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Xu, Xia

Publications and source records attributed to Xu, Xia.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Magnetocaloric alloys useful for magnetic refrigeration applications

This invention relates to magnetocaloric materials comprising alloys useful for magnetic refrigeration applications. In some embodiments, the disclosed alloys may be Cerium, Neodymium, and/or Gadolinium based compositions that are fairly inexpensive, and in some cases exhibit only 2nd order magnetic phase transitions near their curie temperature, thus there are limited thermal and structural hysteresis losses. This makes these compositions attractive candidates for use in magnetic refrigeration applications. Surprisingly, the performance of the disclosed materials is similar or better to many of the known expensive rare-earth based magnetocaloric materials.

Ihnfeldt, Robin↗
DOE OSTI2021

Long-term measurements in a mixed-grass prairie reveal a change in soil organic carbon recalcitrance and its environmental sensitivity under warming

Soil respiration, the major pathway for ecosystem carbon (C) loss, has the potential to enter a positive feedback loop with the atmospheric CO2 due to climate warming. For reliable projections of climate-carbon feedbacks, accurate quantification of soil respiration and identification of mechanisms that control its variability are essential. Process-based models simulate soil respiration as functions of belowground C input, organic matter quality, and sensitivity to environmental conditions. However, evaluation and calibration of process-based models against the long-term in situ measurements are rare. Here, we evaluate the performance of the Terrestrial ECOsystem (TECO) model in simulating total and heterotrophic soil respiration measured during a 16-year warming experiment in a mixed-grass prairie; calibrate model parameters against these and other measurements collected during the experiment; and explore whether the mechanisms of C dynamics have changed over the years. Calibrating model parameters against observations of individual years substantially improved model performance in comparison to pre-calibration simulations, explaining 79–86% of variability in observed soil respiration. Interannual variation of the calibrated model parameters indicated increasing recalcitrance of soil C and changing environmental sensitivity of microbes. Overall, we found that (1) soil organic C became more recalcitrant in intact soil compared to root-free soil; (2) warming offset the effects of increasing C recalcitrance in intact soil and changed microbial sensitivity to moisture conditions. Furthermore, these findings indicate that soil respiration may decrease in the future due to C quality, but this decrease may be offset by warming-induced changes in C cycling mechanisms and their responses to moisture conditions.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Gene-informed decomposition model predicts lower soil carbon loss due to persistent microbial adaptation to warming

Abstract Soil microbial respiration is an important source of uncertainty in projecting future climate and carbon (C) cycle feedbacks. However, its feedbacks to climate warming and underlying microbial mechanisms are still poorly understood. Here we show that the temperature sensitivity of soil microbial respiration ( Q 10 ) in a temperate grassland ecosystem persistently decreases by 12.0 ± 3.7% across 7 years of warming. Also, the shifts of microbial communities play critical roles in regulating thermal adaptation of soil respiration. Incorporating microbial functional gene abundance data into a microbially-enabled ecosystem model significantly improves the modeling performance of soil microbial respiration by 5–19%, and reduces model parametric uncertainty by 55–71%. In addition, modeling analyses show that the microbial thermal adaptation can lead to considerably less heterotrophic respiration (11.6 ± 7.5%), and hence less soil C loss. If such microbially mediated dampening effects occur generally across different spatial and temporal scales, the potential positive feedback of soil microbial respiration in response to climate warming may be less than previously predicted.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Experimental warming amplified opposite impacts of drought vs. wet extremes on ecosystem carbon cycle in a tallgrass prairie

Climate warming is leading to greater precipitation variability, resulting in increased frequency and intensity of both drought and wet extremes. However, how these extreme events interact with climate warming and hay-harvest in grasslands to impact ecosystem functions has not yet been well explored. In this study, we took advantage of a long-term experiment to examine how climate warming and clipping (i.e., mimicking hay harvest) regulated impacts of naturally occurring drought and wet extremes on ecosystem CO 2 fluxes of a tallgrass prairie in the Great Plains, USA. Warming resulted in net ecosystem carbon release (i.e., positive net ecosystem CO 2 exchange, NEE) in the extreme drought year of 2011, but significantly enhanced net carbon uptake in the extremely wet year of 2015 in comparison with NEE in normal years. Warming-induced carbon release in the drought year was due to significantly enhanced ecosystem respiration (ER) from mid-summer to early-autumn, whereas warming-enhanced NEE in the wet year was due to an increase in aboveground net primary production (ANPP) compared to those in normal years. Drought diminished warming-induced increases in ANPP to about one sixth of that in the wet year in the unclipped plots. Interestingly, clipping offset the drought-mediated ecosystem carbon loss by increasing GPP and weakened the wet-enhanced ANPP. Altogether, our results suggest that a future, warmer climate may exacerbate carbon losses in terrestrial ecosystems during drought extremes but stimulate the ecosystem carbon sink under wet extremes.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Evaluating the simulated mean soil carbon transit times by Earth system models using observations

One known bias in current Earth system models (ESMs) is the underestimation of global mean soil carbon (C) transit time ( τ soil ), which quantifies the age of the C atoms at the time they leave the soil. However, it remains unclear where such underestimations are located globally. Here, we constructed a global database of measured τ soil across 187 sites to evaluate results from 12 ESMs. The observations showed that the estimated τ soil was dramatically shorter from the soil incubation studies in the laboratory environment (median = 4 years; interquartile range = 1 to 25 years) than that derived from field in situ measurements (31; 5 to 84 years) with shifts in stable isotopic C( 13 C ) or the stock-over-flux approach. In comparison with the field observations, the multi-model ensemble simulated a shorter median (19 years) anda smaller spatial variation (6 to 29 years) of τ soil across the same site locations. We then found a significant and negative linear correlation between the in situ measured τ soil and mean annual air temperature. The underestimations of modeled τ soil are mainly located in cold and dry biomes, especially tundra and desert.Furthermore, we showed that one ESM (i.e., CESM) has improved its τ soil estimate by incorporation of the soil vertical profile. These findings indicate that the spatial variation of τ soil is a useful benchmark for ESMs, and we recommend more observations and modeling efforts on soil C dynamics in regions limited by temperature and moisture.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Microbial community structure and functions differ between native and novel (exotic-dominated) grassland ecosystems in an 8-year experiment

Aims Grasslands dominated by non-native (exotic) species have replaced purely native-dominated areas in many parts of the world forming ‘novel’ ecosystems. It is still poorly understood how soil microbial communities and their functions are affected between these native- and exotic-dominated sites. Methods We sampled 64 experimental grassland plots in central Texas with plant species mixtures of either all native or all exotic species; half with summer irrigation. We tested how native vs. exotic plant species mixtures and summer irrigation affected bacterial and fungal community composition and structure, microbial phylotype co-occurrence, and rates of mineralization across sampling years. Results Plant origin significantly affected fungal community composition and structure, but not diversity, throughout the length of the study, while changes in bacterial communities were limited to certain years. Nitrogen and phosphorus mineralization rates were higher under native plant mixtures, and correlated with changes in particular fungal species. Microbial communities were more structured in exotic than native grassland plots, especially for the fungal community. Conclusions The results indicate that conversion of native to exotic grasslands will more strongly impact fungal than bacterial communities structure. Furthermore, these impacts can alter ecosystem functioning belowground via changes in nitrogen and phosphorus cycling.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Activated mitochondrial apoptosis in hESCs after dissociation involving the PKA/p-p53/Bax signaling pathway

Highlights: • hESC dissociation causes mitochondrial apoptosis. • H89 and Pifithrin α suppress dissociation-induced mitochondrial apoptosis. • PKA/p-p53/Bax signaling pathway is involved in dissociation-induced hESC apoptosis. Human embryonic stem cells (hESCs) are highly fragile with massive cell death after dissociation into single cells, which seriously hampers their applications. The mechanism underlying the massive cell death after dissociation still remains elusive. Here, the expression of apoptosis-related proteins, cell survival and mitochondrial membrane potential in dissociated hESCs before and after the treatments with a protein kinase A (PKA) inhibitor H89 and p53 inhibitor Pifithrin α were investigated, respectively. Protein interactions were identified by immunoprecipitation and immunofluorescence. The results show that the dissociation causes Caspase-dependent apoptosis in hESCs mediated by mitochondrial pathway with the up-regulation of pro-apoptotic proteins, decrease in mitochondrial membrane potential and elevation in pro-apoptotic Cyto c release, which are obviously suppresses by H89. The dissociation-induced increase of phosphorylated p53 Ser15 (p-p53) is suppressed by Pifithrin α which also rescues the elevated levels of pro-apoptotic proteins in mitochondrial pathway. During the dissociation-induced apoptosis, PKA/p-p53/Bax signaling pathway is identified by immunoprecipitation and immunofluorescence showing the most likely interaction between them. These results indicate that dissociation induces mitochondrial apoptosis in hESCs involving PKA/p-p53/Bax signaling pathway, which not only give new insights into the apoptosis mechanism of dissociated hESCs, but also provide clues for developing potential strategies to promote hESC survival after dissociation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Successional change in species composition alters climate sensitivity of grassland productivity

Succession theory predicts altered sensitivity of ecosystem functions to disturbance (i.e., climate change) due to the temporal shift in plant community composition. However, empirical evidence in global change experiments is lacking to support this prediction. Here, we present findings from an 8-year long-term global change experiment with warming and altered precipitation manipulation (double and halved amount). First, we observed a temporal shift in species composition over 8 years, resulting in a transition from an annual C 3 -dominant plant community to a perennial C 4 -dominant plant community. This successional transition was independent of any experimental treatments. During the successional transition, the response of aboveground net primary productivity (ANPP) to precipitation addition magnified from neutral to +45.3%, while the response to halved precipitation attenuated substantially from -17.6% to neutral. However, warming did not affect ANPP in either state. The findings further reveal that the time-dependent climate sensitivity may be regulated by successional change in species composition, highlighting the importance of vegetation dynamics in regulating the response of ecosystem productivity to precipitation change.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2018

Temperature sensitivity of soil organic carbon decomposition increased with mean carbon residence time: Field incubation and data assimilation

Abstract Temperature sensitivity of soil organic carbon ( SOC ) decomposition is one of the major uncertainties in predicting climate‐carbon (C) cycle feedback. Results from previous studies are highly contradictory with old soil C decomposition being more, similarly, or less sensitive to temperature than decomposition of young fractions. The contradictory results are partly from difficulties in distinguishing old from young SOC and their changes over time in the experiments with or without isotopic techniques. In this study, we have conducted a long‐term field incubation experiment with deep soil collars (0–70 cm in depth, 10 cm in diameter of PVC tubes) for excluding root C input to examine apparent temperature sensitivity of SOC decomposition under ambient and warming treatments from 2002 to 2008. The data from the experiment were infused into a multi‐pool soil C model to estimate intrinsic temperature sensitivity of SOC decomposition and C residence times of three SOC fractions (i.e., active, slow, and passive) using a data assimilation ( DA ) technique. As active SOC with the short C residence time was progressively depleted in the deep soil collars under both ambient and warming treatments, the residences times of the whole SOC became longer over time. Concomitantly, the estimated apparent and intrinsic temperature sensitivity of SOC decomposition also became gradually higher over time as more than 50% of active SOC was depleted. Thus, the temperature sensitivity of soil C decomposition in deep soil collars was positively correlated with the mean C residence times. However, the regression slope of the temperature sensitivity against the residence time was lower under the warming treatment than under ambient temperature, indicating that other processes also regulated temperature sensitivity of SOC decomposition. These results indicate that old SOC decomposition is more sensitive to temperature than young components, making the old C more vulnerable to future warmer climate.

Zhou, Xuhui↗
DOE OSTI2016

Unchanged carbon balance driven by equivalent responses of production and respiration to climate change in a mixed‐grass prairie

Abstract Responses of grassland carbon (C) cycling to climate change and land use remain a major uncertainty in model prediction of future climate. To explore the impacts of global change on ecosystem C fluxes and the consequent changes in C storage, we have conducted a field experiment with warming (+3 °C), altered precipitation (doubled and halved), and annual clipping at the end of growing seasons in a mixed‐grass prairie in Oklahoma, USA , from 2009 to 2013. Results showed that although ecosystem respiration ( ER ) and gross primary production ( GPP ) negatively responded to warming, net ecosystem exchange of CO 2 ( NEE ) did not significantly change under warming. Doubled precipitation stimulated and halved precipitation suppressed ER and GPP equivalently, with the net outcome being unchanged in NEE . These results indicate that warming and altered precipitation do not necessarily have profound impacts on ecosystem C storage. In addition, we found that clipping enhanced NEE due to a stronger positive response of GPP compared to ER , indicating that clipping could potentially be an effective land practice that could increase C storage. No significant interactions between warming, altered precipitation, and clipping were observed. Meanwhile, we found that belowground net primary production ( BNPP ) in general was sensitive to climate change and land use though no significant changes were found in NPP across treatments. Moreover, negative correlations of the ER / GPP ratio with soil temperature and moisture did not differ across treatments, highlighting the roles of abiotic factors in mediating ecosystem C fluxes in this grassland. Importantly, our results suggest that belowground C cycling (e.g., BNPP ) could respond to climate change with no alterations in ecosystem C storage in the same period.

Xu, Xia↗
DOE OSTI2016

Toward more realistic projections of soil carbon dynamics by Earth system models

Soil carbon (C) is a critical component of Earth system models (ESMs), and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the third to fifth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. First, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by first-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic soil organic C (SOC) dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Second, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool-and flux-based data sets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Third, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. Furthermore, we recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable data sets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2015

Magnetic self-assembly for the synthesis of magnetically exchange coupled MnBi/Fe–Co composites

Exchange coupled hard/soft MnBi/Fe–Co core/shell structured composites were synthesized using a magnetic self-assembly process. MnBi particles were prepared by arc-melting, and Fe–Co nanoparticles were synthesized by an oleic acid assisted chemical reduction method. Grinding a mixture of micron-sized MnBi and Fe–Co nanoparticles in hexane resulted in MnBi/Fe–Co core/shell structured composites. The MnBi/Fe–Co (95/5 wt%) composites showed smooth magnetic hysteresis loops, enhanced remanent magnetization, and positive values in the ΔM curve, indicating exchange coupling between MnBi and Fe–Co particles. - Graphical abstract: Both MnBi and Fe–Co particles were dispersed in hexane for grinding. Because of the oleic acid used during the Fe–Co nanoparticle synthesis, they could be well dispersed in hexane. During the grinding, the size of MnBi particles was decreased, hexane was evaporated, and the Fe–Co nanoparticles were concentrated in the solvent and magnetically attracted by MnBi particles, forming a core/shell structure. - Highlights: • Exchange coupled MnBi/Fe–Co composites are synthesized through magnetic selfassembly. • Magnetic exchange coupling is demonstrated by smooth magnetic hysteresis loops, enhanced remanent magnetization, and dominant positive peak in the ΔM curve. • The experimental results in magnetic properties are close to the theoretical calculation results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Selective adsorption in two porous triazolate–oxalate-bridged antiferromagnetic metal-azolate frameworks obtained via in situ decarboxylation of 3-amino-1,2,4-triazole-5-carboxylic acid

Solvothermal reactions of metal salts, 3-amino-1,2,4-triazole-5-carboxylic acid (H{sub 2}atzc) and ammonium oxalate in different temperature produced two metal azolate frameworks, namely, [Cu{sub 3}(atzc){sub 2}(atz)(ox)]·1.5H{sub 2}O (1) and [Co{sub 5}(atz){sub 4}(ox){sub 3}(HCOO){sub 2}]·DMF (2) (H{sub 2}atzc=3-amino-1,2,4-triazole-5-carboxylic acid, Hatz=3-amino-1,2,4-triazole, and ox=oxalate), in which the atzc precusor was in situ decarboxylated. Structural determination reveals that 1 contains [Cu{sub 3}(atzc){sub 2}(atz)]{sup 2−} layers of mixed μ{sub 4}-atzc and μ{sub 3}-atz ligands, which are pillared by ox{sup 2−} groups to form a 3D porous framework. Compound 2 contains 2D layers with basic spindle-shaped decanuclear units, which extended by ox{sup 2−} and formates to form 3D porous framework. Gas adsorption investigation revealed that two kinds of frameworks exhibited selective CO{sub 2} over N{sub 2} sorption. Moreover, activated 2 shows H{sub 2} storage capacity. Additionally, magnetic properties of both the compounds have been investigated. - Graphical abstract: Solvothermal reactions of metal salts, 3-amino-1,2,4-triazole-5-carboxylate and oxalate produced two metal azolate frameworks, which could store gas molecules, especially H{sub 2} due to small pores. in situ decarboxylation of precursor was observed. - Highlights: • Two MAFs were synthesized via in situ decarboxylation of H{sub 2}atzc. • Both activated frameworks exhibited selective CO{sub 2} over N{sub 2} sorption. • Activated 2 could adsorb H{sub 2}, which makes it promising candidates for gas storage.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Synthesis and characterization of hollow mesoporous BaFe{sub 12}O{sub 19} spheres

A facile method is reported to synthesize hollow mesoporous BaFe{sub 12}O{sub 19} spheres using a template-free chemical etching process. Hollow BaFe{sub 12}O{sub 19} spheres were synthesized by conventional spray pyrolysis. The mesoporous structure is achieved by alkaline ethylene glycol etching at 185 °C, with the porosity controlled by the heating time. The hollow porous structure is confirmed by SEM, TEM, and FIB-FESEM characterization. The crystal structure and magnetic properties are not significantly affected after the chemical etching process. The formation mechanism of the porous structure is explained by grain boundary etching. - Graphical abstract: Hollow spherical BaFe{sub 12}O{sub 19} particles are polycrystalline with both grains and grain boundaries. Grain boundaries have less ordered structure and lower stability. When the particles are exposed to high temperature alkaline ethylene glycol, the grain boundaries are etched, leaving small grooves between grains. These grooves allow ethylene glycol to diffuse inside to further etch the grains. As the grain size decreases, gaps appear on the particle surfaces, and a porous structure is finally formed. - Highlights: • Two-step synthesis method for hollow mesoporous BaFe{sub 12}O{sub 19} spheres is proposed. • Porosity of the product can be regulated by controlling the second step of chemical etching. • The crystal structure and magnetic properties are examined to be little affected during the chemical etching. • The mesoporous structure formation mechanism is explained by grain boundary etching.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗