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• Bacterial assembly was dominated by drift (stochastic processes). • Fungal assembly was more governed by heterogeneous selection (deterministic processes). • Understory removal rapidly altered fungal community structure. • Understory removal had minor effects on the community assembly of bacteria and fungi. There is little known about how understory vegetation affects the soil microbial community assembly, although it exerts strong controls on the soil microbial community composition. We attempted to clarify this question by establishing an experiment in which the understory vegetation was either removed or maintained as controls in a primary tropical forest in south China. The results showed that removing the understory markedly altered the structure of fungal rather than bacterial communities. The bacterial assembly in control sites was dominated by drift (stochasticity), whereas the fungal assembly was governed more by heterogeneous selection (determinism). Removal of the understory had minor effects on the community assembly of bacteria and fungi. These results indicate that the absence of understory vegetation in tropical forests can quickly alter the fungal community structure without influencing the community assembly. [ABSTRACT FROM AUTHOR]

• Bacterial assembly was dominated by drift (stochastic processes). • Fungal assembly was more governed by heterogeneous selection (deterministic processes). • Understory removal rapidly altered fungal community structure. • Understory removal had minor effects on the community assembly of bacteria and fungi. There is little known about how understory vegetation affects the soil microbial community assembly, although it exerts strong controls on the soil microbial community composition. We attempted to clarify this question by establishing an experiment in which the understory vegetation was either removed or maintained as controls in a primary tropical forest in south China. The results showed that removing the understory markedly altered the structure of fungal rather than bacterial communities. The bacterial assembly in control sites was dominated by drift (stochasticity), whereas the fungal assembly was governed more by heterogeneous selection (determinism). Removal of the understory had minor effects on the community assembly of bacteria and fungi. These results indicate that the absence of understory vegetation in tropical forests can quickly alter the fungal community structure without influencing the community assembly. [ABSTRACT FROM AUTHOR]

• Microbial biomass increased significantly with subtropical forest succession. • The microbial community structure remained stable with subtropical forest succession. • Microbial community structure, but not biomass, was sensitive to N addition. • Forest succession increased the sensitivity of cyc/pre to N addition. Global nitrogen (N) deposition has been having broad impacts on soil microorganisms. It remains unclear how soil microbial biomass and community structure respond to elevated N deposition during forest succession, especially in tropical forest ecosystems. In this study, we conducted a field manipulative experiment of long-term N addition (14 years) at three levels (CK, LN, MN, which stand for 0, 50 kg N ha−1 yr−1 and 100 kg N ha−1 yr−1) along a 3-step succession sequence of tropical forests (early stage: a pine conifer forest, CF; middle stage: a mixed pine conifer and broad-leaved forest, MF; and late stage: a monsoon evergreen broad-leaved forest, BF), and investigated the responses of microbial composition, biomass and community structure using fumigation-extraction and phospholipid fatty acid (PLFA) methods. We found that: 1) Microbial biomass (microbial biomass carbon and nitrogen and PLFA contents) remained stable under long-term N addition, even though N addition significantly increased soil N nutrients (P < 0.05); it increased significantly with forest succession (P < 0.05), and was dominated by soil water content, which was not affected by N treatment; 2) microbial community structure was significantly changed by N addition, but not by forest succession. N addition, especially MN, significantly increased the ratio of cyclopropyl to its precursor (cyc/pre) in the MF and BF (P < 0.05), and cyc/pre of the BF showed the highest correlations with ammonium-N, nitrate-N and electrical conductivity (>0.85, P < 0.05). Consistently, the pure N treatment effect could explain 71.13% of the variance of cyc/pre; and 3) N treatment contributed more variance to the fungal community than to the bacterial community. Our results indicated that soil microbial community structure was more sensitive to N addition than community biomass under long-term N addition, and the structure would be more severely altered under N addition during forest succession or with the increase in N input. [ABSTRACT FROM AUTHOR]

The impact of human activities on soil carbon (C) storage in tropical forests has aroused wide concern during the past decades, because these ecosystems play a key role in ameliorating global climate change. However, there remain uncertainties about how land-use history alters soil organic carbon (SOC) stability and storage in different forests. In this study, we measured the C content and mass distributions of soil aggregates, density fractions, mineral-bound C and microbial biomass C in the organic horizon, 0-10 cm and 10-20 cm soil layers in coniferous forest and evergreen broadleaf forest at Dinghushan Biosphere Reserve in tropical China. The broadleaf forest had larger SOC stocks than the coniferous forest, but the proportion of SOC stored in different density fractions at 0-10 cm soils was similar between forest types, while a greater proportion of SOC was stored in microaggregates in the coniferous forest. Most of the SOC was held as light fraction C in the organic horizon in the coniferous forest, whereas the concentrations of mineral-bound C were higher in the broadleaf forest. These findings indicate clear differences in the protection of SOC between broadleaf and coniferous forests growing on the same soil type. We propose that historic conversion of broadleaf forest to coniferous forest has reduced soil C sequestration capacity by altering the diversity and quality of plant inputs to the soil, which in turn affected macroaggregate formation, soil chemical properties and microbial biomass. Our results thus demonstrate that changes in forest tree species composition could have long-lasting effects on soil structure and carbon storage, providing crucial evidence for policy decisions on forest carbon sink management. • Historic conversion of broadleaf forest to coniferous forest altered distribution of soil aggregates and density fractions. • Broadleaf forest had a greater proportion of mineral-bound C and macroaggregate-C than that in coniferous forest. • Translating tropical broadleaf forest to coniferous forest reduced SOC stocks and C sequestration capacity. [ABSTRACT FROM AUTHOR]