Plant functional trait dissimilarity drives plant mixture effects on fine root biomass and trait variations.

Fine roots are critical for soil resource uptake, yet how declining plant diversity alters fine root biomass (RB) and traits remains unclear. We tested how plant functional trait dissimilarity in shade tolerance, drought tolerance, growth rate, and nitrogen-fixation ability drives variation in root attributes. Using a global meta-analysis, we examined how functional dissimilarity affects RB and community-weighted mean rooting depth (WRD), root length density (RLD), and specific root length (SRL). Shade-tolerant dissimilarity reduced RB and SRL but increased RLD. Drought-tolerant dissimilarity decreased RLD, growth-rate dissimilarity increased SRL, and nitrogen-fixation dissimilarity increased both RB and RLD. Over time, drought-tolerant dissimilarity increasingly suppressed RLD, growth-rate dissimilarity switched from negative to positive effects on RB but from weak positive to negative effects on RLD, and nitrogen-fixation dissimilarity showed progressively stronger positive effects on RB and RLD. With increasing soil depth, shade-tolerant dissimilarity effects intensified (more negative on RB, more positive on RLD), growth-rate dissimilarity effects shifted from positive to negative on RB, and nitrogen-fixation dissimilarity effects on RLD weakened. Climate did not modulate these responses. These findings highlight that functional trait dissimilarity mediates diversity effects on belowground processes and offer insights for enhancing root productivity and ecosystem carbon sequestration.