Objectives This study investigated the effects of biochar application on tree growth and root nutrient use efficiency, aiming to provide theoretical foundations and technical references for the sustainable management of plantation forests and the enhancement of forest soil functions.

Methods An experiment was conducted using one-year-old seedlings of Masson pine (Pinus massoniana) and Chinese fir (Cunninghamia lanceolata) as the research subjects. Four levels of biochar application were set: 0, 5, 10, and 20 t/hm², representing the control (CK), low (LB), medium (MB), and high (HB) application rates, respectively. The root architecture and soil microbial community structure were examined, and root nutrient use efficiency of the seedlings were calculated on Masson pine and Chinese fir.

Results The three biochar application rates significantly increased the root biomass of Masson pine by 19.9%−30.7%, while only the LB treatment increased the root biomass of Chinese fir by 13.6%. Compared with CK, the total fine root length of Masson pine and Chinese fir increased by 24.3% and 12.7%, respectively, under the HB treatment. The fine root surface area of Masson pine increased by 20.7% and 41.2%, and the volume increased by 24.9% and 51.5% under the MB and HB treatments, respectively. The MB treatment reduced the fine root surface area and volume of Chinese fir by 13.5% and 16.7%, respectively. The LB treatment decreased the nitrogen content in the fine roots of Masson pine and Chinese fir by 16.1% and 12.2%, respectively. The LB and MB treatments reduced the phosphorus content in the fine roots of Masson pine by 18.6% and 22.0%, respectively. The LB and MB treatments significantly increased the total soil microbial biomass of Masson pine by 28.9% and 25.5%, respectively, while the LB treatment increased the total soil microbial biomass of Chinese fir by 51.8%. Compared with CK, under the LB treatment, fungi showed a significant negative correlation with root length, root surface area, root volume, and root nitrogen uptake. Under the MB treatment, mycorrhizal fungi exhibited a highly significant positive correlation with root length, root surface area, root volume, and the uptake and utilization efficiency of root nitrogen and phosphorus, while other components of the microbial community structure showed significant or highly significant negative correlations. Under the HB treatment, only fungi showed a significant negative correlation with root biomass. In the CK treatment, nitrogen uptake and utilization efficiency were significantly influenced by fungi and actinomycetes, while phosphorus uptake efficiency was significantly influenced by fungi. Under the LB treatment, nitrogen uptake efficiency was significantly influenced by root biomass, G⁻ bacteria, and actinomycetes. Under the MB treatment, nitrogen uptake was significantly affected by root length, root surface area, and root volume, while phosphorus uptake was significantly influenced by root surface area, root volume, root nitrogen content, bacteria, and actinomycetes. Under the HB treatment, nitrogen uptake efficiency was influenced by root biomass, root volume, and mycorrhizal fungi, while phosphorus uptake efficiency was significantly influenced by root biomass, root length, and root surface area.

Conclusions Biochar application has a positive promoting effect on the growth and nutrient uptake of subtropical plantation forests. The amount of biochar applied affects the total quantity and structure of soil microorganisms, which in turn influences the uptake and utilization of nitrogen and phosphorus by roots. At low biochar application rates, bacteria and actinomycetes have a greater impact on the uptake and utilization efficiency of root nitrogen and phosphorus. In contrast, medium and high application rates reduce microbial biomass, increase the proportion of fungi, and make nitrogen and phosphorus uptake and utilization primarily dependent on mycorrhizal fungi. High biochar application rates are suitable for Masson pine, while Chinese fir benefits more from low biochar application rates.