Abstract: Lines of evidence show a saturated rate of nitrogen (N) addition for enhancing aboveground biomass (AGB), with phosphorus (P) limitation as one of the most important casual factors. However, whether belowground biomass (BGB) can mirror the responses of AGB remains unknown. Moreover, it is unclear that whether simultaneous additions of N and P could alleviate the limitation caused by single nutrient addition. To clarify the response patterns of BGB to N and P additions, we conducted a fouryear manipulative experiment (N addition: 10 and 40 g N·m^-2·a^-1; P addition: 5 and 10 g P·m^-2·a^-1) in a temperate typical steppe of Inner Mongolia, China. The results showed that total BGB and that of surface layer (0-5 cm) tended to increase with increasing rates of N addition. However, only the high rate of N addition (40 g N·m^-2·a^-1) led to significant promotion of BGB in some years. P addition did not affect BGB, irrespective of the addition rate. Moreover, P addition did not affect the response of BGB to nitrogen addition. Nitrogen addition distinctly reduced or increased the extent to which BGB responded to P addition in a few cases. Nitrogen addition had relatively insignificant influence on BGB changes induced by P addition. Our results provide a better understanding of the responses of grassland belowground biomass to the enhanced N and P deposition, with important implications for grassland management.

Key words: belowground biomass, nitrogen addition, phosphorus addition, Inner Mongolia temperate steppe.