Description: Concern about the functional consequences of unprecedented loss in biodiversity has prompted biodiversity–ecosystem functioning (BEF) research to become one of the most active fields of ecological research in the past 25 years. Hundreds of experiments have manipulated biodiversity as an independent variable and found compelling support that the functioning of ecosystems increases with the diversity of their ecological communities. This research has also identified some of the mechanisms underlying BEF relationships, some context-dependencies of the strength of relationships, as well as implications for various ecosystem services that humankind depends upon. In this chapter, we argue that a multitrophic perspective of biotic interactions in random and non-random biodiversity change scenarios is key to advance future BEF research and to address some of its most important remaining challenges. We discuss that the study and the quantification of multitrophic interactions in space and time facilitates scaling up from small-scale biodiversity manipulations and ecosystem function assessments to management-relevant spatial scales across ecosystem boundaries. We specifically consider multitrophic conceptual frameworks to understand and predict the context-dependency of BEF relationships. Moreover, we highlight the importance of the eco-evolutionary underpinnings of multitrophic BEF relationships. We outline that FAIR data (meeting the standards of findability, accessibility, interoperability, and reusability) and reproducible processing will be key to advance this field of research by making it more integrative. Finally, we show how these BEF insights may be implemented for ecosystem management, society, and policy. Given that human well-being critically depends on the multiple services provided by diverse, multitrophic communities, integrating the approaches of evolutionary ecology, community ecology, and ecosystem ecology in future BEF research will be key to refine conservation targets and develop sustainable management strategies.

Description: 1. Plant diversity is an important driver of belowground ecosystem functions, such as root growth, soil organic matter (SOM) storage, and microbial metabolism, mainly by influencing the interactions between plant roots and soil. Dissolved organic matter (DOM), as the most mobile form of SOM, plays a crucial role for a multitude of soil processes that are central for ecosystem functioning. Thus, DOM is likely to be an important mediator of plant diversity effects on soil processes. However, the relationships between plant diversity and DOM have not been studied so far. 2. We investigated the mechanisms underlying plant diversity effects on concentrations of DOM using continuous soil water sampling across 6 years and 62 plant communities in a long‐term grassland biodiversity experiment in Jena, Germany. Furthermore, we investigated plant diversity effects on the molecular properties of DOM in a subset of the samples. 3. Although DOM concentrations were highly variable over the course of the year with highest concentrations in summer and autumn, we found that DOM concentrations consistently increased with plant diversity across seasons. The positive plant diversity effect on DOM concentrations was mainly mediated by increased microbial activity and newly sequestered carbon in topsoil. However, the effect of soil microbial activity on DOM concentrations differed between seasons, indicating DOM consumption in winter and spring, and DOM production in summer and autumn. Furthermore, we found increased contents of small and easily decomposable DOM molecules reaching deeper soil layers with high plant diversity. 4. Synthesis. Our findings suggest that plant diversity enhances the continuous downward transport of DOM in multiple ways. On the one hand, higher plant diversity results in higher DOM concentrations, on the other hand, this DOM is less degraded. The present study indicates, for the first time, that higher plant diversity enhances the downward transport of dissolved molecules that likely stimulate soil development in deeper layers and therefore increase soil fertility.

Description: This data set contains environmental variables (imposed cumulative precipitation, measurements of radiation, air relative humidity and air temperature), measurements of soil water content, root water uptake estimated from soil moisture contents and community evapotranspiration derived from root water uptake as well as from weight changes, plant trait data (specific leaf area, leaf dry matter content, leaf water potential measured at predawn and midday, stomatal conductance, leaf greenness, height of species, aboveground biomass, species abundances, community leaf area index, vegetation and soil surface cover) and root trait data (biomass, length, diameter, surface area, tips, forks and crossings) from the 12 macrocosms used 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. These data were used to investigate the characteristics of root water uptake profiles in grassland plant communities with different species richness (4-species and 16-species mixtures) and productivity to gain a deeper understanding of how plant species richness affects efficient use of available soil water.

Description: This dataset contains community evapotranspiration derived from root water uptake as well as from weight changes from the 12 macrocosms used 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 from the Jena Experiment 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 from the twelve plots 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. Ecosystem evapotranspiration (ET) was measured from the lysimeter weight changes to validate the ET estimated with a water balance method. The weight measurements (6 minutes resolution) were smoothed using a moving average over 30 minutes to reduce noise due to the experimental setup (Milcu et al. 2016). A water balance method was used to estimate daily root water uptake profiles and thus daily ecosystem ET from diurnal fluctuation of soil water content measurements (Guderle & Hildebrandt, 2015; doi:10.5194/hess-19-409-2015). The method consists in applying a running regression over multiple time steps on soil water content time series of each measurement depth. Here we used measurements with a temporal resolution of 1 minute from 10 cm, 20 cm, 30 cm and 60 cm depth. We split up the time series by fitting a linear function to each day and night branch of the time series in order to disentangle soil water flow and actual root water uptake. In a prior investigation we found the main transpiration time lasted from 5:30 am to 6:30 pm so that the onset of the day and night branch was fixed to these times. Night time transpiration was low (〈 23 % of the day time transpiration) and therefore neglected (Milcu et al. 2016). Subsequently, the root water uptake profile was integrated over the entire soil profile to determine the ET per one m² surface and day. The modelled ET was furthermore multiplied by the factor two in order to upscale the modelled ET to the surface of one lysimeter which is two m². Evapotranspiration values estimated from weight changes between 5:00 am and 6:30 pm of the respective day are provided for 25 June 2012, 28 June 2012 and 29 June 2012. Evapotranspiration values estimated from root water uptake are provided for the days 25 June 2012, 28 June 2012, 29 June 2012, 17 July 2012 and 18 July 2012.

Description: This dataset contains root water uptake estimated from soil moisture contents from the 12 macrocosms used 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 from the Jena Experiment 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 from the twelve plots 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. A water balance method was used to estimate daily root water uptake profiles and thus daily ecosystem ET from diurnal fluctuation of soil water content measurements (Guderle & Hildebrandt, 2015; doi:10.5194/hess-19-409-2015). The method consists in applying a running regression over multiple time steps on soil water content time series of each measurement depth. Here we used measurements with a temporal resolution of 1 minute from 10 cm, 20 cm, 30 cm and 60 cm depth. We split up the time series by fitting a linear function to each day and night branch of the time series in order to disentangle soil water flow and actual root water uptake. In a prior investigation we found the main transpiration time lasted from 5:30 am to 6:30 pm so that the onset of the day and night branch was fixed to these times. Night time transpiration was low (〈 23 % of the day time transpiration) and therefore neglected (Milcu et al. 2016). Subsequently, the root water uptake profile was integrated over the entire soil profile to determine the ET per one m² surface and day. The modelled ET was furthermore multiplied by the factor two in order to upscale the modelled ET to the surface of one lysimeter which is two m². Root water uptake values are provided as absolute values (cm d-1) and as percentage values for the days 25 June 2012, 28 June 2012, 29 June 2012, 17 July 2012 and 18 July 2012 for 10 cm, 20 cm, 30 cm and 60 cm depth. Root water uptake as percentage was calculated as ratio of root water uptake of individual layers to the sum of total root water uptake from 10 cm to 60 cm depths multiplied by 100.

Description: This dataset contains cumulative precipitation imposed 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. The precipitation imposed in the Jena-Ecotron experiment aimed at reconstructing the field-measured values recorded in Jena in year 2007. This year was chosen as a proxy of average climatic conditions at the Jena field site. Note that owing to the higher evapotranspiration rates in the Ecotron, higher precipitation (+28%) was imposed in the Ecotron relative to year 2007. Precipitation was applied to each lysimeter by manual watering using a hose equipped with a sprinkler and a flowmeter. The water was purified by microfiltration (1µ) and reverse osmosis followed by ozone and UV treatments. Precipitation values are provided as cumulative values from 28th of March 2012 to 30th of July 2012.

Description: This dataset contains measurements of soil water content from the 12 macrocosms used 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 from the Jena Experiment 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 from the twelve plots 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. Soil water contents used for estimating root water uptake was measured with time domain reflectometry (TDR) sensors TRIME-PICO 32 (IMKO Micromodultechnik GmbH, Germany), which were placed horizontally in six soil depths into the soil monoliths (one sensor at 10 cm, 20 cm, 30 cm, 60 cm, 100 cm, and 140 cm, respectively). The used sensors had a rod length of 110 mm and a rod diameter of 3.5 mm. The total length of the sensor was 328 mm. The measurements were taken every minute from mid of June to end of July (13.06.-28.07.2012). The sensors were installed with a minimum distance of 22 cm to other sensor installations, and the edge of the lysimeter steel wall. As with the other climatic variables in the Jena-Ecotron experiment, we aimed to recreate the environmental conditions measured in the field at the Jena Experiment site in year 2007.

Description: This dataset contains measurements of radiation, air relative humidity and air temperature from the 12 macrocosms used 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 from the Jena Experiment 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 m**2 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 from the twelve plots 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. The respective climatic variables imposed in the Ecotron facility during the Jena-Ecotron experiment aimed at reconstructing the field-measured values recorded in Jena in year 2007. This year was chosen as a good proxy of average spring-summer conditions in the Jena Experiment. In order to recreate the air temperature and humidity daily profiles from the Jena Experiment, field-recorded weather data was used as set points in the Ecotron (at 10 min interval). Air relative humidity and temperature were measured with the DT269 digital sensor (Michell 706 Instruments Ltd, Ely, UK) in the outlet conduit of the macrocosm, 20 cm above lysimeter/experimental ground level. Radiation was measured outside of the Ecotron domes with a BF5 Sunshine Sensor (Delta-T Devices, Cambridge, UK) 1.5m above the lysimeter/experimental ground level. The radiation within the macrocosms was calculated as 56% of the outside measured values based on the fact that 44% of the radiation was blocked by the Ecotron cover and the additional mesh installed in order to bring the radiation values to the same level as in the Jena Experiment. All tree time series are given with a temporal resolution of 12 min from 28th of March 2012 to 31st of July 2012.

Description: This dataset contains leaf water potential measured at predawn and midday of species 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. Leaf water potential was measured with a pressure chamber (Pressure Chamber Instruments Model 600, PMS Instrument Company, Albany, Oregon, USA) at predawn (04:00 am - 06:00 am) and midday (01:30 pm - 04:00 pm) for the most abundant species on each plot from 17-19 July 2012. Two to three species were chosen in the 4-species mixtures, and five to eight species in the 16-species mixtures. Measurements were carried out on young, but fully expanded leaves from four individuals per species per plot. Stomatal conductance and leaf greenness were measured on three leaves from different individuals of all available species in each experimental unit. Values of leaf water potential of the individual measurements were averaged per species per plot.

Description: This dataset contains vegetation and soil surface cover 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. Cover abundance per sown species and total cover of sown species, weeds, bare ground, mosses, litter were estimated on the whole Ecotron-plot area shortly before the destructive harvest of the experimental units on 15 July 2017. Species-level data were estimated using decimal scale (Londo 1976): 1: 〈= 1%, 2: 〈= 5%, 10: 6-15%, 20: 16-25%, 30: 26-35%, 40: 36-45%, 50: 46-55%, 60: 56-65%, 70: 66-75%, 80: 76-85%, 90: 86-100%). Community cover data were directly estimated as percentage (cover 〈1% is coded as 0.5).