐8100) was used to measure Rs in situ from July 2018 to December 2018 in the Luoyugou watershed on the Chinese Loess Plateau. The average Rs rate showed decreased and increased periods with stand age. Changes in Rs were mainly induced by soil nutrient status, which accounted for 44.5% of the heavy weight in principal component analysis. The Q10 values decreased with stand age after revegetation and followed the order EB (4.96) > 0‐ to 5‐yr apple orchard (3.02) > 5‐ to 10‐yr apple orchard (2.25) > 10‐yr apple orchard (1.67). Our results suggest that Rs is substantially influenced by fungal richness and soil microbial biomass carbon (C), and the Q10 values are explained by the nonmetric multidimensional scaling ordinations of bacteria, which in turn are influenced by labile organic C. We demonstrate that converting EBs to apple orchards could be an effective way to decrease Rs and Q10 values. The fluctuating patterns of Rs with stand age should be considered when accurately evaluating the global C balance and predicting future climate change.

Tidal effects on ecosystem CO2 exchange at multiple timescales in a salt marsh in the Yellow River Delta

The tide plays a crucial role in maintaining the carbon sink strength in salt marsh ecosystems. Furthermore, the effects of tides on ecosystem carbon exchange could vary with different timescales. Using the eddy covariance technique combined with wavelet analysis, we analyzed the time-frequency characteristics of net ecosystem CO2 exchange (NEE) to address the tidal effects on NEE at multiple timescales. The wavelet analysis showed that NEE displayed a tidal-driven pattern with distinct characteristics at the multiday scale (i.e., 8–16 days) and the seasonal scale (i.e., 64–128 days). Moreover, a more significant controlling effect of light rather than air temperature on NEE was found at the diel scale. Tides also affected the variation of the diurnal pattern of NEE. Tidal flooding inhibited nighttime CO2 emissions (NEEnighttime) as well as decreased temperature sensitivity (Q10) of NEEnighttime from 1.37 to 1.16. In contrast, the response of daytime NEE (NEEdaytime) to tidal activities was more complicated, as the NEEdaytime reacted differently with different months during the growing season. Overall, our findings can contribute to a better understanding of tidal effects on ecosystem carbon exchange in salt marshes.

Short Legacy Effects of Growing Season Nitrogen Addition and Reduced Precipitation alter Soil Respiration during Nongrowing Season

The short legacy effects of growing season nitrogen (N) addition and reduced precipitation on nongrowing season soil respiration (Rs), autotrophic respiration (Ra), and heterotrophic respiration (Rh) are still unclear. Therefore, a field manipulative experiment to determine the responses of nongrowing season Rs and its components to growing season N addition and reduced precipitation was conducted in a temperate forest. The results show that growing season N addition and reduced precipitation significantly increased nongrowing season Rs by regulating the response of Ra and Rh. The combination of N addition and reduced precipitation also showed a much stronger effect on Rs and its components, but the magnitude and direction largely depended on the snowpack thickness. The effects of growing season N addition and reduced precipitation on nongrowing season Rs and its components were mediated by different sampling periods. N addition significantly decreased Rs by decreasing Rh in early winter and significantly increased Rs by increasing Ra in deep winter and late winter. All treatments decreased temperature sensitivity (Q10) of Rs and Rh. Our findings contribute to a better understanding of how nongrowing season Rs and its components will change under growing season N addition and reduced precipitation and could improve predictions of the future states of the soil C cycle in response to climate change.

Nitrogen depositions increase soil respiration and decrease temperature sensitivity in a Moso bamboo forest

Nitrogen (N) deposition plays an important role in regulating forest productivity and microbial biomass and activities, ultimately influencing soil respiration (Rs). However, the effects of increasing atmospheric N depositions on Rs in subtropical Moso bamboo forests remain poorly understood. Here, we conducted a 4-year field experiment in a subtropical Moso bamboo forest to quantify the effect of simulated N depositions at four rates (0, 30, 60 and 90 kg N ha−1 yr-1) on Rs. The mean Rs rate of the control was 353.17 ± 53.23 mg CO2 m-2 h-1 or 30.75 ± 2.38 t CO2 ha-1 yr-1. Soil respiration showed significantly higher sensitivity (Q10) to soil temperature than to air temperature, and the Rs rate was significantly positively related to soil microbial biomass carbon, soil temperature, and NO3-. In response to N addition treatments of 30, 60, and 90 kg N ha-1 yr-1, the mean annual Rs increased by approximately 45.7%, 37.7%, and 13.0%, respectively, compared with the control. Nitrogen depositions decreased the temperature sensitivity of Rs, leading to predictions that they may be able to mitigate the priming effects of future climate warmings on Rs in Moso bamboo forests in the coming decades. Combined models based on the significant relationships between Rs rates, daily mean air temperatures, and hourly soil temperatures at a depth of 5 cm may reliably and feasibly estimate annual soil CO2 efflux. On average, soil emitted 470 kg CO2 ha-1 yr-1 per 1 kg N ha-1 yr-1 added, which declined when N addition surpassed the N saturation threshold of 60 kg N ha-1 yr-1. Our findings provide a method for estimating annual soil CO2 efflux and new insights into the effects of N deposition rates on soil CO2 efflux in Moso bamboo forests.

Preparation and swelling behavior of pH/temperature responsive semi-IPN hydrogel based on carboxymethyl xylan and poly(N-isopropyl acrylamide)

In this study, carboxymethyl xylan was synthesized from wheat straw xylan and then pH/temperature dual-responsive hydrogels based on the carboxymethyl xylan (CMX) and poly(N-isopropylacrylamide) (PNIPAm) were firstly synthesized through semi-interpenetrating polymer network synthetic route. The temperature-responsive behavior was investigated by swelling experiment and DSC test, showing that the CMX content played an important role in the phase transition, and the CMX–PNIPAm hydrogel had higher pH sensitivity compared with PNIPAm hydrogel. The CMX content and the interconnected pore structure within the network improved the swelling, and the CMX–PNIPAm hydrogels had a high de-swelling rate. Addition of AAc in the hydrogels can improve pH sensitivity but decrease temperature sensitivity. The excellent swelling reversibility in response to temperature was verified through the oscillatory experiments of swelling and de-swelling. It is expected that the CMX–PNIPAm hydrogels could be used in biomedical fields as an intelligent material, especially for drug delivery.

Long‐term changes in the impacts of global warming on leaf phenology of four temperate tree species

Contrary to the generally advanced spring leaf unfolding under global warming, the effects of the climate warming on autumn leaf senescence are highly variable with advanced, delayed, and unchanged patterns being all reported. Using one million records of leaf phenology from four dominant temperate species in Europe, we investigated the temperature sensitivities of spring leaf unfolding and autumn leaf senescence (ST , advanced or delayed days per degree Celsius). The ST of spring phenology in all of the four examined species showed an increase and decrease during 1951-1980 and 1981-2013, respectively. The decrease in the ST during 1981-2013 appears to be caused by reduced accumulation of chilling units. As with spring phenology, the ST of leaf senescence of early successional and exotic species started to decline since 1980. In contrast, for late successional species, the ST of autumn senescence showed an increase for the entire study period from 1951 to 2013. Moreover, the impacts of rising temperature associated with global warming on spring leaf unfolding were stronger than those on autumn leaf senescence. The timing of leaf senescence was positively correlated with the timing of leaf unfolding during 1951-1980. However, as climate warming continued, the differences in the responses between spring and autumn phenology gradually increased, so that the correlation was no more significant during 1981-2013. Our results further suggest that since 2000, due to the decreased temperature sensitivity of leaf unfolding the length of the growing season has not increased any more. These finding needs to be addressed in vegetation models used for assessing the effects of climate change.

Effect of a smectic liquid crystal polymer as new β-nucleating agent on crystallization structure, melting, and rheological behavior of isotactic polypropylene

A smectic liquid crystal polysiloxane containing cholesteryl groups (SLCP-NA) was synthesized and used as a new kind of β-nucleating agent to induce the formation of β-crystal in isotactic polypropylene (iPP). The nucleating ability of SLCP-NA was investigated using wide-angle X-ray diffraction and differential scanning calorimetry. The content of SLCP-NA and crystallization temperature could lead to substantial changes to crystallization behavior of iPP. As the SLCP-NA content and crystallization temperature increased, the relative content of β-crystal (Kβ) tended to first increase and then decrease. The Kβ value reached a maximum value of 66.3% when the mass fraction of SLCP-NA and the crystallization temperature were 0.2 wt% and 120 °C, respectively. In addition, the rheological behavior of the iPP/SLCP-NA blends was studied. Exhilaratingly, a little mass fraction of SLCP-NA was found to greatly improve the flow ability of the blends, and the melt flow rate increased from 2.9 g/10 min of pure iPP to 43.9 g/10 min of the iPP blend with 0.3 wt% SLCP-NA. Moreover, the addition of SLCP-NA could decrease temperature sensitivity and increase shear sensitivity.

Spatial variations of soil respiration and temperature sensitivity along a steep slope of the semiarid Loess Plateau.

The spatial heterogeneity of soil respiration and its temperature sensitivity pose a great challenge to accurately estimate the carbon flux in global carbon cycling, which has primarily been researched in flatlands versus hillslope ecosystems. On an eroded slope (35°) of the semiarid Loess Plateau, soil respiration, soil moisture and soil temperature were measured in situ at upper and lower slope positions in triplicate from 2014 until 2016, and the soil biochemical and microbial properties were determined. The results showed that soil respiration was significantly greater (by 44.2%) at the lower slope position (2.6 μmol m–2 s–1) than at the upper slope position, as were soil moisture, carbon, nitrogen fractions and root biomass. However, the temperature sensitivity was 13.2% greater at the upper slope position than at the lower slope position (P < 0.05). The soil fungal community changed from being Basidiomycota-dominant at the upper slope position to being Zygomycota-dominant at the lower slope position, corresponding with increased β-D-glucosidase activity at the upper slope position than at the lower slope position. We concluded that soil respiration was enhanced by the greater soil moisture, root biomass, carbon and nitrogen contents at the lower slope position than at the upper slope position. Moreover, the increased soil respiration and decreased temperature sensitivity at the lower slope position were partially due to copiotrophs replacing oligotrophs. Such spatial variations along slopes must be properly accounted for when estimating the carbon budget and feedback of future climate change on hillslope ecosystems.

Improvement in performance properties of asphalt using a novel boron-containing additive

In current times, many additives are used to improve the performance characteristics of asphalt. In this study, a new additive that contains boron, given the short name of cyclic borate ester (CBE), was chemically synthesised in laboratory conditions. It was added to the asphalt binder as 1%, 2%, 4% and 6% ratios of asphalt by weight, and modified asphalt samples were produced. The effects of CBE on the conventional properties of asphalt binder (penetration, softening point, ductility, flash point, Fraass breaking point) were examined using a dynamic shear rheometer, rotational viscosity, a rolling thin film oven, a pressure aging vessel and bending beam rheometer test methods. Results showed that the use of CBE additive increased the hardness, softening point, viscosity, flash-point value and rutting resistance of asphalt binder and decreased temperature sensitivity. CBE also enhanced elastic responses (increased complex shear modulus and decreased phase angle), aging resistance and low-temperature cracking resistance and did not change the cohesion property.

Nanoparticles and Apparent Activation Energy of Portland Cement

Although chemically inert nanosize mineral fillers have been shown to modify early cement hydration kinetics, with the effects dependent upon usage rate, particle size, and dispersibility, the effects of such fillers on the “apparent activation energy” (Ea) of cement has not been previously examined. Here, cementEawas calculated from isothermal calorimetry performed at different temperatures with two different types of fillers (i.e., titanium dioxide and limestone) using a linear method as well as a modifiedASTMC1074 method. The use of both types of nanoparticles increased the rate of cement hydration as well as accelerated the reaction rate, due to heterogeneous nucleation effect, as previously demonstrated.Eaincreased in the presence of nanosized fillers, demonstrating an increased temperature sensitivity of the filler‐cement composites relative to ordinary cement. These results show that chemically inert nanoparticles behave fundamentally differently compared with supplementary cementitious materials such as fly ash and silica fume which instead decrease temperature sensitivity. The increased temperature sensitivity could thus be used to modify and optimize the reaction mechanism and kinetics of cement hydration, especially to increase the rate of cement hydration, to decrease setting time, and to achieve faster strength gain accounting for higher or lower temperatures during curing.

Thermal acclimation of organic matter decomposition in an artificial forest soil is related to shifts in microbial community structure

Thermal acclimation of soil organic matter (SOM) decomposition is frequently observed and has often been attributed to substrate depletion under warming, but other mechanisms, such as changes in microbial community structure and functioning, have received less attention. In order to determine whether shifts in microbial community structure and functioning are involved in thermal acclimation of SOM decomposition, a laboratory incubation experiment was conducted using an artificial forest soil. Samples were first subjected to different temperatures of 5, 15, and 25 °C during a 72-day pre-incubation period and then half of the microcosms from each pre-incubation temperature were incubated at 5 or 25 °C for a period of 11 days. Substantial thermal acclimation of SOM decomposition was observed, with the SOM decomposition in soils pre-incubated at higher temperatures being less sensitive to temperature. Along with the reduced temperature sensitivity in response to warming, significant changes in microbial community PLFAs, microbial biomass carbon (MBC), and the potential activities of 11 enzymes were also observed. Nevertheless, shifts in microbial community PLFAs and particular enzyme activities provided the most explanatory power for the decreased temperature sensitivity with warming, as revealed by a multivariate regression analysis. The microbial community structure shifts were mainly manifested as an increase in the relative abundance of Gram-positive bacteria and decreases in the relative abundances of Gram-negative bacteria and fungi. Microbial communities pre-incubated under lower temperatures experienced greater shifts in their structure. Substrate depletion did not occur in this short-term incubation experiment, since neither total organic carbon (TOC) nor dissolved organic carbon (DOC) decreased with increasing temperature. Our results suggest that shifts in microbial community structure and functioning may underlie the thermal acclimation of SOM decomposition and should be taken into account when predicting the response of soil CO2 efflux to global warming.

Responses of soil respiration to N addition, burning and clipping in temperate semiarid grassland in northern China

Soil respiration is an important part of the global carbon (C) cycle and the largest component of C flux from terrestrial ecosystems to the atmosphere. Global change and anthropogenic perturbations can profoundly impact soil respiration. A field experiment examined the seasonal variability of soil respiration in response to the addition of nitrogen (N), burning, clipping and their possible interactions throughout an entire growing season from April to October 2011, in semiarid grassland in northern China. Results showed that N addition and burning significantly increased mean soil respiration by 35.8% and 11.0%, respectively, and that burning and N addition synergistically enhanced soil respiration. However, the effects of N addition and burning on soil respiration were mediated by season. Clipping had no significant effect on soil respiration. Soil moisture was primarily responsible for the seasonal changes in soil respiration, whereas the positive responses of soil respiration to burning and N addition were attributable to elevated soil temperature, plant growth, root and microbial activity and respiration. In unfertilized plots, burning decreased temperature sensitivity (Q10) of soil respiration by 10.0%. In plots with N addition, burning and clipping decreased Q10 by 15.4% and 11.6%, respectively. We therefore conclude that burning can, and clipping may, decrease the dependence of soil respiration on temperature. We further observed that the magnitude of positive feedback in soil respiration to temperature increase weakened in the burned plots, and that the availability of N might regulate the degree of this weakening. The different mechanisms by which N addition, burning and clipping influence soil respiration and its sensitivity to temperature identified in this study may facilitate the simulation and prediction of C cycling in semiarid grassland under future scenarios of global change.

Consistent effects of nitrogen amendments on soil microbial communities and processes across biomes

AbstractEcosystems worldwide are receiving increasing amounts of reactive nitrogen (N) via anthropogenic activities with the added N having potentially important impacts on microbially mediated belowground carbon dynamics. However, a comprehensive understanding of how elevated N availability affects soil microbial processes and community dynamics remains incomplete. The mechanisms responsible for the observed responses are poorly resolved and we do not know if soil microbial communities respond in a similar manner across ecosystems. We collected 28 soils from a broad range of ecosystems in North America, amended soils with inorganic N, and incubated the soils under controlled conditions for 1 year. Consistent across nearly all soils, N addition decreased microbial respiration rates, with an average decrease of 11% over the year‐long incubation, and decreased microbial biomass by 35%. High‐throughput pyrosequencing showed that N addition consistently altered bacterial community composition, increasing the relative abundance of Actinobacteria and Firmicutes, and decreasing the relative abundance of Acidobacteria and Verrucomicrobia. Further, N‐amended soils consistently had lower activities in a broad suite of extracellular enzymes and had decreased temperature sensitivity, suggesting a shift to the preferential decomposition of more labile C pools. The observed trends held across strong gradients in climate and soil characteristics, indicating that the soil microbial responses to N addition are likely controlled by similar wide‐spread mechanisms. Our results support the hypothesis that N addition depresses soil microbial activity by shifting the metabolic capabilities of soil bacterial communities, yielding communities that are less capable of decomposing more recalcitrant soil carbon pools and leading to a potential increase in soil carbon sequestration rates.