This data set contains soil carbon measurements (Organic carbon, inorganic carbon, and total carbon; all measured in dried soil samples) 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 by bi-annual weeding and mowing. Stratified soil sampling to a depth of 1 m was repeated in April 2007 (as had been done before sowing in April 2002). Three independent samples per plot were taken of all plots in block 2 using a motor-driven soil column cylinder (Cobra, Eijkelkamp, 8.3 cm in diameter). Soil samples were dried at 40°C and segmented to a depth resolution of 5 cm giving 20 depth subsamples per core. All samples were analyzed independently. All soil samples were passed through a sieve with a mesh size of 2 mm. Because of much higher proportions of roots in the soil, the samples in 2007 were further sieved to 1 mm according to common root removal methods. No additional mineral particles were removed by this procedure. Total carbon concentration was analyzed on ball-milled subsamples (time 4 min, frequency 30 s**-1) by an elemental analyzer at 1150°C (Elementaranalysator vario Max CN; Elementar Analysensysteme GmbH, Hanau, Germany). We measured inorganic carbon concentration by elemental analysis at 1150°C after removal of organic carbon for 16 h at 450°C in a muffle furnace. Organic carbon concentration was calculated as the difference between both measurements of total and inorganic carbon.

The reliability of interfering the organic carbon content from measured total and measured inorganic carbon after removal of organic carbon was supported by measurements of certified reference soil material (HEKAtech GmbH, Wegberg, Germany) on a regular basis. The repeated measurements of the soil standard resulted in an excellent relative standard deviation for organic carbon concentration measurements of 1.2%. High intensity sampling campaigns were conducted only in block 2 and were scheduled to be repeated every 5 years. Every two years measurements of soil carbon in a depth of up to 0.3 m are available from plots of all blocks in a separate time series. 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 thus cannot be measured. Missing values that resulted from methodological problems, sampling errors, or lost samples/data are marked with "-9999".In addition to measurement of soil carbon, measurements of dissolved organic carbon in soil solution are available.This datasets is part of a time series of soil carbon measurements in the Main Experiment since 2002 (doi:10.1594/PANGAEA.848946).