Results
We used data from 98 grassland patches in the Pyrenees (Fig. S1) to
build a Diversity Interaction model (DIM) able to disentangle the effect
of plant guild proportions (identity effects) and pairwise interactions
(guild interaction effects) after controlling the influence of a set of
climate, bedrock, topography and livestock management predictors (Table
S1). Model results and ternary plots showing the predicted SOC values
across grass, legume and forb proportions (Table 1 and Fig. 1) revealed
the complex role of legumes.
Legumes enhanced SOC storage when mixed with either grasses or forbs,
but only at low to moderate legume proportions (Fig. 1). SOC storage
increased with legume proportion, up to 15-20% legumes, depending on
neighbors (Fig. 2). At legume proportions above 15-20%, SOCS decreased
with increased legume proportion (Fig. 2).
The enhancement effect of legumes on SOC storage was higher for grasses
than for forbs (Figs. 1 & 2). The strongest legume enhancement effects
on SOC occurred when grasses were dominant and legume proportion was
around 10-15% (Fig. 2). When forbs were dominant, the enhancement
effect of legumes on SOC storage was lower than in the grass-dominance
or the grass:forb codominance scenarios, with peak values around 15-20%
legumes (Figs 1 & 2). Thus, the SOC maximum peak with legume
enhancement was higher and reached faster (at lower legume proportions)
under grass dominance than under codominance and forb-dominance
scenarios (Figs. 1 & 2).
Differently to grass, legume and forb effects, the effect of sedge
proportion on SOC was independent of the proportions of other plant
guilds. However, this guild was rare and reached low proportions in the
grasslands. Finally, we did not find significant interactions between
plant guilds and other environmental SOC drivers, suggesting that the
revealed diversity-function patterns held under the wide array of
environmental conditions tested.