Abstract: Fine root decomposition is a key way for plants to return carbon (C) and nutrients to the soil. The increases in exogenous nitrogen input caused by increased global atmospheric nitrogen deposition is continuously interfering with root decomposition process, thereby affecting the nutrient cycle of forest ecosystems. To explore the mechanisms underlying the effects of nitrogen addition on decomposition rates and nutrient release of fine roots across different root orders, we selected Larix gmelinii and Fraxinus mandshurica, the dominant afforestation tree species in Northeast China, as research materials. The root-order classification approach was adopted to separate fine roots (diameter ≤2 mm) into three groups: first- and second-order roots were combined as low-order roots, third- to fifth-order roots as medium-order roots, and sixth-order roots as high-order roots. There were four nitrogen treatments, including inorganic nitrogen (IN), organic nitrogen (ON), mixed inorganic and organic nitrogen (IN+ON), and a control (CK). The litterbag method was used to investigate decomposition and nutrient release of fine roots with different root orders for the two tree species. The results showed that after 350 days of decomposition, there was a significant interspecific difference in root decomposition rate between the two tree species (P<0.05), which was manifested as F. mandshurica>L. gmelinii. Nitrogen application significantly promoted root decomposition rate in both species, but there were differences in the promotion effects of different nitrogen forms. The specific manifestations were: IN had the best effect, followed by ON, IN+ON had no significant effect. There was no significant difference in the decomposition rate of each fine root order across different nitrogen treatments (P>0.05). There was a significant differentiation in nutrient release characteristics for both species. The C release rate of low-order roots was significantly lower than that of medium- and high-order roots, while the N release rate was significantly higher than that of medium- and high-order roots. In addition, nitrogen application significantly promoted the release of fine root C, but did not affect N release. Our results can help understand  the material cycle process in forest ecosystems under nitrogen deposition.

Key words: nitrogen addition, Fraxinus mandshurica, Larix gmelinii, fine root decomposition, nutrient release