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.