Soil conditions drive belowground trait space in temperate agricultural grasslands

Plant belowground 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 belowground trait space also exists at the community level
in grasslands. Furthermore, we tested whether the position of grassland
communities in belowground trait space relates to environmental variables.
For a total of 313 species, we collected data on eight belowground 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 ten traits for 150 temperate
grassland plots to investigate belowground plant-trait dimensionality and
its variation along ten 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. In
conclusion, 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.