Fine root morphology and chemical responses to N addition depend on root function and soil depth in a Korean pine plantation in Northeast China

Fine roots play a crucial regulatory role in the subsurface carbon (C) and nitrogen (N) cycles of terrestrial ecosystems and are potentially influenced by atmospheric N deposition. However, there is still a lot of uncertainty regarding the effect of long-term N deposition on fine root traits in forest ecosystem. Our study aims to research the responses to N addition on fine root biomass, morphology and stoichiometry at different soil layers in a temperate Korean pine (Pinus koraiensis Sieb. et Zucc.) plantation. N addition experiments were initiated in Korean pine plantations for 3 consecutive years. Twenty plots (four treatments with five replicates) were assigned: Control, Low-N, Medium-N and High-N (0, 20, 40 and 80 kg N ha−1 yr−1). The coring method was applied to quantify absorptive and transport roots morphological and chemical traits and biomass along soil profiles (0–20 cm and 20–40 cm) and to assess the responses of these root traits to a gradient of N additions. N addition increased the specific root length (SRL), root N and P concentrations, and decreased the root tissue density (RTD) and C/N ratio of absorptive roots in the two soil layers. N addition increased the diameter, decreased RTD and the C/N ratio of transport roots in the two soil layers. N addition increased absorptive and transport roots biomass, and decreased the ratio of absorptive root biomass to transport root biomass. N addition significantly increased shallow root biomass and changed the vertical distribution pattern. N deposition increased the nutrient absorption and transportation efficiency of Korean pine by adjusting root morphology and chemistry. N deposition increased the amount of C allocated to fine roots. N deposition perhaps delayed the decomposition of roots and improved the C and nutrient retention in soil. In general, adjusting fine root functional traits to acquire water and nutrients is an effective strategy for plants to adapt to the environmental changes.