Stand density management can influence understory natural regeneration and plant diversity during forest rewilding. Changes in plant diversity drive shifts in soil microbial communities and soil organic carbon (SOC) sequestration. However, the response of soil microbes, especially microbial carbon (C) metabolic functions, and SOC fractions to changes in stand density and plant diversity remains poorly understood. This study examined naturally regenerated Chinese fir (Cunninghamia lanceolata) stands after 30 years under three sprout density treatments (1200, 850, and 500 stems ha^–1). We investigated variations in plant diversity (tree species, structural, and functional), SOC fractions (particulate and mineral-associated organic C), microbial diversity, CO₂ fixation pathways, and carbohydrate-active enzymes, as well as the linkage among these variables in the topsoil (0–10 cm) using metagenomic sequencing. Tree species, structural, and functional diversities, as well as fungal alpha diversity, increased with decreasing sprout density, whereas bacterial alpha diversity remained unchanged. The abundances of most C fixation pathways and genes involved in labile C degradation increased with decreasing sprout density. Microbial diversity and C fixation/degradation genes were primarily influenced by tree species diversity. The contents of SOC fractions increased with reduced sprout density and exhibited positive correlations with plant and fungal diversities, most C fixation pathways (e.g., Calvin cycle, rTCA cycle, DC/4-HB cycle, and 3-HP/4-HB cycle), and labile C (e.g., cellulose and peptidoglycan) degradation genes. These findings highlight that reducing stand density significantly enhances tree species diversity over the long term, which in turn promotes SOC accumulation by influencing microbial C fixation and degradation potential. Our study provides new insights into how tree species diversity mediates microbial regulation of SOC sequestration during forest rewilding.

Mangrove forests are characterized by highly stressful conditions and are thought to be resistant to plant invasions (the ‘mangrove invasion resistance paradigm’). Nevertheless, the exotic mangrove Sonneratia apetala is encroaching into the native shrubby mangroves and invasive salt marshes (Spartina alterniflora) in southern China, forming a widespread marsh-mangrove ecotone. The mechanisms driving the rapid invasion of S. apetala remain unknown. Using a trait-based approach, we compared the trait-environment interactions between invasive S. apetala and native Avicennia marina through field transplant experiments and greenhouse shading trials. We found (1) S. apetala exhibited 5–10 times higher relative growth rate than A. marina across light gradients; (2) S. apetala achieved over 50% establishment in native shrubby mangrove stands and S. alterniflora meadows vs. zero for A. marina, while establishment of both species was zero in closed-canopy tree stands; (3) A trait syndrome combining with fast growth capacity, acquisitive leaf traits (higher photosynthetic rate and specific leaf area, and shorter leaf lifespan), and shoot-biased biomass allocation linked light availability to establishment of S. apetala, whereas A. marina’s conservative traits (higher leaf dry mass content and longer leaf lifespan) and root-biased biomass allocation decoupled from growth and establishment. S. apetala’s trait syndrome, which maximizes whole-plant growth in low-light shrubby mangrove and S. alterniflora meadow habitats, challenges the standard mangrove invasion resistance paradigm. Furthermore, multi-dimensional trait-environment-performance interactions may underlie the success of highly invasive species worldwide. Our results suggest that management priorities should be removing S. apetala seedlings before they reach escape height and protecting intact native mangrove vegetation.

It is unclear whether and how tree-ring width is associated with photosynthetic productivity and nutrient recycling in forest ecosystems. Here, we aim to demonstrate how leaf turnover and nitrogen (N) resorption are crucial in regulating the seasonality of carbon (C) availability, and how tree radial growth is controlled by C supply in an alpine treeline forest. The seasonal litterfall, N resorption, leaf N concentration, leaf and twig non-structural carbohydrate (NSC) contents across eight trees were monitored every two weeks during the growing season, and radial increments were recorded by automatic dendrometers at Abies georgei var. smithii treeline. Leaf N showed a positive correlation with previous bi-weekly litterfall and N-resorption in the early season before late June, but with soil temperature in the later season. Leaf NSC typically peaked in the early growing season and showed significant relationships with previous bi-weekly litterfall and N-resorption and the following bi-weekly radial increment. There was a lagged chain from previous bi-weekly litterfall and N-resorption to current NSC in leaves, and then from current NSC in leaves to the following bi-weekly radial increment. As a significant C-storage pool, twig NSC reached a maximum in mid-July, and showed no correlation with litterfall, N-resorption and radial growth rate. N resorption and C supply regulated by leaf turnover controls tree radial growth at the alpine treeline. Our results highlight the significant role of nitrogen recycling and leaf source NSC production in driving tree radial growth, as compared to stored NSC.

CO₂ released into the atmosphere through soil respiration represents the second-largest carbon flux between terrestrial ecosystems and the atmosphere. While extensive research has concentrated on surface soils, limited studies have explored CO₂ emission patterns and their primary drivers across varying soil depths. In this study, soil CO₂ emissions were measured using static chambers at six different depths (10 cm, 50 cm, 100 cm, 200 cm, 300 cm, and 400 cm) in a primary forest. Additionally, potential influencing factors, including soil physical and chemical properties, microbial diversity, and community structure and function, were assessed. The results demonstrated that soil nutrients, along with fungal and bacterial diversity, generally declined with increasing soil depth. Soil CO₂ emissions also decreased significantly with depth, driven primarily by biotic factors like fungal and bacterial alpha diversity and abiotic factors such as ammonium nitrogen and available phosphorus. These findings provide new insights into the mechanisms of carbon cycling within deep soil layers in forest ecosystems.

Despite nitrogen (N) and phosphorus (P) being biologically coupled and controlling many biochemical reactions, few studies have examined the N: P patterns and controls of legumes and non-legumes at a global scale. Herein, we explored how the ratio of N and P in legumes and non-legumes responds to environmental factors, globally. Our results indicated that legumes exhibit stronger N-P coupling (R² = 0.39, P < 0.0001) in warm-humid environments (mean precipitation: 988.94 mm, temperature: 12.63 °C), and the N:P is negatively affected by the soil total P (scored at = –0.25). In contrast, non-legumes are more flexible in N and P (R² = 0.23, P < 0.0001) in semihumid regions (precipitation = 785.01 mm, temperature = 8.85 ℃), where soil total N (scored at = –0.22) and biodiversity (scored at = 0.16) emerge as dominant drivers for the N:P. Although legumes are expected to be more soil P-limited, our findings reveal that the leaf N and P are more coupled in legumes than in non-legumes, which offers a unique perspective on resource utilization and survival strategies in different plant functions.