Anthropogenic exploitation of the natural environment causes dramatic changes in the composition of ecological communities (Chapin et al. 2000; Hooper et al. 2005). The decline of plant diversity is one of the major issues in this context, as plants are the primary producers in terrestrial ecosystems. After two decades of conducting biodiversity experiments, it is well established that plants produce more above-ground biomass when they grow in mixtures than would be expected from their monocultures (Cardinale et al. 2007). Enhanced biomass production in diverse communities has been explained by selection and/or complementarity effects (Loreau & Hector 2001). The selection effect assumes that the most productive species dominates the biomass of the species mixtures as a result of chance. Complementarity or facilitation occur when species exhibit niche differentiation allowing for resources capture that is complementary in space or time. Cardinale et al. (2007) statistically showed that above-ground overyielding is largely due to species complementarity.
To date it is unclear whether above-ground overyielding and complementarity are mirrored underground, as most experiments have focused on shoot biomass only. Only a few biodiversity studies quantified total root biomass and report mixed results, ranging from 50% below-ground overyielding (Tilman et al. 2001; Dimitrakopoulos & Schmid 2004; Reich et al. 2004) to no overyielding (Spehn et al. 2000; Gastine, Scherer-Lorenzen & Leadley 2003), or even reduced biomass partitioning to roots in mixtures compared to monocultures (Bessler et al. 2009). Until now, below-ground complementarity has never been studied in diverse communities, as relative abundance at the species level was unknown for root mixtures. This omission was largely due to a lack of technical possibilities to explore root distribution in diverse communities. We recently developed a molecular method to quantify the proportional abundance of different species in mixed-species root samples (Mommer et al. 2008), allowing for tests of below-ground complementarity.
One commonly proposed mechanism to reach functional complementarity is vertical niche differentiation below-ground. This, for example, is assumed to occur between deep-rooting dicots and shallower-rooting grasses (Parrish & Bazzaz 1976; Berendse 1981, 1983; Dimitrakopoulos & Schmid 2004; von Felten & Schmid 2008; Levine & HilleRisLambers 2009). Complementarity would increase if roots of a given species grew away from zones of intense nutrient competition with neighbours (Gersani, Abramsky & Falik 1998; Semchenko, John & Hutchings 2007b), leading to a further vertical segregation of roots of the different species, reminiscent of the two barnacle species in the classic example of realized niche differentiation (Connell 1961). Nutrient depletion would then be a driving force behind niche differentiation (Casper, Schenk & Jackson 2003). Dicots are thus expected to root even deeper and grasses even shallower in species-rich mixtures than in monocultures. Such functional complementarity in vertical root distribution of different species should in theory lead to a deeper and more even distribution of roots for the community as a whole, and may lead to below-ground overyielding. This overproduction allows a more complete exploitation of the soil column and its available nutrients and in the end a higher above-ground biomass production.
Here, we report about a biodiversity experiment with four grassland species – two grasses and two dicots – with a below-ground focus. We use the molecular technique of Mommer et al. (2008) to unravel species abundances in root mixtures to test the following specific hypotheses: (i) vertical root distribution will be different among the four species, and these differences will be more pronounced in mixtures than in monocultures, leading to vertical segregation of root systems of the different species. As it takes time for root systems to establish, the vertical segregation will increase over time. (ii) Vertical segregation of root systems results in a deeper and more even distribution of roots at community level, and leads to below-ground overyielding.
As net nitrogen mineralization typically decreases with soil depth, most of the roots are present in the topsoil layer. Therefore, competition for nutrients, and thus nutrient depletion, is expected to be most intense in the topsoil. To investigate whether rooting patterns were driven by soil nutrient depletion, we included a treatment with a nutrient-poor topsoil layer, to simulate intense nutrient competition and experimentally speed up vertical niche differentiation. The third hypothesis is, therefore, as follows: (iii) If nutrient depletion is intensifying root competition and driving vertical niche differentiation, segregation of rooting patterns of the species should be enhanced in the poor topsoil treatment compared to the nutrient-rich control.