Description: This data set contains measurements of belowground biomass productivity, i.e. coarse and fine root biomass production (N-concentration of the fine roots are attached to this dataset as well). Data presented here is from the Main Experiment plots of a large grassland biodiversity experiment (the Jena Experiment; see further details below). In the Main experiment, 82 grassland plots of 20 x 20 m were established from a pool of 60 species belonging to four functional groups (grasses, legumes, tall and small herbs). In May 2002, varying numbers of plant species from this species pool were sown into the plots to create a gradient of plant species richness (1, 2, 4, 8, 16 and 60 species) and functional richness (1, 2, 3, 4 functional groups). Plots were maintained in general by bi-annual weeding and mowing. Since 2010, plots were weeded three times per year. In July 2007, five soil cores with a 4.8 cm diameter per plot were removed to 30 cm depth. The removed soil was replaced by root-free soil from the field site. The ingrowth cores were removed in June 2008 and cut with scissors until root fragments were 〈 1 cm in length. For root washing, a 210 g subsample of the bulk material was taken and soaked in water. The soil was removed from the roots by rinsing over a sieve with 0.5 mm mesh width. Then, organic debris and remaining soil particles were removed by hand. The remaining roots were dried at 70 °C and weighed. In 2008, ingrowth cores were separated in depth increments of 0-5, 5-10, 10-20 and 20-30 cm depth. Roots were seperated in coarse (diameter 〉 2 mm) and fine roots before cutting the bulk material. Fine roots were grinded in a ball mill and 20 mg of the grinded material was used to determine N-concentration of the fine roots with VarioMax CNS (Elementar).