Abstract: The increase of nitrogen deposition has significantly altered ecosystem functions and processes. Numerous studies showed that nitrogen addition could increase ecosystem productivity. However, most of these studies focused on aboveground biomass, much less on the belowground biomass and root:shoot ratio. We conducted a 7year nitrogen addition experiment in a typical steppe in Inner Mongolia. Six treatments were set up in 2006, namely N0 (0), N1 (5.6 g N·m^-2), N2 (11.2 g N·m^-2), N3 (22.4 g N·m^-2), N4 (39.2 g N·m^-2), N5 (56 g N·m^-2). Aboveground biomass and belowground biomass in soil depths of 0-10, 10-30, 30-50 and 50-100 cm of plant communities were measured in 2012. We examined the responses of aboveground and belowground biomass and their ratio to nitrogen addition. The results showed that: (1) Low nitrogen addition levels (N1 and N2) did not significantly affect aboveground biomass. High nitrogen addition levels (N3-N5) significantly increased aboveground biomass (96%-117%), with no significant difference across the high nitrogen addition levels. (2) Under different nitrogen addition levels, the nitrogen response efficiency (NRE) of aboveground biomass was all greater than 0 and showed a downward trend. The difference in NRE (ΔNRE) increased significantly only at the level of N3, indicating that aboveground biomass reached saturation at the level of N3; (3) Nitrogen addition did not affect 0-100 cm belowground biomass, and kept consistent in all four soil layers (0-10, 10-30, 30-50 and 50-100 cm). N addition did not affect the proportion of belowground biomass in each soil layer. (4) N1 and N2 did not significantly change the rootshoot ratio, but N3-N5 did. These results suggest that increased nitrogen deposition has a Pofitive effect on aboveground biomass. The effects of increased nitrogen deposition on plant roots in different soil layers varied greatly. More researches are needed to clarify the effects of increased nitrogen deposition on belowground productivity.

Key words: belowground biomass, eutrophication, nitrogen deposition, root:shoot ratio, grassland ecosystem.