This data set contains measurements of standing belowground plant biomass. 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. Plot size was reduced to 5 x 6 m since 2010.In 2014, standing root biomass was sampled in June. Three soil cores with a 4.0 cm diameter per plot were taken to 40 cm depth and pooled plot-wise. The cores were immediately stored cool until further handling. For root washing, the bulk sample was soaked in water and then repeatedly rinsed with tap water over a 0.2 mm sieve. Remaining soil particles were removed by hand. Roots were dried at 60 - 70 °C and weighed subsequently. In 2014, soil cores were separated in depth increments of 0-5, 5-10, 10-20, 20-30 and 30 - 40 cm depth and the corresponding layers were pooled plot-wise. Roots were not seperated in coarse (diameter > 2 mm) and fine roots and only total root biomass is shown in this dataset.

The Main experiment on the field site of the Jena Experiment comprises a split-plot design in which additional treatments (e.g. no weeding allowing for invasion of other species, or application of fertilizer) have been applied to subplots along the outer margin of the plots (see Treatment description in the section "further details"). While in this dataset only data from the core area (mown and weeding of all none-sown species twice a year, no additional treatments) is reported, all potential treatments are listed as the first parameters in the data file to guarantee compatibility with other datasets that include different treatments. There are two types of missing values contained in the dataset. Empty cells represent missing values that result from the design of the experiment when the respective value does not occur and could thus not be measured. For example, in the columns of species-specific biomass cells are left blank, when the species was not sown into the respective plot. Missing values that resulted from methodological problems, sampling errors, or lost samples/data are marked with "-9999".This dataset is part of a collection of measurements of belowground plant biomass and morphological root parameters in the Jena Experiment. Similar data from other experiments (e.g. Main Experiment, Dominance Experiment; see details above), other years, or other methods quantifying related data might be available (https://doi.pangaea.de/10.1594/PANGAEA.880330).