Most work on plant community ecology has been performed above ground, neglecting the processes that occur in the soil. DNA metabarcoding, in which multiple species are computationally identified in bulk samples, can help to overcome the logistical limitations involved in sampling plant communities belowground. However, a major limitation of this methodology is the quantification of species’ abundances based on the percentage of sequences assigned to each taxon. Using root tissues of five dominant species in a semi‐arid Mediterranean shrubland (Bupleurum fruticescens, Helianthemum cinereum, Linum suffruticosum, Stipa pennata and Thymus vulgaris), we built pairwise mixtures of relative abundance (20%, 50% and 80% biomass), and implemented two methods (linear model fits and correction indices) to improve estimates of root biomass. We validated both methods with multispecies mixtures that simulate field‐collected samples. For all species, we found a positive and highly significant relationship between the percentage of sequences and biomass in the mixtures (R2 = .44–.66), but the equations for each species (slope and intercept) differed among them, and two species were consistently over‐ and under‐estimated. The correction indices greatly improved the estimates of biomass percentage for all five species in the multispecies mixtures, and reduced the overall error from 17% to 6%. Our results show that, through the use of post‐sequencing quantification methods on mock communities, DNA metabarcoding can be effectively used to determine not only species’ presence but also their relative abundance in field samples of root mixtures. Importantly, knowledge of these aspects will allow us to study key, yet poorly understood, belowground processes.

Roots are assumed to play a major role in structuring soil microbial communities, but most studies exploring the relationships between microbes and plants at the community level have only used aboveground plant distribution as a proxy. However, a decoupling between belowground and aboveground plant components may occur due to differential spreading of plant canopies and root systems. Thus, soil microbe-plant links are not completely understood. Using a combination of DNA metabarcoding and spatially explicit sampling at the plant neighbourhood scale, we assessed the influence of the plant root community on soil bacterial and fungal diversity (species richness, composition and b-diversity) in a dry Mediterranean scrubland. We found that root composition and biomass, but not richness, predict unique fractions of variation in microbial richness and composition. Moreover, bacterial b-diversity was related to root b-diversity, while fungal b-diversity was related to aboveground plant b-diversity, suggesting that plants differently influence both microbial groups. Our study highlights the role of plant distribution both belowground and aboveground, soil properties and other spatially structured factors in explaining the heterogeneity in soil microbial diversity. These results also show that incorporating data on both plant community compartments will further our understanding of the relationships between soil microbial and plant communities.

1. Water is the most limiting resource for plant survival and growth in arid environments, but the diversity of water-use strategies among coexisting species in dryland communities is not well understood. There is also growing interest in assessing whether a whole-plant coordination exists between traits related to water-use and the leaf economic spectrum (LES).2. We used water stable isotopes (δ 2 H, δ 18 O) to quantify water uptake proportions from different soil depths by 24 species in a Mediterranean shrubland. Leaf traits associated with water-use efficiency, stomatal regulation (δ 13 C, δ 18 O) and the LES (SLA, N, P, K concentrations) were also measured. We assessed potential trade-offs between the above-mentioned leaf traits, water uptake depth and their relationship with species abundance.3. We found distinct ecohydrological niche segregation among coexisting species.Bayesian models showed that our shrubland species used a median of 37% of shallow soil water (0-30 cm) and 63% of deep water (30-100 cm). Still, water source proportions varied considerably among species, as shallow soil water-use ranged from a minimum of 6.4% to a maximum of 68%. Interspecific variability in foliar carbon investment (SLA) and nutrient concentrations was remarkably high, indicating diverse nutrient-use strategies along the LES. Leaf δ 18 O, δ 13 C and δ 15 N values also differed widely among species, revealing differences in stomatal regulation, water-use efficiency and nitrogen acquisition mechanisms.After accounting for evolutionary history effects, water uptake depth was coordinated with the LES: species using shallower soil water from fertile topsoil layers exhibited a more acquisitive carbon-and nutrient-use strategy, whereas water uptake from deeper but less fertile soil layers was linked to a more conservative nutrient-use strategy. Leaf-level water-use traits significantly influenced species abundance, as water-savers with tight stomatal regulation and high water-use efficiency were dominant. Synthesis.Greater utilisation of water stored in nutrient-rich topsoil layers favoured a more acquisitive nutrient-use strategy, whereas a deeper water uptake

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