Arbuscular mycorrhizal fungi play an important role in mediating plant-soil interactions across succession stages. However, AMF community dynamics which about the change of community composition and member activity remain unclear. To complete the gap knowledge about microbial community dynamics during restoration succession, soil AMF community composition was studied within a tropical forest ecosystem in the Ganshiling nature reserve using high throughput sequencing methods. The results revealed that soil AMF communities characteristics about speices diversity, species composition and microbial correlation network showed significant differences between shrubland (SC) and secondary forest ecosystems, but the same differences were not found between 40-year recovery secondary forest (SF40) and 60-year recovery secondary forest (SF60). Plant community dynamics were the key factor for regulating soil AMF communities among succession stages. An important biotic factor explaining variance in AMF community composition was root biomass. The correlation network analysis showed that although the nodes were similar among succession stages, the complexity of networks was significant higher in SF40 than in SC and SF60, suggesting that AMF communities were more active in SF40, which verified the hypothesis of intermediate disturbance hypothesis. This study provides new insights into AMF community dynamics and their driving factors across succession stages, as well as expanding knowledge of the ecological value of AMF for tropical forest restoration processes.

IntroductionSoil organic carbon (SOC) accumulation changed with forest succession and hence impacted the SOC storage. However, the variation and underlying mechanisms about SOC during tropical forest succession are not fully understood.MethodsSoil samples at four depths (0–10 cm, 10–20 cm, 20–40 cm and 40–60 cm), litter, and roots of 0–10 cm and 10–20 cm were collected from three forest succession stages (plantation forest, secondary forest, and old– growth forest) in the Jianfengling (JFL) National Nature Reserve in Hainan Island, China. The SOC, soil enzyme activities, physiochemical properties, the biomass of litter and roots were analyzed.ResultsResults showed that forest succession significantly increased SOC at 0–10 cm and 10–20 cm depth (from 23.00 g/kg to 33.70 g/kg and from 14.46 g/kg to 22.55 g/kg, respectively) but not at a deeper depth (20–60 cm). SOC content of the three forest succession stages decreased with increasing soil depth and bulk density (BD). With forest succession from plantation to secondary and old–growth forest, the soil pH at 0–10 cm and 10–20 cm depth decreased from 5.08 to 4.10 and from 5.52 to 4.64, respectively. Structural equation model (SEM) results showed that the SOC at depths of 0–20 cm increased with total root biomass but decreased with increasing soil pH value. The direct positive effect of soil TP on SOC was greater than the indirect negative effect of decomposition of SOC by soil acid phosphatase (AP).DiscussionTo sum up, the study highlighted there was soil P– limited in tropical forests of JFL, and the increase in TP and total root biomass inputs were main factors favoring SOC sequestration during the tropical forest succession. In addition, soil acidification is of great importance for SOC accumulation in tropical forests for forest succession in the future. Therefore, forest succession improved SOC accumulation, TP and roots contributed to soil C sequestration.

Soil carbon (C) distribution, which is deeply correlated with soil extracellular enzyme activity and its temperature sensitivity (Q10), are important for predicting the soil organic carbon (SOC) pools under climate warming. However, the high spatial heterogeneity in tropical forest soils makes the predicted results vary significantly. Thus, a total of 87 soil samples of three forest types, eight slope orientations, and four vegetation types were selected from Wuzhishan in Hainan Island, China. SOC distribution, the activities of six soil enzymes, and Q10 at 10, 20, 30, and 40 °C were investigated to supplement the tropical data. The results showed that: (1) SOC ranged from 24.82 to 87.72 g/kg. SOC in the primary forest was significantly higher than that of the secondary forest, and SOC of the cloud forests and hilltop scrub at higher elevations was significantly higher than that of the lowland rainforests and montane rainforests at lower elevations. However, the amount of recalcitrant carbon in the primary forest was the lowest. (2) Under lab temperature conditions, the acid phosphatase and β-1,4-glucosidase in the plantation forest were significantly higher than that of the secondary forest, and the polyphenol oxidase and catalase in the plantation forest were significantly higher than that of the primary forest. Enzyme stoichiometry analysis indicated that microbials were limited by nitrogen in the study area. The six soil enzyme activities were strongly correlated with SOC and total nitrogen. (3) The Q10 of soil enzymes ranged from 0.61 to 1.92 under three temperature gradients. Most hydrolases enzymes (Q10 > 1) showed a positive response with temperatures from 10 to 30 °C, and showed a negative response (Q10 < 1) with temperatures from 30 to 40 °C. We concluded that the negative response of enzyme Q10 with global warming would slow down the SOC decomposition. Primary tropical forests could still sequester SOC; however, their ability to do so may be vulnerable to climate change, as the amount of soil C quality index was low.