Soil conditions drive below‐ground trait space in temperate agricultural grasslands

Plant below‐ground organs perform essential functions, including water and nutrient uptake, anchorage, vegetative reproduction and recruitment of mutualistic soil microbiota. Recently, multivariate analyses showed that root traits of species can largely be linked to a ‘conservation’ and a ‘collaboration’ gradient. Here, we tested whether this species‐level bidimensional below‐ground trait space also exists at the community level in grasslands. Furthermore, we tested whether the position of grassland communities in below‐ground trait space relates to environmental variables. For a total of 313 species, we collected data on eight below‐ground traits in greenhouse and common garden experiments and supplemented it with data on bud‐bank size and specific leaf area from databases. We calculated community weighted means (CWMs) of these 10 traits for 150 temperate grassland plots to investigate below‐ground plant‐trait dimensionality and its variation along 10 soil and land‐use parameters. Using PCA, we found that about 55% of variance in CWMs was explained by two main dimensions, corresponding to a mycorrhizal ‘collaboration’ and a resource ‘conservation’ gradient. Frequently overlooked traits such as rooting depth, bud‐bank size and root‐branching intensity were largely integrated in this trait space. The two plant‐strategy gradients were partially dependent on each other, with communities that do ‘outsourcing’ of resource uptake to mycorrhizal fungi along the collaboration gradient also being more ‘slow’ along the conservation gradient. (i.e. high root tissue density and high root weight ratio). ‘Outsourcing’ communities were also more often deep rooting and associated with soil parameters, such as low moisture and sand content, high topsoil pH, high C:N and low δ¹⁵N. ‘Slow’ communities had large bud banks and were associated with low land‐use intensity, high topsoil pH and low nitrate but high ammonium concentration in the soil. Surprisingly, we did not find an association of phosphorus availability with the mycorrhizal ‘collaboration’ gradient. Synthesis. The ‘collaboration’ and ‘conservation’ gradients previously identified among species scale up to the community level in grasslands, encompass more traits than previously described, and vary with the environment.