Plasticity of fine-root functional traits in the litter layer in response to nitrogen addition in a subtropical forest plantation

Background and aims Fine-root traits mediate the capacity of plants to acquire soil resources in different environments. This study aimed to examine the changes of fine-root traits when roots proliferate into the litter layer vs. mineral soils, and to determine fine-root trait plasticity of these roots in response to nitrogen (N) addition. Methods A one-year N addition experiment was conducted in a 22-year-old broadleaf Mytilaria laosensis (Hamamelidaceae) plantation in subtropical China. Newly produced fine roots were collected monthly from the litter layer and upper mineral soil (0–10 cm) layer to measure root morphological traits and nutrient concentrations. Fine-root production was determined using ingrowth mesh screens in the litter layer. Results Fine-root production in the litter layer in the Mytilaria laosensis plantation was 2.6 g m⁻² yr.⁻¹ but increased 3- to 5-fold with N addition. Significant differences in fine-root morphological traits and nutrient concentrations were found between the litter layer and 0–10 cm mineral soil layer. Fine roots in the litter layer were thinner, with higher specific root length (SRL), higher specific root area (SRA), a higher proportion of fine-root biomass in lower, more absorptive root orders, and lower root tissue density (RTD) than those in 0–10 cm mineral soil layer. Higher carbon (C), N, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) concentrations and lower C:N ratio (C/N) were also observed in fine roots in the litter layer, compared to the 0–10 cm mineral soil layer. Nitrogen addition significantly increased root P, K, and Ca concentrations, but had no effect on Mg concentration. Nitrogen addition did not affect most fine-root morphological traits but did result in decreased root diameter. Conclusions Compared with the mineral soil, roots produced in the litter layer generally reflected a more absorptive strategy with smaller root diameter and lower RTD and with higher SRL, SRA, and nutrient concentrations which together are generally associated with more metabolically active, but shorter lived roots. Strong responses of fine-root production and nutrient concentrations to N addition also suggest that N may be a driving factor for fine-root growth into the litter layer. Further studies are required to identify the effect of fine-root growth into the litter layer on microbial activity.