Abstract: The biomass dynamics and conversion efficiency to the total solar radiation of rhizotaxy of grass+forbs steppe in southern fringe of Mu Us sandland were studied in the paper. The variation of rhizotaxy biomass, in the measuring period, expressed a two peak curve. The maximum value, occurred on May 15, was 707.53 g·m^-2 (Dry material) and the second maximum value, appeared on July 15, was 677.29 g·m^-2. The maximum value of biomass of living roots, came out in the middle ten days of July, was 99.56 g·m^-2; while that of dead roots, appeared in the middle of May, was 610.69 g·m^-2. The biomass of dead roots in the measuring period averagely made up 88% of the rhizotaxy biomass, which constitut ed the majority of total rhizotaxy biomass. The NPP (Net Primary Productivity) of rhizotaxy of grass+forbs steppe in southern fringe of Mu Us sandland was 232.65 g·m^-2·a^-1, but from the energy point of view, the NPP of that was 473.99 kJ·m^-2·a^-1. The discrepancy of vertical distribut ion of rhizotaxy biomass in different soil layers was obvious and showed a "T" style distribution in the soil. More than 70% of the rhizotaxy, including 63.6% of the living roots and 71.4% of the dead roots distributed in the 0~20 cm soil layer, while only 8% of the total roots existed in the 40~50 cm soil layer. Th is phenomenon showed the general characteristics of rhizotaxy distribution of gramineous plants and the therophyte. The relationship between the rizotaxy biomass and the depth of soil layer could be simulated by the power equation which were Y_L=10.4532x^-0.7537,Y_D=110.480x^-0.8678,Y_T=120.712x^-0.8540, representing the relationships btween the biomass of living roots, dead roots, the total roots and the depth of soil layer respectively. The grow th rate of rhizotaxy biomass in vegetative period of the community was higher th an that in reproductive period. And the variation regularity of RGR (Rate of Growth) of the rhizotaxy biomass was related to the partitioning of photosynthe tic material. Between the aboveground and underground parts of the community in different growth period of the plants, the maximum RGR of living roots of the community appeared before the green turning period so as to storage nutrients for the community turning green. The conversion efficiency to total solar radi ation for rhizotaxy biomass was 0.008%. The efficiency to physiological radiation for it was 0.016% and that of growing period was 0.028%. The maxim um efficiency of rhizotaxy to total radiation could reached 0.24% and occurred in the end of growing period of the community.

Key words: Grass+forbs steppe, Rhizotaxy biomass, Conversion efficiency to solar radiation Mu Us sandland