Productivity in experimental grassland communities is often positively correlated with plant species richness (Tilman et al. 2001; Cardinale et al. 2007; Marquard et al. 2009), and this so-called overyielding has been explained by a more complete use of niche space (Berendse 1982; Fargione & Tilman 2005; Hooper et al. 2005; Levine & HilleRisLambers 2009). The niche complementarity hypothesis has recently been challenged by putting forward an alternative ‘pathogen niche’ hypothesis, suggesting that build-up of specific soil biota decreases plant productivity in species-poor grasslands (Westover & Bever 2001; Bever 2003; Reynolds et al. 2003; Petermann et al. 2008). Results from two recent biodiversity experiments indeed suggest that soil pathogens affect the relationship between plant species diversity and biomass production. Maron et al. (2011) and Schnitzer et al. (2011) showed that the positive relationship between biodiversity and productivity largely disappeared, as removing soil biota by sterilization or fungicide application increased the productivity of the low-diversity treatments more than productivity of the high-diversity treatments. Overyielding in biodiversity experiments, thus, appears to be due to the release from pathogens in mixtures compared to monocultures (Kulmatiski, Beard & Heavilin 2012).
Soil biota have very specific effects on plant growth and species competitive performance, and in general, plants suffer more from their own soil biota than from the soil biota of other plant species (e.g. van der Putten, Van Dijk & Peters 1993; Bever 1994; Kardol et al. 2007; Kulmatiski et al. 2008; Harrison & Bardgett 2010). However, we do not know how these species-specific interactions between plant community and soil community affect overyielding. The experimental treatments of soils as performed by Maron et al. (2011) and Schnitzer et al. (2011) revealed the effects of soil biota on plant diversity–functioning relationships by altering soil communities irrespective of plant species identity and thus left open the question whether these effects were due to a dilution of the species-specific infective potential of the soil.
To answer this question, we combined a biodiversity experiment with a plant–soil feedback experiment and investigated the interactions between soil communities and plant species on overyielding. A plant–soil feedback approach makes use of soil conditioning by plant species affecting the composition and proportional contribution of soil biota in the soil community and testing these effects on plant biomass production in a follow-up experiment (Bever, Westover & Antonovics 1997). We tested the hypothesis that plants growing in soil that was preconditioned by conspecifics produce less biomass than plants growing in a mixture of soils conditioned by heterospecific plant species, thus leading to overyielding in plant species mixtures. To test this hypothesis, we performed the plant–soil feedback experiment with monocultures and mixtures of four plant species that were grown in soil conditioned by single plant species (soil monocultures) or in soil composed of a 1 : 1 : 1 : 1 mixture of those soils (soil mixtures). This factorial design enabled us to study the performance of plant species in monocultures and mixtures as a function of monoculture soils and a mixture of these soils and to quantify any additional effects of mixing plant species on top of effects of mixing species-specific soil biota.
We analysed biomass of plants in all combinations in both non-sterilized and sterilized soils in which all soil biota were eliminated (Brinkman et al. 2010). A limitation of plant–soil feedback experiments is that the results may be influenced by nutrient flushes due to soil sterilization (Kulmatiski et al. 2008) or due to different nutrient uptake during the soil conditioning phase (Kardol, Bezemer & van der Putten 2006). Therefore, we quantified the effects of sterilization on nutrient availability and carried out an additional nutrient experiment to test whether growth limitations under non-sterilized conditions were due to low nutrient availability (Troelstra et al. 2001). Comparing the effects of non-sterilized and sterilized soils on plant biomass gives the opportunity to compare soils that differ in soil biota (and possibly nutrients) with soils that differ in nutrients only. We hypothesized that only the species-specific effects of soil biota (in non-sterilized soil) will result in overyielding, rather than the non-specific effects of differences in mineral nitrogen amongst the sterilized soils, and analysed how the underlying biomass hierarchies of the different species resulted in overyielding.