This data set contains measurements of carbon and nitrogen concentrations in aboveground 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, plot size was reduced to 5 x 6 m and plots were weeded three times per year.Aboveground plant biomass was harvested twice in June and August at estimated peak standing biomass before mowing. Plants were clipped at 3 cm above ground level in four rectangles of 20 x 50 cm size per plot. All material was sorted to species, weeds and rest (dead). Samples were dried at 70 °C for at least 48 h and weeds and rest were thrown away. All other material from one plot was combined to one sample and cut up with an analysis mill (Kinematica, Littau, Schweiz). A small subsample of this material was milled to fine powder in a ball-mill (Retsch 2000 M). From the milled material 20 mg were weighed in for CN analysis. Carbon and nitrogen concentrations of this subsample were then determined with an elemental analyzer (VarioEL CHNS).

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 element concentrations in plants 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.887388).