This dataset contains root traits (biomass, length, diameter, surface area, tips, forks and crossings) in the Jena-Ecotron Experiment in 2012. This experiment was conducted in the Montpellier European Ecotron (CNRS, France), an advanced controlled environment facility for ecosystem research, and aimed at understanding the impact of plant species richness (4 vs. 16 species) for ecosystem carbon and water fluxes.The soil monoliths used in this experiment contained plant communities originating from the long- term Jena Experiment (50°57.1' N, 11°37.5' E, 130 m above sea level; mean annual temperature 9.3°C, mean annual precipitation 587 mm) established in May 2002. Twelve plots were selected for the Jena-Ecotron study according to the following criteria: (1) the four functional groups grasses, legumes, small and tall herbs were present, (2) realized species numbers were close to sown species richness, and (3) plots were equally distributed across the experimental field site to account for different soil textures. Large monoliths (2 m² surface area, diameter of 1.6 m, 2 m depth with a weight of 7 to 8 tons) including intact soil and vegetation were excavated in December 2011 and placed in lysimeters. In March 2012, before the start of the vegetation growth, the lysimeters were transported and installed in the Macrocosms platform of the Montpellier European Ecotron.Three soil cores (diameter 3.5 cm) were sampled to 60 cm depth in each plot. Each core was divided into six layers (0-5, 5-10, 10-20, 20-30, 30-40 and 40-60 cm). The respective layers were pooled per plot, and washed with tap water over a sieve (mesh size 200 µm) to separate roots from soil. Cleaned roots were weighed and a subsample of the fresh roots (approx. 2 g) was stored in 70 % ethanol before it was dyed (with neutral red solution) and scanned (Scanner Optical STD4800 Regent Instruments Inc. with top light unit and an image resolution of 600 dpi). Root length density and root surface area were estimated with WinRhizo (Reg 2009c, Regents Instruments Inc.). The smooth off area of WinRhizo was set to 0.0001 and the length-width ratio to 3.0. Belowground biomass was estimated as described above but the samples were then dried at 65°C for three days.

There are two types of missing values contained in datasets from the Jena Experiment. Empty cells represent missing values that result from the design of the experiment. Empty cells result when the respective value does not occur in the design and could thus not be measured. For example, in the case of species-specific biomass cells are left blank, when the species was not sown in 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 the Jena-Ecotron Experiment, which was part of the Jena Experiment.