Nitrogen (N) is an essential macronutrient for plant function and growth and a key component of amino acids, which form the building blocks of plant proteins and enzymes. However, misuse and overuse of N can have many negative impacts on the ecosystem, such as reducing soil exchangeable base cations (BCs) and causing soil acidification. In this research, we evaluated clonal Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) seedlings grown with exponentially increasing N fertilization (0, 0.5, 1, 2 g N seedling⁻¹) for a 100-day trial in a greenhouse. The growth of seedlings, their nutrient contents, and soil exchangeable cations were measured. We found that N addition significantly increased plant growth and N content but decreased phosphorous (P) and potassium (K) contents in plant seedlings. The high nitrogen (2 g N seedling⁻¹) treated seedlings showed a negative effect on growth, indicating that excessive nitrogen application caused damage to the seedlings. Soil pH, soil exchangeable base cations (BCs), soil total exchangeable bases (TEB), soil cation exchange capacity (CEC), and soil base saturation (BS) significantly decreased following N application. Our results implied that exponential fertilization resulted in soil acidification and degradation of soil capacity for supplying nutrient cations to the soil solution for plant uptake. In addition, the analysis of plants and BCs revealed that Na⁺ is an important base cation for BCs and for plant growth in nitrogen-induced acidified soils. Our results provide scientific insights for nitrogen application in seedling cultivation in soils and for further studies on the relationship between BCs and plant growth to result in high-quality seedlings while minimizing fertilizer input and mitigating potential soil pollution.

Shading is one of the management practices for preventing the damage or injury of plant seedlings during extreme weather and climate events, such as very high temperatures and heat stress. In this study, we investigated the effects of different shading conditions on the photosynthetic characteristics of the endangered plant Horsfieldia hainanensis in Guangxi, China. The H. hainanensis seedlings in this study underwent five shading treatments, including 20% (L1), 40% (L2), 60% (L3), 80% (L4), and 100% (control) of full sunlight. The net growth of their diameter and height, and photosynthetic gas exchange parameters including their photosynthesis rate (Pn), transpiration rate (Tr), intercellular CO₂ concentration (Ci), and water use efficiency (WUE) were measured for the examined seedlings. The OJIP curve and 820 nm light absorption curve, and the osmotic substances and products of membrane lipid peroxidation were employed to assess photosynthetic capacity, identify the factors constraining photosynthetic carbon assimilation, and investigate the mechanisms influencing photosystem II (PSII) and photosystem I (PSI) in the seedlings under shade stress. The results showed that the seedlings in the L2 treatments had the highest net growth and Pn, the best photosynthetic performance, and the best coordination between PSII and PSI. The net photosynthesis (Pn) levels exhibited a declining trend in the following order: L2 > L3 > L4 > L1. In the L1 treatment, non-stomatal factors emerged as the primary determinant affecting the Pn of the seedlings. The performance index (potential) of PSII, representing the conservation of absorbed photon energy to intersystem electron acceptor reduction (PI_ABS and ΔI/I₀) of the seedlings, decreased in the order of L2 > L3 > L4 > L1. The photosystem performance and the coordination between PSII and PSI (Φ_(PSI/PSII)) of the seedlings decreased in the order of L2 > L1 > L3 > L4. Under the low and moderate shading stresses (L1–L3), more serious damages occurred in PSII than in PSI, including on the donor side of PSII and in the electron transfer from Q_B to the acceptor side of PSI. In contrast, more considerable injury occurred in PSI than in PSII under the stress of the heavy shading treatment (L4). Considering the alterations in their leaf osmotic regulatory substances and membrane lipid peroxidation products, our findings indicate that the L2 treatment was the most conducive to the growth of the H. hainanensis seedlings. In contrast, the L1 treatment subjected H. hainanensis seedlings to the most significant stress, resulting in substantial damage to their growth and photosynthetic mechanisms. Our research provides a scientific insight into and a practical guide for the selection of an appropriate light intensity for the conservation and cultivation of endangered plant species, such as H. hainanensis.

Hopea chinensis (Merr.) Hand.-Mazz. is a Class II national key protected plant and Plant Species with Extremely Small Populations in China. In order to further analyze why H. chinensis is endangered and optimize conservation techniques, we carried out a study on the effects of seed biological characteristics and environmental factors on the seed germination and seedling growth of H. chinensis. The results show that there were no significant differences in seed morphology between four populations in southern China, but there were significant differences in calyx lobe morphology and seed germination. The removal or retention of the calyx lobes or the seed coat had no significant effect on seed germination. The weight of individual H. chinensis seeds was mainly >1.0 g, with small seeds (2.0 g had the highest germination rate and fastest seedling growth. H. chinensis produces typical recalcitrant seeds, being sensitive to natural dehydration, with dehydration for 8 d reducing seed viability by 50%, whereas dehydration for 16 d resulted in seeds with zero viability. H. chinensis seeds are light-neutral and capable of germination at a temperature of 30 °C and on a substrate with good water retention and aeration. The seeds are not tolerant of drought, salinity, or flooding. In conclusion, the unique biological characteristics of H. chinensis seeds and the specific habitat which they inhabit contribute to a significant loss of viable seeds, which negatively affects the population’s ability to regenerate and achieve maintenance.

It is well established that forest type can have a profound impact on soil physicochemical properties but the associated changes in soil microbial communities and the mechanisms by which soil quality is improved by various plantations are not fully understood. In this study, soil physicochemical properties and microbial and enzyme activities were investigated in four forest types–Castanopsis hystrix pure forests (CHPF), C. hystrix–Pinus elliottii mixed forests (CHPEF), C. hystrix–Michelia macclurei mixed forests (CHMMF), and C. hystrix–Mytilaria laosensis mixed forests (CHMLF) in the subtropical region of China. The purpose of this study was to assess the effects of afforestation types on characteristics of soil–its physical, chemical, and biological properties. The results showed that the contents of soil total organic carbon (TOC), soil total nitrogen (TN), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) were significantly improved in both CHMMF and CHMLF mixed forest stands when compared to the CHPF pure stand. Soil enzyme activities were enhanced in the mixed forests. In particular, high phosphatase activity was observed in CHMLF stands, leading to the transformation of soil phosphorus to available phosphorus in this forest type. Our study demonstrated that the broad–leaved mixed forests, but not coniferous mixed forests, could significantly improve soil quality in the study region. Our research provides a scientific insight into the promotion of vegetation restoration and plantation forest management in plantation regions of subtropical areas.

Liquidambar formosana Hance, a tree species in subtropical broad-leaved forests, exhibits a striking autumn leaf coloration. However, how drought stress during this period influences leaf color change remains poorly understood. In this study, two-year-old seedlings were subjected to four drought gradients. Leaf color parameters, pigment contents, cellular structure, photosynthetic physiology, and hydraulic properties were systematically measured throughout the leaf color transition period. The results show that, with increasing drought severity, leaf red-green coordinate a* increased significantly during early-to-middle stress (S1–S3), while lightness L* and yellow-blue coordinate b* increased at late stress (S4). Chlorophyll (Chl) content continuously decreased, anthocyanins (Ant) peaked at mid-stress, and carotenoids (Car) became enriched at late stress. Leaf cellular structure and hydraulic parameters declined, photosynthetic function was inhibited, and antioxidant enzyme activities showed an initial increase followed by a decrease. Correlation analysis and Random Forest models revealed that L* was strongly associated with superoxide dismutase (SOD) activity, carotenoid-to-chlorophyll (Car/Chl) ratio, and net photosynthetic rate (P_n); a* was closely linked to osmotic potential at full saturation (Ψ_sat), relative water content at the turgor loss point (RWCtlp), SOD activity, Car/Chl ratio, anthocyanin-to-chlorophyll (Ant/Chl) ratio, Ant content, transpiration rate (T_r), P_n, and main vein thickness (Mvt), while b* was primarily correlated with Ψ_sat, Car/Chl ratio, SOD activity, Ant/Chl ratio, and P_n. These statistical associations suggest multiple physiological processes are involved in leaf color change. Based on these findings, we propose a hypothetical sequence: drought initially disrupts leaf water status, leading to structural atrophy and hydraulic decline, followed by photosynthetic inhibition, activated antioxidant defense, and altered pigment accumulation, which are correlated with the sequential leaf color transition from green to red to yellow-orange in this species.