Characteristics of soil microbial biomass and community composition in <i>Pinus yunnanensis</i> var. <i>Tenuifolia</i> secondary forests

<p><i>Pinus yunnanensis</i> var. <i>Tenuifolia</i> is an important species of timber and grease in southern China, but the characteristics of the soil microbial community in <i>P. yunnanensis</i> var. natural secondary forests are still poorly understood. Using a fumigation-extraction method and phospholipid fatty acid (PLFA) analysis, we study microbial biomass and community composition in the topsoil (0–10 cm) of three types of secondary forests (PYI, PYII, PYIII) dominated by <i>P. yunnanensis</i> var. to varing degrees. Microbial biomass carbon and nitrogen, total PLFA, and PLFA contents of bacterial, fungal, and arbuscular mycorrhizal fungi were significantly lower in PYI than PYII or PYIII, and there were significant differences in the monounsaturated/saturated fatty acid ratio among the tested forests. Principal component analysis indicated that the soil microbial community structure of the tested forests differed significantly. The changes in soil microbial biomass and community composition were positively correlated with soil water content, pH, organic matter (SOM), total nitrogen (TN), and total phosphorus. Season did not significantly affect the soil microbial community structure, but significantly affected soil microbial biomass, SOM, and TN, which were higher in the dry season than in the wet season.</p

Effect of Low pH and Aluminum Toxicity on the Photosynthetic Characteristics of Different Fast-Growing <i>Eucalyptus</i> Vegetatively Propagated Clones

<div><p>Knowing how acid soils and aluminum in soils may limit the growth of <i>Eucalyptus</i> trees in plantations is important because these plantations grow in many tropical and subtropical regions. Seedlings of four vegetatively propagated <i>Eucalyptus</i> clones, <i>E</i>. <i>grandis × E</i>. <i>urophylla</i> ‘GLGU9’(G9), <i>E</i>. <i>grandis × E</i>. <i>urophylla</i> ‘GLGU12’ (G12), E. <i>urophylla × E</i>. <i>camaldulensis</i> ‘GLUC3’ (G3) and <i>E</i>. <i>urophylla</i> ‘GLU4’(G4), were subjected to liquid culture with Hoagland nutrient solution for 40 days, then treated with four different treatments of acid and aluminum for 1 day. The four treatments used either pH 3.0 or 4.0 with or without added aluminum (4.4 mM) in all possible combinations; a control used no added aluminum at pH 4.8. Subsequently, the photosynthetic parameters and morphology of leaves from eucalypt seedlings were determined and observed. The results showed that the tested chlorophyll content, net photosynthetic rate, transpiration rate and water use efficiency were apparently inhibited by aluminum. Under uniform Al concentration (4.4 mM), the Al-induced limitation to photosynthetic parameters increased with pH, indicating acid stimulation to Al toxicity. Among all treatments, the most significant reduction was found in the combination of pH 3.0 and 4.4 mM Al. The photosynthetic and transpiration rates showed similar trends with G9 > G12 > G3 > G4, suggesting that G9 and G12 had higher Al-tolerance than other two clones. Microscopic observation revealed changes in leaf morphology when exposed to Al stress; for example, a reduced thickness of leaf epidermis and palisade tissue, the descendant palisade tissue/spongy tissue ratio and leaf tissue looseness. Overall, the acid and aluminum stress exerted negative effects on the photosynthetic activity of eucalypt seedlings, but the differences in tolerance to Al toxicity between the clones were favorable, offering potential to improve <i>Eucalyptus</i> plantation productivity by selecting Al tolerant clones.</p></div

Changes in seedling chlorophyll content of four eucalypt clones under different acid aluminum treatments.

<p>G3 represents <i>E</i>. <i>urophylla × E</i>. <i>camaldulensis</i> ‘GLUC3’; G4, <i>E</i>. <i>urophylla</i> ‘GLU4’; G9, <i>E</i>. <i>grandis × E</i>. <i>urophylla</i> ‘GLGU9’; G12, <i>E</i>. <i>grandis × E</i>. <i>urophylla</i> ‘GLGU12’. Treatments: K_0-3, treatment with 0 mM Al³⁺, pH 3.0; K_4.4–3, 4.4 mM Al³⁺, pH 3.0; K_0-4, 0 mM Al³⁺, pH 4.0; K_4.4–4, 4.4 mM Al³⁺, pH 4.0; CK, control treatment, 0 mM Al³⁺, pH 4.8. Capital letters represent significant differences between treatments within an eucalypt clone at the 0.01 level, small letters are at the 0.05 level, n = 3.</p

Changes in transpiration rates of four eucalypt clones under different acid aluminum treatments.

<p>G3 represents <i>E</i>. <i>urophylla × E</i>. <i>camaldulensis</i> ‘GLUC3’; G4, <i>E</i>. <i>urophylla</i> ‘GLU4’; G9, <i>E</i>. <i>grandis × E</i>. <i>urophylla</i> ‘GLGU9’; G12, <i>E</i>. <i>grandis × E</i>. <i>urophylla</i> ‘GLGU12’.Treatments: K_0-3, treatment with 0 mM Al³⁺, pH 3.0; K_4.4–3, 4.4 mM Al³⁺, pH 3.0; K_0-4, 0 mM Al³⁺, pH 4.0; K_4.4–4, 4.4 mM Al³⁺, pH 4.0; CK, control treatment, 0 mM Al³⁺, pH 4.8. Capital letters represent significant differences between treatments within an eucalypt clone at the 0.01 level, small letters are at the 0.05 level, n = 3.</p