Biomass allocations between aboveground and belowground organs provide pivotal information for connecting aboveground productivity and belowground carbon sequestration. As accurate measurement of belowground biomass is essential for determining the biomass allocation, we first reviewed the methods in quantifying belowground biomass and their merits. We then presented the major advances on plant biomass allocations between aboveground and belowground organs, as well as the potential drivers such as precipitation, warming, atmospheric CO_2 concentration, and nitrogen deposition. We finally provided a list of challenges in studying belowground biomass allocation for the future. This review has important implications for studies on carbon cycling in grassland ecosystems under the changing climate.