Short-rotation forestry is a common practice worldwide for efficient timber harvesting. However, determining the appropriate rotation period to minimize disturbance to plantation-soil ecosystems remains a controversial topic. The microbial community structure is a sensitive indicator of soil quality in plantation forests. Therefore, we analyzed the soil bacterial community composition, co-occurrence network, functional characteristics and influencing factors in subtropical Eucalyptus plantation forests of different ages and revealed the mechanisms through which soil ecosystem function is affected. Soil carbon (C), nitrogen ( N) and phosphorus (P) decreased and then increased with forest age, and there was temporal variability in the composition and function of the soil bacterial communities; the bacterial community composition and diversity and function of young and overmature forest stands contributed most. Changes in total nitrogen (TN), dissolved organic nitrogen (DON), total phosphorus (TP), and available phosphorus (AP) supported the bacterial community pattern, which was dominated by Acidobacteria and Proteobacteria. The distributions of Acidobacteria and Proteobacteria were stable during the growth phase of the Eucalyptus plantation. The low-abundance bacterial communities WPS-2, Actinobacteria, Firmicutes, and Bacteroidetes contributed to the main functions of the bacterial communities, among which WPS-2 and Actinobacteria dominated the soil C fixation and N cycle functions. This study evaluated the response of bacterial communities to soil environmental factors during plantation forest development and elucidated the microbial controls on soil multifunctionality, which is important for improving ecosystem simulation models in subtropical Eucalyptus plantation forests and planning sustainable management during rotational logging periods.

Research on the variations in soil aggregate stability and ecological stoichiometry at aggregate scales by stand type is of great significance to investigating the distribution, limitation, balance, and cycling of chemical elements. We measured the variations in soil aggregate stability, organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) contents and stocks, in addition to their stoichiometric ratios by stand type (mixed stands of Chinese fir and Mytilaria laosensis and Chinese fir and Michelia macclurei and pure stand of Chinese fir), and aggregate size (<0.25, 0.25-1, 1-2, and >2 mm) in soil profile (0-20 and 20-40 cm) in Guangxi, China. Soil OC and TN contents and C/N, C/P and N/P ratios were more distributed in aggregates of <0.25 mm in the three stands; in contrast, soil OC, TN, and TP stocks were more obvious in aggregates of >2 mm. In comparison to the pure stand, the mixed stands showed significantly higher levels of soil aggregate stability, aggregate-associated OC and TN contents with stocks, TP content, C/N and C/P ratios. The C/N ratio, TN and TP contents, and TP stocks were the key factors in the stand types affecting the stability of soil aggregate changes. Hence, constructing mixed stands and applying appropriate amounts of N and P fertilization can improve soil fertility and promote degraded soil restoration.

We measured bidirectional N transfer and quantified the amount of transferred N between Eucalyptus urophylla × grandis (Eucalyptus) and Dalbergia odorifera (Dalbergia), to determine whether the facilitation process from N transfer could improve Eucalyptus productivity and nodulation of Dalbergia. A 15N leaf-labeling study was conducted by using a pot experiment with Dalbergia and , and 15N was traced in the labeled species as well as in neighboring trees at 90, 135 and 180 days after labeling. Bidirectional N occurred between Eucalyptus and Dalbergia, the amount of net transferred N was 21.8–127.0 mg plant-1 N from Dalbergia to Eucalyptus, which was equal to 1.5–21.2 % of the total N accumulated in Eucalyptus plants to increase the biomass of Eucalyptus. The results also showed that the nodule number of Dalbergia increased by 19.4–107.4 % in the intercropping system and further improved its N2-fixation. The concentration of soil nitrogen, especially the concentration of N-NH4+, was a major factor affecting N transfer in this system. This study indicated that the advantage in Eucalyptus and legume intercrops relates to ‘complementary N use’, which provide a significant amount of N required for Eucalyptus productivity, and maintain ecological sustainability.