Effects of long-term nitrogen addition on fine root biomass, production and morphological characteristics of Fraxinus mandshurica and Larix gmelinii plantations

The objective of this study was to reveal the effects of long-term nitrogen addition on the different soil layers of the biomass, production and morphology of absorption roots and transportation roots, providing valuable insights for further understanding the adaptation strategies of fine roots under nitrogen deposition. Based on the long-term (19 years) nitrogen addition experimental platform of Fraxinus mandshurica and Larix gmelinii plantations in Maoershan, Heilongjiang Province, China, we investigated the effect of nitrogen addition on the biomass, production and morphology of absorption roots and transportation roots in different soil depths by root drilling method and in-growth core approach. Long-term nitrogen addition significantly increased the specific root length of absorption roots of F. mandshurica by 26.5% in the 0～10 cm soil layer. Additionally, nitrogen addition significantly increased root tissue density of absorption roots of F. mandshurica by 30.8% in the 10～20 cm soil layer. In contrast, nitrogen addition had no significant effects on the morphological traits of transportation roots of F. mandshurica and fine roots of L. gmelinii across all soil depths. In the 0～30 cm soil layer, the biomass of absorption roots of F. mandshurica significantly increased by 76.9%. In the 10～20 cm and 20～30 cm soil layers, both absorption and transportation roots biomass significantly increased, and the ratio of absorption to transportation root biomass in the 10～20 cm layer also significantly increased(P<0.05). For L. gmelinii, the biomass of absorption roots in the 10～20 cm soil layer decreased significantly(P<0.05), while the production of absorption root significantly increased in the 20～30 cm layer from 4.98 g/m² to 14.07 g/m². F. mandshurica responded positively to the prolonged N addition by adjusting morphological plasticity of absorption roots and vertical distribution of biomass. L. gmelinii exhibited a decrease in absorption root biomass in the subsurface layer and an increase in production in deeper layers. Overall, the responses of fine root biomass, production and morphology of F. mandshurica and L. gmelinii to long-term nitrogen addition were significantly different among species and soil layers, and the absorption roots were more sensitive than the transport roots.