Long‐term study of root biomass in a biodiversity experiment reveals shifts in diversity effects over time

Abstract

Biodiversity experiments generally report a positive effect of plant biodiversity on aboveground biomass (overyielding), which typically increases with time. Various studies also found overyielding for belowground plant biomass, but this has never been measured over time. Also, potential underlying mechanisms have remained unclear. Differentiation in rooting patterns among plant species and plant functional groups has been proposed as a main driver of the observed biodiversity effect on belowground biomass, leading to more efficient belowground resource use with increasing diversity, but so far there is little evidence to support this. We analyzed standing root biomass and its distribution over the soil profile, along a 1-16 species richness gradient over eight years in the Jena Experiment in Germany, and compared belowground to aboveground overyielding.In our long-term dataset, total root biomass increased with increasing species richness but this effect was only apparent after four years. The increasingly positive relationship between species richness and root biomass, explaining 12% of overall variation and up to 28% in the last year of our study, was mainly due to decreasing root biomass at low diversity over time. Functional group composition strongly affected total standing root biomass, explaining 44% of variation, with grasses and legumes having strong overall positive and negative effects, respectively. Functional group richness or interactions between functional group presences did not strongly contribute to overyielding. We found no support for the hypothesis that vertical root differentiation increases with species richness, with functional group richness or composition. Other explanations, such as stronger negative plant-soil feedbacks in low-diverse plant communities on standing root biomass and vertical distribution should be considered.Currently, there is consensus about the importance of plant species diversity for ecosystem functioning (Hooper et al. 2005, Cardinale et al. 2012. Plant diversity is known to increase aboveground productivity, leading to overyielding of plant species in mixtures compared to monocultures (Cardinale et al. 2012). Aboveground overyielding has often been shown to increase over time Berendse 2009, Reich et al. 2012). A meta-analysis of biodiversity experiments showed that aboveground overyielding is mainly attributable to mathematically derived complementarity effects (Cardinale et al. 2007), but the biological underlying mechanisms are poorly understood. Complementarity effects have been hypothesized to occur not only aboveground (for example, by means of canopy stratification) but also belowground, by means of spatial niche differentiation. Here, root systems of different plant species or functional groups segregate vertically, leading to a more complete exploitation of belowground resources (Walter 1971,

Plant diversity increases soil microbial activity and soil carbon storage

“…In contrast, the positive plant diversity effect on soil carbon storage (Table 1), root biomass 39 and soil microbial biomass 40 was also found for deeper soil layers in The Jena Experiment. Despite the consistency of the plant diversity effect, it may be strongest in the topsoil and decrease with soil depth 25,39 , which may be due to the fact that root carbon inputs 24,39 and microbial activity 41 decrease with soil depth. Therefore, the mechanism proposed for the topsoil is very likely to also be relevant for deeper soil layers, but the decreased biological activity at the deeper soil layers has to be taken into account.…”

“…Eisenhauer et al 20 and Ravenek et al 39 showed that plant species richness was not closely linked to basal respiration and root standing biomass in the first years of the experiment; therefore, we did not use data from the first 2 years in the present analyses. Missing values were replaced by the mean.…”

“…Longer term, we expect this link to become even more important through both an increase of the positive plant diversity effect on plant (root) biomass production 6,39 and an increased microbial contribution to soil carbon storage by elevated soil microbial activity 20 . Both mechanisms in concert might be responsible for an increase of the plant diversity effect on carbon storage, as indicated by the time series path model ( Supplementary Fig.…”

Linking ecology and plant pathology to unravel the importance of soil-borne fungal pathogens in species-rich grasslands

“…Although there is now consensus among ecologists that in general monocultures perform less well than plant species mixtures (Cardinale et al 2012), the main underlying mechanisms remain debated. On one hand, ecologists have focused on plant-plant interactions, with resource partitioning and facilitation between plants as the most likely explanations for the differential plant productivity (Jesch et al 2018;Mueller et al 2013;Ravenek et al 2014;Wright et al 2017). On the other hand, interactions between plants and pathogenic soil biota have emerged as an important mechanism for the positive biodiversity effect (de Kroon et al 2012;Maron et al 2011;Mommer et al 2018;Schnitzer et al 2011).…”

“…The spread of Phytopthora blight between pepper rows was reduced with a higher degree of root intermingling and even totally blocked in the treatment with the tightest root intermingling. Root systems in species-rich grassland communities are often dense and tightly intermingled as well (Kesanakurti et al 2011;Ravenek et al 2014;Frank et al 2015), thus potentially reducing spread of fungal pathogens.…”