Linking Root Traits and Soil Organic Matter Pools in a Grassland Diversity Experiment

Plant roots contribute substantially to soil organic matter (SOM) formation via litter inputs, exudation, and stimulation of microbial activity. Increasing plant species richness has been proposed as a strategy to enhance soil organic carbon (SOC) storage by promoting greater variation in root functional traits and associated carbon (C) inputs. Root traits may influence C partitioning between particulate organic matter (POM) and mineral-associated organic matter (MAOM) pools. Specific root length (SRL) reflects a do-it-yourself resource acquisition strategy characterized by fine, short-lived roots that primarily contribute plant-derived inputs, whereas greater root diameter (RD) indicates increased investment in microbial symbionts that can enhance microbial processing and stabilization of C in mineral-associated pools.  

Here, we examined whether root morphological traits associated with the root economic space (RES) collaboration gradient provide a mechanistic link between plant roots and SOM partitioning, and the role of plant diversity therein. We hypothesized that root traits regulate SOC fractions, with SRL positively associated with POM, and RD associated with MAOM. We utilised the 1- and 12-species mixtures in BioCliVE, a large-scale grassland diversity experiment on sandy soil at Utrecht University (Netherlands). We measured community-level root traits for each plot, performed wet sieving to separate soils into particulate and mineral-associated fractions, and quantified C concentrations using elemental analysis to derive particulate organic carbon (POC) and mineral-associated organic carbon (MAOC).  

Initial analyses showed that RD had a marginal positive association with MAOC, supporting the conceptual expectation that root traits linked to microbial collaboration may influence MAOM formation. SRL, however, was not related to POC, contrary to our hypothesis. There was no detectable effect of plant species richness on C stored in either the particulate or mineral-associated pools. Our research demonstrates the complexity of SOM partitioning and suggests that trait-based and diversity-driven controls on SOM are limited or context-dependent in sandy grassland soils. By testing root trait pathways within a diversity experiment, our study contributes to ongoing discussions on the mechanisms governing SOM across ecosystems.