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Water-use efficiency (WUE) is a key determinant of carbon and water fluxes at scales ranging from individual plants to continents and thus a key driver of hydro-climatic changes and carbon models. Multiple lines of evidence suggest that WUE of plants increases with atmospheric CO2, pointing to potential changes in physiological forcing of global carbon and hydrological cycles. Although the increase in forest WUE with atmospheric CO2 is widespread, declines in tree growth have been observed in different forests. These controversial results highlight the need to re-evaluate the historical trend of forest WUE.13C signatures (i.e., δ13C) of plant organic matter is a useful tool for estimating WUE at temporal scales ranging from days to centuries. This estimation relies on photosynthetic 13C discrimination models that have different assumptions about the components of isotope discrimination. Mesophyll conductance is a key uncertainty in estimated WUE owing to its influence on diffusion of CO2 to sites of carboxylation.We developed a 13C-based WUE model that takes into account the effect of mesophyll conductance and tested its performance with experimental data. We applied this model to 464 δ13C chronologies in tree-rings of 143 species spanning global biomes. Adjusted for mesophyll conductance, mean sensitivity of WUE to atmospheric CO2 (0.15 ppm ppm-1) was considerably smaller than those estimated from conventional modelling (0.23-0.30 ppm ppm-1). Our results showed a 10-ppm gain in WUE during the 20th century. Ratios of internal-to-atmospheric CO2 (ci/ca) in leaves maintained constant over time but differed among biomes and plant taxa. Our results also suggest that ratios of chloroplastic-to-atmospheric CO2 (cc/ca) are constrained over time, but this result is associated with the sensitivity of gs /gm ratio to CO2 which need further evaluation.Over the last century, a general increase in forest WUE across the globe has been confirmed. However, our study shows that forest WUE may have not increased as much as previously suggested and that projections of CO2 fertilization may need to be adjusted accordingly.

Mesophyll conductance (g叶肉导度(

International audience; Carbon isotope discrimination (Delta) has been used widely to infer intrinsic water-use efficiency (iWUE) of C-3 plants, a key parameter linking carbon and water fluxes. Despite the essential role of mesophyll conductance (g(m)) in photosynthesis and Delta, its effect on Delta-based predictions of iWUE has generally been neglected.Here, we derive a mathematical expression of iWUE as a function of Delta that includes g(m) (iWUE(mes)) and exploits the g(m)-stomatal conductance (g(sc)) relationship across drought-stress levels and plant functional groups (deciduous or semideciduous woody, evergreen woody and herbaceous species) in a global database. iWUE(mes) was further validated with an independent dataset of online-Delta and CO2 and H2O gas exchange measurements with seven species.Drought stress reduced g(sc) and g(m) by nearly one-half across all plant functional groups, but had no significant effect on the g(sc) : g(m) ratio, with a well supported value of 0.79 +/- 0.07 (95% CI, n = 198). g(m) was negatively correlated to iWUE. Incorporating the g(sc) : g(m) ratio greatly improved estimates of iWUE, compared with calculations that assumed infinite g(m).The inclusion of the g(sc) : g(m) ratio, fixed at 0.79 when g(m) was unknown, proved desirable to eliminate significant errors in estimating iWUE from Delta across various C-3 vegetation types.

Carbon isotope discrimination (Δ) has been used widely to infer intrinsic water-use efficiency (iWUE) of C

SummaryThe carbon isotope discrimination (Δ) has been used widely to infer intrinsic water-use efficiency (iWUE) of C3 plants, a key parameter linking carbon and water fluxes. Despite the essential role of mesophyll conductance (gm) in photosynthesis and Δ, its effect on Δ-based predictions of iWUE has generally been neglected.Here, we derive a mathematical expression of iWUE as a function of Δ that includes gm (iWUEmes) and exploits the gm-stomatal conductance (gsc) relationship across drought-stress levels and plant functional groups (deciduous or semi-deciduous woody, evergreen woody and herbaceous species) in a global database. iWUEmes was further validated with an independent dataset of online-Δ and CO2 and H2O gas exchange measurements with seven species.Drought stress reduced gsc by 52% and gm by 45% averaged over all plant functional groups, but had no significant effect on the gsc/gm ratio, suggesting a well-constrained gsc/gm ratio of 0.79±0.07 (95%CI, n=198) across plant functional groups and drought-stress treatments. Due in part to the synchronous behavior of gsc and gm, gm was negatively correlated to iWUE. Incorporating the gsc/gm ratio in the iWUEmes model significantly improved the estimation of iWUE compared to the simple model.The inclusion of gm effects, even using a fixed gsc/gm ratio of 0.79 when gm is unknown, proved desirable to eliminate significant bias in estimating iWUE from Δ across various C3 vegetation types.

This study aims to elucidate the effect of magnetic graphene oxide (GO) alignment in a polymer matrix on the ionic conduction and methanol barrier properties of the resulting composites. Magnetic iron oxide (Fe3O4) nanoparticles were fabricated and anchored on GO nanosheets. The magnetic GO-Fe3O4 nanofillers were incorporated into a quaternized polyvinyl alcohol (QPVA) matrix to form a polymeric composite. By applying a magnetic field (MF) along the in-plane direction during the drying and film formation steps, the GO-Fe3O4 nanosheets were aligned in an orientation parallel to the direction of the MF and connected to each other in elongated domains, as confirmed using transmission electron microscopy, field emission scanning electron microscopy/energy dispersive X-ray spectroscopy mapping, and laser scanning microscopy. The MF-aligned QPVA/GO-Fe3O4 electrolyte possessed higher conductivity and lower permeability than the un-aligned QPVA/GO-Fe3O4 membrane and pristine QPVA film. The aligned QPVA/GO-Fe3O4 composite exhibited a peak power density of 172.4 mW cm−2 at 60 °C, which is higher than those of the composite without the MF treatment (67.7 mW cm−2) and the pristine QPVA (50.3 mW cm−2). This research demonstrates the extraordinary benefits of MF assisted GO-Fe3O4 nanofillers in facilitating the ion conduction and barrier property of a polymeric composite at 0.1% filler loading.

The magnetite nanoparticles on graphene oxide (Fe3O4@GO) nanofiller are synthesized, and incorporated in a polymeric matrix to produce effective electrolyte membrane for direct alkaline methanol/ethanol fuel cells. Fe3O4@GO nanofillers (0.1%) are gradiently distributed along the through-plane direction in a quaternized poly(vinyl alcohol) (QPVA) membrane by applying an external magnetic field (MF) during the film drying step. The present study is the first work to illustrate a facile method to form gradient distribution of nanofillers in a QPVA matrix via applying MF, resulting in a polymer-rich region in the center and a nanofiller-rich region near the membrane surface. This MF assisted QPVA/0.1%Fe3O4@GO nanocomposite membrane exhibits decreased dimensional changes without sacrificing the potassium hydroxide (KOH) doping level compared to QPVA. This nanocomposite membrane design not only reduces the permeability but also improves the ionic conductivity. Maximum power densities of 200 and 144.3 mW cm−2 are achieved when the fuel cell is fed with 2 M methanol or 3 M ethanol at 60 °C using the magnetic-field assisted nanocomposite electrolyte. The fabrication of high-performance nanocomposite membrane electrolytes using a MF implies promising potential and an innovative future trend for fuel cells.

Electrospun quaternized polyvinyl alcohol (Q-PVA) nanofibers are prepared, and a potassium hydroxide (KOH)-doped nanofiber mat demonstrates enhanced ionic conductivity compared with a dense Q-PVA film with KOH doping. The Q-PVA composite containing 5.98% electrospun Q-PVA nanofibers exhibits suppressed methanol permeability. Both the high conductivity and suppressed methanol permeability are attributed to the quasi-coaxial structure of the electrospun nanofibers. The core of the fibers exhibits a more amorphous region that forms highly conductive paths, while the outer shell of the nanofibers contains more polymer crystals that serve as a hard sheath surrounding the soft core. This shell induces mass transfer resistance and creates a tortuous fuel pathway that suppresses methanol permeation. Such a Q-PVA composite is an effective solid electrolyte that makes the use of alkaline fuel cells viable. In a direct methanol alkaline fuel cell operated at 60 °C, a peak power density of 54 mW cm−2 is obtained using the electrospun Q-PVA composite, a 36.4% increase compared with a cell employing a pristine Q-PVA film. These results demonstrate that highly conductive coaxial electrospun nanofibers can be prepared through a single-opening spinneret and provide a possible approach for high-performance electrolyte fabrication.

A nanocomposite polymer membrane based on quaternized poly(vinyl alcohol)/fumed silica (QPVA/FS) was prepared via a quaternization process and solution casting method. The physico-chemical properties of the QPVA/FS membrane were investigated. Its high ionic conductivity was found to depend greatly on the concentration of fumed silica in the QPVA matrix. A maximum conductivity of 3.50 × 10−2 S/cm was obtained for QPVA/5%FS at 60 °C when it was doped with 6 M KOH. The permeabilities of methanol and ethanol were reduced with increasing fumed silica content. Cell voltage and peak power density were analyzed as functions of fumed silica concentration, temperature, methanol and ethanol concentrations. A maximum power density of 96.8 mW/cm2 was achieved with QPVA/5%FS electrolyte using 2 M methanol + 6 M KOH as fuel at 80 °C. A peak power density of 79 mW/cm2 was obtained using the QPVA/5%FS electrolyte with 3 M ethanol + 5 M KOH as fuel. The resulting peak power densities are higher than the majority of published reports. The results confirm that QPVA/FS exhibits promise as a future polymeric electrolyte for use in direct alkaline alcoholic fuel cells.

Perovskite LaFeO3 nanofillers (0.1%) are incorporated into a quaternized poly(vinyl alcohol) (QPVA) matrix for use as hydroxide-conducting membranes in direct alkaline methanol fuel cells (DAMFCs). The as-synthesized LaFeO3 nanofillers are amorphous and functionalized with cetyltrimethylammonium bromide (CTAB) surfactant. The annealed LaFeO3 nanofillers are crystalline without CTAB. The QPVA/CTAB-coated LaFeO3 composite membrane shows a defect-free structure while the QPVA/annealed LaFeO3 film has voids at the interfaces between the soft polymer and rigid nanofillers. The QPVA/CTAB-coated LaFeO3 composite has lower methanol permeability and higher ionic conductivity than the pure QPVA and QPVA/annealed LaFeO3 films. We suggest that the CTAB-coated LaFeO3 provides three functions to the polymeric composite: increasing polymer free volume, ammonium group contributor, and plasticizer to enhance the interfacial compatibility. The composite containing CTAB-coated LaFeO3 results in superior cell performance. A maximum power density of 272 mW cm−2 is achieved, which is among the highest power outputs reported for DAMFCs in the literature.

This work aims to clarify the effect of magnetic graphene oxide (GO) reorientation in a polymer matrix on the ionic conduction and methanol barrier properties of nanocomposite membrane electrolytes. Magnetic iron oxide (Fe3O4) nanoparticles were prepared and dispersed on GO nanosheets (GO-Fe3O4). The magnetic GO-Fe3O4 was imbedded into a quaternized polyvinyl alcohol (QPVA) matrix and crosslinked (CL-) with glutaraldehyde (GA) to obtain a polymeric nanocomposite. A magnetic field was applied in the through-plane direction during the drying and film formation steps. The CL-QPVA/GO-Fe3O4 nanocomposite membranes were doped with an alkali to obtain hydroxide-conducting electrolytes for direct methanol alkaline fuel cell (DMAFC) applications. The magnetic field-reoriented CL-QPVA/GO-Fe3O4 electrolyte demonstrated higher conductivity and lower methanol permeability than the unoriented CL-QPVA/GO-Fe3O4 membrane or the CL-QPVA film. The reoriented CL-QPVA/GO-Fe3O4 nanocomposite was used as the electrolyte in a DMAFC and resulted in a maximum power density of 55.4 mW·cm−2 at 60 °C, which is 73.7% higher than that of the composite without the magnetic field treatment (31.9 mW·cm−2). In contrast, the DMAFC using the CL-QPVA electrolyte generated only 22.4 mW·cm−2. This research proved the surprising benefits of magnetic-field-assisted orientation of GO-Fe3O4 in facilitating the ion conduction of a polymeric electrolyte.