Effects of slow-release fertilizer and mycorrhizal fungi on the root growth and architecture of Cyclobalanopsis gilva container seedlings

Objectives We studied the effects of slow-release fertilizer and mycorrhizal fungi inoculation on the root growth and architecture of one-year-old container seedlings of Cyclobalanopsis gilva (C. gilva), aiming to propose fertilization measurements for health cultivation of container seedlings.

Methods Two factorial complete design was adopted to carry out a pot experiment, the cultural substrate was made of peat, grain husks and perlite (in volume ratio of 6∶1∶3), the tested mycorrhizal fungi was Pisolithus tinctorius (P. tinctorius), slow-release fertilizer contained total N 180 g/kg, available P 60 g/kg, total K 120 g/kg, and the nutrient release period was 6−7 months. The application rates of slow release fertilizer were 1.50, 2.25, 3.00, 3.75 and 4.50 kg/m³. When the container seedlings grew for 3 months, each fertilizer rate treatment was divided into two groups, one group was inoculated with 10 mL P. tinctorius solution, with concentration of 36.28 mg/mL, and the other group was not inoculated. The seedlings grew for further 4 months after the inoculation, and then harvested (the end of container seedlings growth season) for investigation of root growth and architecture indexes.

Results Slow-release fertilizer application rate within 1.50−3.75 kg/m³ did not heavily affect the root growth and architecture of C. gilva seedlings, but fertilizer rate 4.50 kg/m³ significantly reduced the root length, regardless of P. tinctorius inoculation. Inoculation of P. tinctorius showed significant promotion effect on the root development of C. gilva, especially under low fertilizer rate. Compared with non-inoculation, inoculation increased the root biomass, root surface area, mean root diameter, root volume and number of root tips by 10.94%, 18.73%, 18.23%, 42.77% and 72.33% on average, but decreased the topological index and average link length by 3.11% and 6.10%, respectively. The highest promoting effect exhibited in combination treatment of P. tinctorius inoculation and slow-release fertilizer rate 3.00 kg/m³, in which the length and surface area of roots at 0−0.5 mm diameter class were significantly higher than those in the other treatments. According to the correlation analysis results, the longer the root length of container seedling, the larger the root biomass and the larger of root surface area, and the higher the fractal dimension, and the better the root system development, respectively.

Conclusions Inoculation of P. tinctorius can promote the root growth and architecture of container seedlings of C. gilva, and compensate the possible adverse effects of low or high fertilizer application on the root surface area, average root diameter and root volume. In the production of container seedlings, P. tinctorius inoculation is recommended for improving the utilization efficiency of slow-release fertilizer and the development of roots, so producing high-quality container seedlings.