[Effects of weeding and fertilization on soil nitrogen mineralization and leaching in the mixed forest of Dalbergia odorifera and Santalum album]. / é“²è‰å’Œæ–½è‚¥å¯¹é™é¦™é»„æª€ä¸Žå°åº¦æª€é¦™æ··äº¤æž—åœŸå£¤æ°®ç´ çŸ¿åŒ–æ·‹æº¶çš„å½±å“.

The mixed plantation of precious tree species Dalbergia odorifera and Santalum album is an important model for the development of plantations in South China. Based on an experiment with four treatments, i.e., CK (without any treatment), weeding, fertilization, weeding+fertilization, we investigated the effects of weeding and fertilization on soil net mineralization rate, net nitrification rate, net ammonification rate, and nitrogen leaching rate. Among the four treatments, the maximum net nitrogen mineralization rate in the 0-10 cm soil layer was 18.92 and 18.13 mg·kg-1·month-1 in spring and autumn, respectively; the maximum nitrification rate was 20.35 and 18.85 mg·kg-1·month-1in spring and autumn, respectively; the maximum ammonium mineralization rate was 0.22 and 0.26 mg·kg-1·month-1 in summer and winter, respectively; the maximum nitrogen leaching was 15.98 mg·kg-1·month-1 in autumn and 86.69 mg·kg-1·year-1 in the whole year. The treatments of weeding, fertilization, weeding+fertilization reduced net mineralization rate and net nitrification rate. The annual nitrogen mineralization decreased by 26.2%, 16.1% and 6.3%, respectively, and the annual nitrogen nitrification decreased by 17.1%, 16.6%, and 1.4%, respectively. Moreover, the treatments of weeding, fertilization, weeding+fertilization inhibited soil ammonium accumulation and reduced annual nitrogen leaching by 25.2%, 8.6%, and 6.1%, respectively. Compared with weeding, fertilization and weeding+fertilization, seasonal factors had more significant effects on soil nitrogen mineralization and leaching. Weeding, fertilization and weeding+fertilization decreased soil nitrogen nitrification and ammonification to a certain extent, reduced soil nitrogen mineralization and leaching loss, which contribute to the preservation of soil fertility and nitrogen accumulation.

Genetic Diversity and Population Structure Analysis of DalbergiaOdorifera Germplasm and Development of a Core Collection Using Microsatellite Markers.

Dalbergia odorifera T. Chen (Fabaceae) is a woody tree species indigenous to Hainan Island in China. Due to its high medicinal and commercial value, this tree species has been planted over 3500 ha² in southern China. There is an urgent need for improvement of the D. odorifera germplasm, however, limited information on germplasm collection, conservation, and assessment of genetic resources is available. Therefore, we have built a database of 251 individuals collected across the whole of southern China, which included 42 wild trees and 210 cultivated trees, with the following objectives. (1) Evaluate genetic diversity and population structure of the database using 19 microsatellite markers and (2) develop a core collection for improvement and breeding programs. Totally, the 19 microsatellite markers harbored 77 alleles across the database with the polymorphic information content (PIC) ranging from 0.03 to 0.66. Medium genetic diversity level was inferred by Nei's gene diversity (0.38), Shannon's information index (0.65), and observed (0.33) and expected heterozygosity (0.38). Structure analysis showed that four was the optimum cluster size using the model-based Bayesian procedure, and the 251 D. odorifera individuals were grouped into five populations including four pure ones (RP1-4) and one mixed one (MIX) based on their maximum membership coefficients. Among these populations, the expected heterozygosity varied from 0.30 (RP3) to 0.38 (RP4). Analysis of molecular variance (AMOVA) showed 11% genetic variation existed among populations, and moderate population differentiation was inferred by the matrix of pairwise Fst (genetic differentiation among populations), which was in the range of 0.031 to 0.095. Moreover, a core collection of 31 D. odorifera individuals including six wild and 25 cultivated trees was developed, which was only 12.4% of the database but conserved the whole genetic diversity. The results of this study provided additional insight into the genetic structure of the large D. odorifera germplasm, and the core collection will be useful for the efficient and sustainable utilization of genetic resources, as well as efficient improvement in breeding programs.

Characterization of the chloroplast genome of Cassia siamea Lain, a rosewood species from southeast China.

Cassia siamea is a rosewood species in Southwest China with high wood and medicinal value. To clarify genetic background of C. siamea, we sequenced chloroplast genome by Illumina Hiseq and PacBio Sequel. The whole genome was 148,437 bp in length, containing a large single copy region (77,723 bp), a small single copy region (18,462 bp) and a pair of inverted repeats regions (26,126 bp). The cp genome contained 102 genes (71 protein-coding genes, 27 tRNAs and 4 rRNAs). The phylogenetic analysis indicated that C. siamea is close to Senna tora within Cassiinae/Caesalpiniaceae. The complete chloroplast genome of C. siamea will provide useful resources for the development and utilization of this species and the phylogenetic study of Fabaceae.

The environment, not space, dominantly structures the landscape patterns of the richness and composition of the tropical understory vegetation.

The mechanisms driving the spatial patterns of species richness and composition are essential to the understanding of biodiversity. Numerous studies separately identify the contributions of the environment (niche process) and space (neutral process) to the species richness or composition at different scales, but few studies have investigated the contributions of both types of processes in the two types of data at the landscape scale. In this study, we partitioned the spatial variations in all, exotic and native understory plant species richness and composition constrained by environmental variables and space in 134 plots that were spread across 10 counties in Hainan Island in southern China. The 134 plots included 70 rubber (Hevea brasiliensis) plantation plots, 50 eucalyptus (Eucalyptus urophylla) plantation plots, and 14 secondary forest plots. RDA based variation partitioning was run to assess the contribution of environment and space to species richness and composition. The results showed that the environmental variables alone explained a large proportion of the variations in both the species richness and composition of all, native, and exotic species. The RDA results indicated that overstory composition (forest type here) plays a leading role in determining species richness and composition patterns. The alpha and beta diversities of the secondary forest plots were markedly higher than that of the two plantations. In conclusion, niche differentiation processes are the principal mechanisms that shape the alpha and beta diversities of understory plant species in Hainan Island.

[Growth effect of eucalyptus-acacia mixed plantation in South China].

Eucalyptus U6 and Acacia crassicarpa were mixed planted with different ratios and modes to investigate the growth parameters of the two tree species. In the 2-3 years old mixed plantation, the wind-throw of A. crassicarpa decreased markedly with increasing ratio of Eucalyptus U6, the decrement being 26.14% when the Eucalyptus U6/A. crassicarpa ratio was 3 : 1, but the survival rates of Eucalyptus U6 and A. crassicarpa had no significant difference under different planting modes. Mixed planting retarded the A. crassicarpa growth to some extent, with the DBH being 90% of that in pure A. crassicarpa stand. The mixed planting had little effects on the height growth of Eucalyptus U6, but promoted its DBH growth markedly, and the beneficial effect increased with increasing ratio of A. crassicarpa. In the 6 years old 1 : 1 Eucalyptus U6/A. crassicarpa plantation, the Eucalyptus U6 individuals with DBH > 15 cm occupied 32.1%; while in pure Eucalyptus U6 stand, they only accounted for 5.83%. Mixed planting with 2 : 1 Eucalyptus U6/A. crassicarpa could obtain a maximum total biomass of 198.8 m3 x hm(-2), which was 118.8% of the total biomass in pure Eucalyptus U6 stand, or 169.9% of that in pure A. crassicarpa stand. Mixture of Eucalyptus with Acacia would be a good choice to produce Eucalyptus trees with larger DBH.