Mixed plantation of Chinese fir (Cunninghamia lanceolata) is an effective artificial forest management for tree productivity. However, the mixing strategies, site conditions, and subsurface properties that affect tree productivity are not yet fully understood. In this study, we conducted a meta-analysis of 96 publications to consolidate insights on the effects of mixing strategies (e.g., planting density, mixing proportion, mixed species, and tree age), site conditions (e.g., mean annual precipitation (MAP), mean annual temperature (MAT), elevation, and total nitrogen (TN) or total phosphorus (TP) of sample sites), and subsurface properties (e.g., soil characteristics, microbial communities, and extracellular enzyme activity) on tree height, diameter at breast height, and individual volume of Chinese fir. We used the Web of Science and China National Knowledge Infrastructure for searching peer-reviewed papers, and the searching words were: (“Cunninghamia lanceolata” OR “Chinese fir”) AND “mix*”. Following the data screening process, the natural logarithm of the response ratio (lnRR) was computed for subsequent analysis. The results showed that introduced companion species generally increased the individual volume of Chinese fir by an average of 20%. Densities ranging from 1200 to 2000 trees per hectare and moderate mixing proportions (1:1 to 3:1) optimized individual tree growth and thereby boosted productivity. Broadleaf species may be beneficial companions, and trees aged 10 to 20 years grew fastest. At sites with low MAT and high MAP, mixed plantations enhanced the tree productivity of Chinese fir. The optimal elevation range for mixed plantations may be 200 to 600 m. Further, mixed plantations significantly changed soil properties by improving soil structure, increasing soil pH and soil water content, and soil total and available N and P, which were crucial for boosting the productivity of Chinese fir. Soil microbial biomass and enzyme activities were also significantly increased by mixed plantations. Overall, these findings highlight the importance of mixing strategies and site conditions in increasing tree productivity of Chinese fir by improving soil physicochemical characteristics, increasing resource availability, and reducing interspecific and intraspecific competition through niche separation.

Rhizosphere effects (REs) play important roles in regulating carbon (C) and nutrient cycling in terrestrial ecosystems. However, little is known about the REs of mature trees in the field, especially at the ecosystem scale. This study aimed to explore the variation and patterns of REs in natural ecosystems. Here, combining soil monoliths with an adhering soil (shaking fine roots) method was adopted to sample paired rhizosphere soil and bulk soil along an altitudinal gradient. Based on the relative REs and the percentage of rhizosphere soil mass, the REs on soil C and net nitrogen mineralization rates (C_min and net N_min) at the ecosystem scale were estimated. Our results showed that the REs on soil processes, soil microbial biomass C and extracellular enzyme activities (β-glucosidase and N-acetyl-glucosaminidase activities), and soil chemical properties (total C, total N, inorganic N, extractable P, K, Ca, Mg, Fe, and Mn) were significantly positive across altitudinal sites, while soil pH was significantly negative. Although the relative REs on investigated variables varied significantly among altitudes, the relative REs did not show a clear trend with the increased altitudes. Across altitudes, the mean magnitude of ecosystem-level REs on C_min and net N_min were 19% (ranging from 4% to 48%) and 16% (ranging from 3% to 34%), respectively. Furthermore, the magnitude of ecosystem-level rhizosphere effects increased linearly with the increased altitudes. The altitudinal patterns of ecosystem-level RE mainly depend on the percentage of rhizosphere soil mass. In conclusion, our results provided a set of new evidence for the REs, and highlighted the need to incorporate REs into land C and N models.

The influence of living roots on soil organic matter decomposition is termed the rhizosphere priming effect (RPE). Although root traits are critical for understanding the RPE, it is unclear how the trade-offs among root traits, exudation and mycorrhizal symbioses mediate the RPE. The RPEs of 12 grassland species were quantified using a natural 13C tracer method in a mesocosm experiment. Ten root functional traits were measured to examine the trade-offs among root traits, and their linkage with the RPEs. All species produced positive RPEs, with legumes and forbs showing larger RPEs than grasses. The magnitude varied from 32% to 350% compared to the unplanted soil. After accounting for root biomass effect, specific RPEs were positively correlated with specific root length, specific root surface area, root exudation rate, and specific rhizosphere respiration, while negatively correlated with root diameter and arbuscular mycorrhizal fungi colonization. These results demonstrate that plants with thinner roots show efficient root morphology and/or more exudation by inducing larger specific RPEs, while plants with thicker roots associate more with mycorrhizal symbioses and induce smaller specific RPEs. Overall, root functional traits play key roles in mediating the species-specific RPEs and have implications for predicting soil organic matter dynamics.