Viburnum japonicum (Thunb.) Sprengel is an endangered species endemic to coastal regions of eastern Asia (China, Japan, and South Korea). However, its systematic position has been controversial. In this study, we present the complete chloroplast (cp) genome of V. japonicum (GenBank OP644292) sequenced using the Illumina NovaSeq platform. The cp genome has a total length of 158,606 bp and a G+C contents of 38.08%. It consists of a large single-copy (LSC) region, a small single-copy (SSC) region, and a pair of inverted repeats (IRs) regions measuring 87,060 bp, 18,510 bp, and 26,516 bp, respectively. A total of 131 genes were identified, including 87 protein-coding genes, 36 transfer RNA genes (tRNAs), and 8 ribosomal RNA genes(rRNAs). Additionally, a total of 44 dispersed repeats were detected, including three types: forward, palindromic, and reverse. Among the 38 SSR loci that were discovered, the majority were mononucleotide loci composed of A/T. Furthermore, we found that 15 genes (accD, atpF, ndhA, ndhB, petB, petD, rpl16, rpl2, rpoC1, rps12, rps16, trnA-UGC, trnI, trnK-UUU, and trnL-UAA) contain one intron, while clpP and ycf3 have two introns. The relative synonymous codon usage (RSCU) analysis detected 31 high-frequency codons, where A/U bases accounted for 93.55% of the total, indicating an asymmetry in chloroplast gene and a presence for A/U bases. Comparative analysis of genome structure and sequences data of V. japonicum chloroplast genomes in comparison with other closely related species demonstrated a high level of conservation in their structure and organization. Furthermore, three mutation hotspots (psbH, rps19, and trnL) were identified, which could be valuable for future phylogenetic and population genetic research. Phylogenetic analysis revealed that the two accessions of V. japonicum are closely related to a group of V. setigerum, V. erosum, and V. fordiae within the Viburnum genus. In conclusion, this study provides important insights for accurately identifying and understanding the phylogeny of Viburnum species through the complete cp genome sequencing of V. japonicum.

A huge amount of CO₂ emissions from the drained drawdown areas of reservoirs overturns the previous results in carbon budget. Three Gorges Reservoir (TGR) has a large drawdown area, which accounts for nearly one third of the total area of the TGR. However, the total CO₂ emissions from the TGR drawdown area have seldom been estimated by far. To demonstrate the contribution of CO₂ emissions from the TGR drawdown areas, the study measured CO₂ emissions from the downstream part of the TGR drawdown areas by the chamber method, and synthesized CO₂ emissions from the other parts of TGR drawdown areas. Both the diel and seasonal variation indicated that CO₂ emission fluxes were significantly higher in the drained season than in the flooded season. The average CO₂ emission flux in the drained season was about 1.7 times higher than that in the inundated season in our experiments, and the ratio increased to 4.8 times when other available data was added. CO₂ emission flux in the drained season was positively correlated with air temperature, soil temperature at 5 cm depth, soil water content, organic carbon, and soil nitrate nitrogen, but negatively correlated with elevations. CO₂ emission from the TGR drawdown area was estimated to be 342.67–495.96 Gg yr⁻¹ in the drained season, and offset about 80% of carbon fixation by vegetation in the TGR drawdown area. Therefore, CO₂ emission from the drained soils should be included in the carbon budget of reservoir systems, especially for these reservoirs with a large drawdown area.

Viburnum japonicum is a rare plant species and endemic to the coastal region of Eastern Asia with extremely small populations. Within mainland China, this species can be only found in narrow habitats of the northeast coastal islands of Zhejiang Province. However, there are scarce conservation genetic studies on V. japonicum, which has limited the effective conservation and management of this rare species. Here, 51 individuals in four natural populations covering the Chinese geographic range of the species were sampled to assess the genetic diversity and population structure. A total of 445,060 high-quality single nucleotide polymorphisms (SNPs) were identified using double digest restriction-site associated sequencing (ddRAD-seq). The overall average values of observed heterozygosity (Ho), expected heterozygosity (He), and average nucleotide diversity (π), were 0.2207, 0.2595, and 0.2741, respectively. The DFS-2 population exhibited the highest level of genetic diversity among all the populations. Genetic differentiation between populations was moderate (F_ST = 0.1425), and there was selfing between populations (F_IS = 0.1390, S = 24.52%). Of the total genetic variation, 52.9% was found among populations through AMOVA analysis. The Mantel test (r = 0.982, p = 0.030) combined with analyses of the Maximum Likelihood (ML) phylogenetic tree, ADMIXTURE, and principal component analysis (PCA), revealed that populations of V. japonicum were genetically segregated and significantly correlated with their geographical distribution. Our study demonstrated that V. japonicum maintained a medium level of genetic diversity and differentiation with a strong population structure, and the results were mainly affected by its island distribution pattern and self-crossing characteristics. These results provide insights into the genetic diversity and population history of V. japonicum, critical information for conserving and sustainably developing its genetic resources.

Carbonaceous greenhouse gases (i.e., CO2, CH4) have often reported to be emitted from the surfaces of reservoirs, while other non-carbonaceous greenhouse gases are not available to many reservoirs. In this study, the spatiotemporal variability of N2O emissions from Xin’anjiang Reservoir was investigated at the different regions over a 13-month period using the method of the static floating chambers combined with gas chromatography. Results showed that the average N2O emission flux was the highest in the downstream river (220.24±153.68 μg m-2·s-1), followed by the upstream river (49.85±48.55 μg m-2·s-1). The reservoir’s main body had the lowest flux (26.61-39.07 μg m-2·s-1). Seasonal variability indicated that the average N2O emission fluxes before the dam reached their maximum in March, which was likely influenced by the disappearance of thermal stratification then, caused a short N2O flux peak at the waterair interface. A thorough investigation should be carried out at the different reservoir regions for a longterm basis to discover the spatiotemporal variability in N2O emission flux in a hydroelectric reservoir system, which could avoid the error of estimating the N2O emissions incorrectly.