This data set contains measurements of phosphorus fractions (Hedley fractions) in soil collected 2007 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. Soil sampling and analysis: Five independent soil samples per plot were taken in a depth of 0-15 cm using a soil corer with an inner diameter of 1 cm. The five samples per plot were combined to one composite sample per plot. A four-step sequential P fractionation (Hedley fractions) was applied. Sequentially, 20 ml NaHCO3 (adjusted to pH 8.5), 30 ml NaOH, and 35 ml HCl were used as extraction solutions for 0.5 g soil. The last step comprised the combustion (550 °C) of the remaining soil to destroy all organic material followed by shaking with 20 ml H2SO4. Organic P concentrations of the respective fractions were calculated as the difference between total dissolved P and inorganic P. Duplicate phosphate concentrations of P fractions in soil were measured photometrically (molybdenum blue-reactive P) with a Continuous Flow Analyzer (Bran&Luebbe, Germany).

Duplicate extractions of P fractions of selected samples yielded the following precision (as standard error of replicate analyses): NaHCO3-Pi 2.2 mg kg-1; NaHCO3-Po 1.6 mg kg-1; NaOH-Pi 5.3 mg kg-1; NaOH-Po 2.7 mg kg-1; and HCl 7.0 mg kg-1. In accordance with Negassa and Leinweber (2009), labile P refers to NaHCO3-Pi and -Po, moderately labile P to NaOH-Pi and -Po and stable P to HCl-P and H2SO4-P fractions.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 case 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".In addition to measurements of phosphorus fractions in soil determined from soil samples, measurements of total phosphorus in soil, measurements of dissolved phosphorus (organic, inorganic, and total) determined from bi-weekly soil solution samples, and measurement of dissolved inorganic phosphorus in soil solution aggregated to seasonal values are available.This datasets is part of a collection of soil phosphorus measurements in the Main Experiment since 2002 (doi:10.1594/PANGAEA.854466).