Plant diversity can alter soil carbon stocks, but the effects are difficult to predict due to the multitude of mechanisms involved. We propose that these mechanisms and their outcomes can be better understood by testing how plant diversity affects particulate organic matter (POM) and mineral-associated organic matter (MAOM) depending on whether MAOM storage is "saturated" and the total soil organic matter pool is limited by plant inputs. Such context-dependency of plant-diversity effects on POM, MAOM, and total soil organic matter helps explain inconsistencies in plant-diversity-soil-carbon relationships across studies. Further illumination of this context-dependency is required to better predict consequences of biodiversity losses and gains, and manage ecosystems as carbon sinks and nutrient stores.

Biology Centre of the Czech Academy of Sciences Institute of Soil Biology and Biogeochemistry Na Sádkách 7 37005 České Budějovice Czech Republic

Department of Biological Geological and Environmental Sciences Cleveland State University Cleveland OH USA

German Centre for Integrative Biodiversity Research Halle Jena Leipzig Puschstraße 4 04103 Leipzig Germany

Institute for Environmental Studies Charles University Benátská 2 Praha 2 Prague Czech Republic

Institute of Biology Leipzig University Leipzig Germany

Max Planck Institute for Biogeochemistry POB 100164 Jena 07701 Germany

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