Premise Phosphorus (P) limitation in subtropical red soil is a major constraint on plant productivity. Understanding how plant species adapt to nutrient-poor conditions is a fundamental question in plant ecology. This study addresses this question by investigating the nutrient utilization strategies of four common subtropical tree species. Methods We conducted a homogeneous garden experiment on phosphorus-limited red soil with four subtropical tree species (Pinus massoniana, Liquidambar formosana, Elaeocarpus decipiens, and Schima superba). We systematically evaluated their leaf functional traits, nitrogen (N) and phosphorus (P) resorption efficiencies (NRE and PRE), and their interrelationships among these parameters. Results (1) Leaf functional traits varied significantly among species, forming a continuous resource-use spectrum from acquisitive (L. formosana) to conservative (P. massoniana), with. E. decipiens and S. superba showed intermediate strategies.2) The trait differentiation was strongly linked to nutrient resorption: the conservative species, P. massoniana exhibited the highest NRE and PRE (54.25% and 54.78%, respectively). Among broadleaved species, E. decipiens and S. superba had significantly higher NRE than L. formosana. All species exhibited strong P limitation (leaf N/P > 20), resulting in generally higher PRE than NRE, supporting the Relative Resorption Hypothesis. 3) Redundancy analysis further confirmed that conservative traits were positively correlated with resorption efficiency, identifying leaf carbon, litter nitrogen, leaf N/P ratio, and thickness as key regulatory factors. Conclusions These findings demonstrate how subtropical trees adapt to nutrient-poor soils through functional trait differentiation and nutrient resorption plasticity, offering a scientific basis for species selection in restoring degraded red soil ecosystems.

Biodiversity–ecosystem functioning relationships (BEFs) have been extensively explored across ecosystems. However, these relationships may change as the forest matures, and the underlying mechanisms remain underexplored. Using large temperate forest datasets from 2,392 permanent plots in northeastern China, we examined the relationships between biodiversity and aboveground biomass (AGB) across different developmental stages from young to over-mature stands. We found the positive BEFs using both species richness and functional diversity, but these positive effects decreased with forest development. However, the effects of community-weighted mean on AGB showed two peaks in young and mature stands. Notably, the effects of community-weighted mean on AGB became larger than the effects of functional diversity after the forests developed to near-mature/mature stands, indicating that BEFs are driven by mass-ratio effects (i.e., dominant species) rather than niche complementarity in old stands. Our findings on how the developmental stage influences the effects of biodiversity on ecosystem functioning in natural forests will help identify effective strategies for maintaining or enhancing ecosystem services at different forest successional stages.

Reforestation after forest clearcutting is an effective measure to increase soil organic carbon (SOC) sequestration. However, the soil C balance and functions of microbial communities under reforestation remain to be determined. Samples of organic (0-2 cm) and mineral (2-10 cm) horizons were collected from the 7-, 15-, 20-, 29-, and 36-year-old forest stands of Chinese fir developed after plantation clearcutting in subtropical climate zone under the condition of phosphorus limitation. Particulate organic carbon (POC), mineral-associated organic carbon (MAOC), microbial phospholipid fatty acids (PLFAs), and enzymatic activities for C, nitrogen (N), and phosphorus (P) acquisition were analyzed. The lowest contents of POC (10 %) and MAOC (13 %) in the organic horizon were found in 7-year-old stands due to the slow tree regrowth and extensive decomposition of SOC in the first years of forest regrowth. POC (2.0x) and MAOC (0.8x) increases in the organic horizon with forest age were attributed to the stand development and accumulation of above and belowground litter. The organic horizon had a higher POC: MAOC ratio than the mineral (0.7-1.1 vs. 0.2-0.5), indicating lower SOC stability in the first one. A positive correlation of the Gram-positive to Gram-negative bacteria (G+:G-) ratio with the POC: MAOC ratio may point to developing specific substrate utilization strategies for microbial communities. Microorganisms were limited by C and P; however, the C limitation was alleviated in the 36-year-old plots in the organic horizon due to increased litter input. Microbial C and P limitations increased with total PLFAs and the G+:G- ratio, indicating the strong influence of community structure on nutrient acquisition from SOC. Thus, soil C sequestration under reforestation of Chinese fir can be controlled by microbial community structure and metabolic limitation, which both shifted with the stand age.

We sought to assess effect of plant environmental adaptation strategies and evolutionary history and quantify the contribution of ecological processes to community assembly by measuring functional traits and phylogenetic composition in local forest community. We selected 18 dominant tree species in a Lithocarpus glaber–Cyclobalanopsis glauca evergreen broad-leaved forest and measured nine leaf functional traits and phylogenetic data of each species. We analyzed the variation in traits and trade-off relationships, tested phylogenetic effects on leaf functional traits, explored the influence of phylogeny and environment on leaf functional traits, and distinguished the relative effects of spatial and environmental variables on functional traits and phylogenetic compositions. The results showed the following: (i) Leaf traits had moderate intraspecific variation, and significant interspecific variation existed especially among life forms. (ii) Significant phylogenetic signals were detected only in leaf thickness and leaf area. The correlations among traits both supported “the leaf economics spectrum” at the species and community levels, and the relationships significantly increased or only a little change after removing the influence of phylogeny, which showed a lack of consistency between the leaf functional trait patterns and phylogenetic patterns. We infer the coexistent species tended to adopt “realism” to adapt to their habitats. (iii) Soil total potassium and phosphorus content, altitude, aspect, and convexity were the most critical environmental factors affecting functional traits and phylogenetic composition. Total environmental and spatial variables explained 63.38% of the variation in functional trait composition and 47.96% of the variation in phylogenetic structures. Meanwhile, the contribution of pure spatial factors was significantly higher than that of the pure environment. Neutral- theory-based stochastic processes played dominant roles in driving community functional trait assembly, but niche-theory-based determinative processes such as environmental filtering had a stronger effect on shaping community phylogenetic structure at a fine scale.

Decomposition of forest litter plays a major role in nitrogen (N) dynamics in soil. But to which extent that forest litter affects soil N and how much soil N is derived from the new litter remains unknown. An in-situ soil column experiment with 14-month litter decomposition was conducted to examine the effect of litter retention on soil N dynamics in a typical forest of subtropical China in 2018. Litter removal in the soil column was used as a control treatment, while natural litter or identical amount of 15N labeled litter was added to soil columns as litter retention treatment. The results showed that litter removal caused a continuous decrease in concentration of soil soluble organic nitrogen (SON) in the first 5 months, and then SON began to accumulate and its concentration went up in spring showing obvious seasonal change. Litter retention accelerated the reduction of soil SON concentration in the first 2 months, while maintained a high concentration after that period. Soil NH4+-N derived from litter was nitrified rapidly, and newly formed NO3–N was quickly immobilized or lost. Only 1.8% of soil SON came from litter N and 98.2% from indigenous soil N under the decomposition of labeled litter. Litter provided supplementation N to form new soil SON continuously, however, only a small part of SON was relatively stable, and SON played the role of reserve and regulatory pool. Soil SON and TN were formed after long-term litter accumulation and decomposition.

How soil quality and microbial communities change in conjunction with stand age in plantations is poorly understood. Here, we evaluated soil quality by using an integrated soil quality index (SQI) and traced the paralleled shifts in fungal community composition by high-throughput sequencing in a chronosequence of Chinese fir (Cunninghamia lanceolata) plantations (stand age of 3, 16, 25, 32, >80 years). Soil properties showed pronounced changes with stand age in the top 0-5 cm. The most prominent increase from 3 to >80-year-old stand was for soil organic carbon (SOC, by 2.1-times), total nitrogen (TN, 1.9-times) and available phosphorus (AP, 2.2-times). SQI increased logarithmically with stand age, with sharper change seen in the 0-5 cm layer than in the 5-15 cm layer. Mycorrhizal fungi increased in abundance initially in younger stands, but then they were gradually replaced by saprotrophic fungi in older stands due to the increase in litter input, which sustains saprotrophs. The positive correlation between saprotrophic fungi and the key soil quality indicators, such as TN, AP and NH4+, showed that higher soil quality was tightly linked with the enrichment of decomposers. Mycorrhizal taxa, such as orders Sebacinales, Thelephorales and Russulales, were positively correlated with acid phosphatase mobilizing P from organic matter. This suggests that the establishment of mycorrhizal fungi sustains tree productivity in younger stands under low soil quality. We conclude that the increase in soil quality throughout the development of Chinese fir plantations is closely linked with the observed transition of fungal communities from mycorrhizae to saprotrophs.

Complementarity in resource use leading to increased resource partitioning is the most commonly proposed mechanism for explaining the positive relationship between plant diversity and productivity. However, we still have a poor understanding of the relationship between plant diversity and root biomass. We used molecular method to identify tree species and to estimate the biomass of fine root (≤ 2 mm in diameter) for each tree species in soil cores sampled from the plots along a tree species gradient elaborated in subtropical forests. Our objectives were to examine whether spatial resource partitioning and symmetric proliferation are responsible for the relationship between aboveground tree species richness (SRA) and fine root biomass. We found that increasing SRA led to higher fine root biomass and a support for symmetric proliferation strategies, but this pattern only appeared in nutrient-rich upper soil layer. Structural equation modelling (SEM) indicated that stand density was the dominant factor to mediate SRA effects on fine root biomass. Specifically, fine root biomass depended on the SRA × stand density interaction, with lower biomass at lower density and low richness, and this effect disappeared in higher density forests. Overall, we found inconsistent support for the vertical niche partitioning, indicating that greater soil volume filling is not the reason for belowground overyielding pattern. Alternatively, density-dependent biotic interactions affecting tree recruitment are an important driver affecting productivity in diverse subtropical forests but the usual root distribution patterns in line with the resource partitioning hypothesis are unrealistic in contexts where soil nutrients are heterogeneously distributed.

Complementarity in resource use leading to increased resource partitioning is the most commonly proposed mechanism for explaining the positive relationship between plant diversity and productivity. However, we still have a poor understanding of the relationship between plant diversity and root biomass. We test whether the hypotheses of spatial resource partitioning and symmetric proliferation are responsible for the phenomena that aboveground tree species richness (SRA) increases fine root (≤ 2 mm in diameter) biomass. We found that increasing SRA led to higher belowground biomass and a support for symmetric root proliferation strategies, but this pattern only appeared in the more nutrient-rich upper soil layer. Fine root biomass depended on the SRA × tree density interaction, with lower biomass at lower density and low richness, and this effect disappeared in mixtures with high density. The results indicate that density-dependent biotic interactions affecting tree recruitment are an important driver to influence productivity.