[Seasonal changes of ammonia-oxidizing bacterial communities during tropical forest restoration]. / 热带森林恢复过程中氨氧化细菌群落的季节变化.

In this study, we used a high-throughput sequencing technology to survey the dry-wet seasonal change characteristics of soil ammonia-oxidizing bacteria (AOB) communities in the three restoration stages [i.e., Mallotus paniculatus community (early stage), Millettia leptobotrya community (middle stage), and Syzygium oblatum community (later stage)] of Xishuangbanna tropical forest ecosystems. We analyzed the effects of soil physicochemical characteristics on AOB community composition and diversity during tropical forest restoration. The results showed that tropical forest restoration significantly affected the relative abundance of dominant AOB phyla and their dry-wet seasonal variation. The maximum relative abundance of Proteobacteria (71.3%) was found in the early recovery stage, while that of Actinobacteria was found in the late recovery stage (1.0%). The abundances of Proteobacteria and Actinobacteria had the maximum ranges of dry-wet seasonal variation in the early and late stages, respectively. The abundance of dominant AOB genera and its dry-wet seasonal variation varied across tropical forest restoration stages. The maximum average relative abundance of Nitrosospira and Nitrosomonas in the late recovery stage was 66.2% and 1.5%, respectively. In contrast, the abundance of Nitrosovibrio reached its maximum (25.6%) in the early recovery stage. The maximum dry-wet seasonal variation in relative abundance of Nitrosospira and Nitrosomonas occurred in the early recovery stage, while that of Nitrosovibrio occurred in the middle recovery stage. The Chao1, Shannon, and Simpson diversity indices of AOB communities increased along the restoration stages, which were significantly higher in the wet season than in the dry season. The results of canonical correspondence analysis showed that soil easily oxidized carbon was the main factor controlling AOB community diversity and Actinobacteria abundance. Soil bulk density and temperature were the main factors affecting Proteobacteria abundance. Soil pH, microbial biomass carbon, water content, ammonium nitrogen, bulk density, and temperature were the main factors controlling the abundances of Nitrosospira, Nitrosomonas, and Nitrosovibrio. Therefore, tropical forest restoration can regulate the change of relative abundance of dominant AOB taxa via mediating the changes of soil temperature, bulk density, and readily oxidized carbon, leading to an increase in soil AOB community diversity.

Mixing of pine and arbuscular mycorrhizal tree species changed soil organic carbon storage by affecting soil microbial characteristics.

Pure and mixed pine forests are found all over the world. The mycorrhizal type affects soil microbial activity and carbon sequestration capacity in pure forests. However, the effects of mycorrhizal type on microbial characteristics and carbon sequestration capacity in pine mixed forests remain untested. Further, making it difficult to predict carbon storage of the conversion from pure pine forests to mixed forests at larger scales. Herein, a meta-analysis showed that the contents of soil microbial biomass, mineral-associated organic carbon, and soil organic carbon in pine mixed forests with introduced arbuscular mycorrhizal tree species (PMAM) increased by 26.41 %, 58.55 %, and 27.41 %, respectively, compared to pure pine forests, whereas those of pine mixed forests without arbuscular mycorrhizal tree species (PMEcM) remained unchanged. Furthermore, the effect size of microbial biomass, mineral-associated organic carbon and organic carbon contents in subsoil of PMAM are 56.48 %, 78.49 % and 43.05 %, respectively, which are higher than those in topsoil. The improvement of carbon sinks throughout the PMAM soil profile is positively correlated with increases in microbial biomass and mineral-associated organic carbon in subsoil, according to regression analysis and structural equation modelling. In summary, these results highlight that the positive effects of introducing arbuscular mycorrhizal tree species rather than ectomycorrhizal tree species into pure pine forests on soil microbial biomass and carbon sequestration. The positive link between microbial biomass, mineral-associated organic carbon, and soil organic carbon suggests an underlying mechanism for how soil microorganisms store carbon in pine mixed forests. Nevertheless, our findings also imply that the soil carbon pool of PMAM may be vulnerable under climate change. Based on the above findings, we propose that incorporating mycorrhizal type of tree species and soil thickness into mixed forests management and biodiversity conservation.

[Effects of ant colonization on spatiotemporal variation of organic carbon mineralization in Xishuangbanna tropical forest soils]. / 蚂蚁筑巢对西双版纳热带森林土壤碳矿化动态的影响.

Ants as ecosystem engineers can increase the input of soil organic matter, change soil physicochemical properties, and stimulate microbial activities through their colonization, thus affecting the spatiotemporal dynamics of soil organic carbon mineralization. We explored the spatiotemporal characteristics of carbon mineralization rates in ant nests and the adjacent soils in Syzygium oblatum community of Xishuangbanna, Yunnan. We analyzed the association of the variation in carbon mineralization rates with soil physicochemical properties. We found that ant colonization had a significant effect on soil organic carbon mineralization. The mean carbon mineralization rate was 19.2% higher in nest soils than that in the surrounding soils. The monthly carbon mineralization rate in nest soils and the reference soils was ranked as June ＞ September ＞ March ＞ December. The highest increase of carbon mineralization rate in ant nests was observed in 10-15 cm soil layer, while that in the reference soils was in 0-5 cm soil depth. Ant colonization had a significant effect on soil physicochemical properties. Compared with reference soils, soil temperature, soil water, soil organic carbon,soil microbial carbon, total nitrogen, hydrolytic nitrogen, nitrate, and ammonium increased by 7.6%, 5.4%, 9.9%, 14.8%, 13.4%, 9.9%, 24.1%, 6.6% and 19.4%, respectively. In contrast, soil bulk density and soil pH were decreased by 1.4% and 2.5%, respectively. Results from correlation coefficients and principal component analysis (PCA) showed that soil organic carbon and soil microbial carbon were the key factors controlling the mineralization of soil organic carbon, followed by total nitrogen, hydrolyzed nitrogen, ammonium, nitrate, temperature, and soil moisture. We conclude that ant colonization mainly alter the substrate components (i.e., soil organic carbon and microbial biomass carbon) of soil organic carbon mineralization and thus affect its spatio-temporal dynamics in Xishuangbanna tropical forests.