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To better understand soil nitrogen (N) sequestration by Acacia mangium Willd. in the plantations of the Southeastern region of Vietnam, a study was conducted to examine soil N content and storage in three different-aged A. mangium stands (4, 7 and 11 years old). Soil samples were collected at different depths from 0-50 cm. Field measurements were taken based on established national standard methods. We used the modified Kjeldahl method to determine soil total N concentration. Soil total N concentration at the various soil depths for each plantation decreased significantly with increasing depth, but increased significantly with plantation age. Soil total N stocks at the topsoil (0-50 cm) increased from 6.13 Mg. N. ha-1 in 4-year-old stands to 9.71 Mg. N. ha-1 in 11-year-old stands. Soil total N storage showed obvious topsoil aggregation with more than 60% of soil total N storage in the 0-30 cm depth for each stand. Hence, protection of total N stocks present in the topsoil of planted forests is very critical in the context of N sequestration. Furthermore, stand characteristic parameters (i.e., stand age, plant biomass, stand density, tree height and diameter at breast height, and canopy closure) significantly affected soil total N storage. The findings from this study indicate that taking stand age into consideration is greatly beneficial for forest soil N storage assessment and highlights the potential of A. mangium for N sequestration in plantation ecosystems. [ABSTRACT FROM AUTHOR]

The present study was conducted to better understand the ecology, spatial patterns and associations of Hopea pierrei – a vulnerable plant in the Vietnam Red Book and on the IUCN’s Red List – in the medium natural evergreen broadleaved forest of Phu Quoc National Park, Kien Giang Province, Vietnam. Data was collected from all woody tree individuals with a diameter at breast height (dbh) ≥ 2.5 cm in three 1-ha plots (100 m × 100 m). The spatial point-pattern analysis method was used to analyze the data with Programita Noviembre 2018 and R 4.1.1 software. Our findings suggested that H. pierrei was aggregated and randomly distributed at various scales in the area where it appeared. The spatial patterns of H. pierrei were influenced by environmental heterogeneity. The intraspecific associations of H. pierrei were mainly attraction associations, while its interspecific associations with sixteen dominant species were mainly independent. Processes such as seed dispersal and self-thinning are the main mechanisms underlying the spatial patterns and associations of H. pierrei. Based on the present findings, it is possible to regulate density and choose an appropriate planting-hole distance when restoring forests or planting new forests with H. pierrei and dominant species with which it grows. [ABSTRACT FROM AUTHOR]

To understand the ecology and spatial distribution patterns of Shorea roxburghii, a vulnerable plant on the IUCN’s Red List, this study was conducted in a Vietnamese evergreen forest. To collect data, a 100 m × 100 m plot was established in the Tan Phu protected forest, in South Vietnam. Relative density, relative basal area, and importance value index (IVI) were determined for each species. Individuals were categorized by juvenile (dbh < 5 cm), subadult (5 cm ≤ dbh < 10 cm) and adult (dbh ≥ 10 cm) life-history stages. We used Excel, Programita Noviembre 2018 and R to analyze data. Spatial analyses focused on overall univariate patterns, dispersion patterns, and hetero-specific association of the species. A total of 111 species was found. Shorea roxburghii has higher basal area, relative density, maximum dbh, IVI and lower median main neighbour. Shorea roxburghii has cluster distribution in the juvenile (5f) and subadult stage (5g), regular or clustered pattern in the adult stage (5h). However, the general trend is a regular pattern as the dbh increases. With the achieved results, this study contributes to the simulation method of forest species ecological characteristics, including distribution characteristics, intra and interspecific relationships in the space of Shorea roxburghii [ABSTRACT FROM AUTHOR]

In this work, MgAl2O4:Cr3+ phosphor has been synthesized by the co-precipitation approach coupled with heat treatment. The phase structure, morphology, composition and luminescent properties of the phosphor were investigated in detail by x-ray diffraction, field emission-scanning electron microscopy, energy dispersive x-ray spectroscopy and steady-state photoluminescence (PL) measurements. The MgAl2O4:Cr3+ phosphor emits a deep red light that peaked at 687 nm, which is attributed to the 2Eg–4A2g state transition of Cr3+ ions located at the sites with D3d local symmetry of host lattice. The phosphor presents a strong absorption with two bands in ultraviolet to visible and yellow regions. The changes of PL and photoluminescence excitation spectra of MgAl2O4:Cr3+ phosphors as a function of sintering temperature and time intervals were observed. The phosphors emit the highest PL intensity when doped with a Cr3+ concentration of about 1.2 mol.%. The International Commission on Illumination chromaticity coordinates and internal quantum efficiency of MgAl2O4:1.2%Cr3+ were measured and calculated. A warm white light emitting diode using the deep red phosphor, has shown a high colour rendering index (CRI, Ra = 78) and a low correlated colour temperature (CCT = 3481 K). [ABSTRACT FROM AUTHOR]

In this work, SrAl2O4:Mn4+ phosphor is prepared by co-precipitation. The phase structure, morphology, composition and luminescent performance of the phosphor are investigated in detail with x-ray diffraction, field emission scanning electron microscopy, steady-state photoluminescence (PL) spectra, and temperature-dependent PL measurements. The phosphor shows a strong red emission peak at ∼ 690 nm, which is due to the transition between electronic levels and the electric dipole transition 2Eg to 4A2g of Mn4+ ions located at the sites with D3d local symmetry. The sample doped with 0.04 mol.% Mn4+ exhibits intense red emission with high thermal stability and appropriate International Commission on Illumination (CIE) coordinates (x = 0.6959, y = 0.2737). It is also found that the phosphor absorption in an extended band from 250 nm to 500 nm has three peaks at 320 nm, 405 nm, and 470 nm, which match well with the emission band of ultraviolet (UV) lighting emission diode (LED) or blue LED chips. These results demonstrate that SrAl2O4:Mn4+ phosphor can play the role of activator in narrow red-emitting phosphor, which is potentially useful in UV (∼ 320 nm) or blue (∼ 460 nm) LED. [ABSTRACT FROM AUTHOR]

The excitonic properties of high-quality ZnO/Zn[sub 0.88]Mg[sub 0.12]O multiquantum wells grown by laser-molecular-beam epitaxy were investigated using temperature-dependent optical absorption spectra from 5 K to room temperature. The strength of exciton-longitudinal-optical (LO) -phonon coupling was deduced from the temperature dependence of the linewidth of the fundamental excitonic peak. Effective reduction of the exciton-LO-phonon coupling with decreasing the well width was observed, which is consistent with the confinement-induced enhancement of the exciton binding energy. The thermal shift of the lowest excitonic energy is independent of well width, indicating that the strain effect is negligible for this material. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

To assess the in vivo sensitivity of Plasmodium falciparum to mefloquine and artesunate in a hyperendemic area of southern Viet Nam, we studied 41 children and 21 adults from a remote commune who had uncomplicated falciparum malaria without previous treatment. Patients were randomly allocated to artesunate (4 mg/kg on day 0 and 2 mg/kg on days 1-4) or mefloquine (10 mg/kg followed by 5 mg/kg at 6 h). Serial assessments were performed over 28 days. Of 31 patients allocated artesunate, nine (29%) redeveloped parasitaemia during follow-up compared with 23% (seven of 30) who received mefloquine. Of the 41 children, 15 (37%) had recrudescence/re-infection compared with only one of 20 adults (5%; p < 0.001). Significantly more children than adults failed on mefloquine treatment (37% vs 0%; p = 0.021) and one case showed RIII resistance. There was no significant difference in the case of artesunate. In regression analysis, parasitaemia was an independent predictor of recrudescence/re-infection after mefloquine (p = 0.02). These data support the use of combination therapy such as artesunate plus mefloquine for falciparum malaria in a hyperendemic area of Viet Nam. Primarily because of their greater parasite densities, children should be given higher doses of mefloquine (e.g. 25 mg/kg).