Long-term root standing biomass development in a biodiversity experiment does not show evidence for vertical niche differentiation belowground

Background/Question/Methods

It is well known that plant biodiversity positively affects ecosystem functioning at multiple trophic levels aboveground and belowground. Evidence is accumulating that the higher biomass production in plant mixtures compared to monocultures (overyielding), a general phenomenon aboveground, also occurs belowground. A major hypothesis underlying overyielding is belowground niche differentiation, with different plant species - or different plant functional groups - having different, more segregated rooting patterns in mixtures than in monocultures. However, very few studies exist that test this hypothesis in (semi-)natural environments. We questioned: How does root standing biomass develop over time in a long-term biodiversity experiment? Do we find evidence at the community level for vertical niche differentiation in rooting patterns? We analyzed root standing biomass in the 0-30 cm soil profile of a diversity gradient (1-16 species; 4 functional groups) over nine years in the Jena Experiment (Germany). We compared above- and belowground effects of diversity on plant production along the diversity gradient. If spatial niche differentiation would occur, we expected increasing mean rooting depth and increasing evenness of root biomass over the soil profile with increasing diversity. 

Results/Conclusions

Aboveground overyielding occurred immediately from the first growing season onwards. Belowground overyielding was three years delayed compared to aboveground, as we found a positive effect of biodiversity on root standing biomass first in the fourth growing season. This positive relationship became progressively stronger towards the ninth year of the experiment. Plant functional group presence had little additional effect on root standing biomass on top of species richness effects. This belowground overyielding does not appear to be the result of spatial niche differentiation. Mean rooting depth over the 0-30 cm soil profile was 9.8 ± 0.30 cm over all communities, and was not affected by species or functional group richness. Evenness in root biomass decreased rather than increased with increasing diversity. We conclude that evidence for belowground vertical niche differentiation is lacking in the 0-30 cm soil profile of the Jena Experiment even after nine years, although we cannot rule out differential root uptake activities of the species involved. Alternative explanations for overyielding, such as temporal or chemical niche differentiation, and plant-soil feedback, should thus be considered.