Plant diversity strongly influences ecosystem functions and services, such as soil carbon storage. However, the mechanisms underlying the positive plant diversity effects on soil carbon storage are poorly understood. We explored this relationship using long-term data from a grassland biodiversity experiment (The Jena Experiment) and radiocarbon (¹⁴C) modelling. Here we show that higher plant diversity increases rhizosphere carbon inputs into the microbial community resulting in both increased microbial activity and carbon storage. Increases in soil carbon were related to the enhanced accumulation of recently fixed carbon in high-diversity plots, while plant diversity had less pronounced effects on the decomposition rate of existing carbon. The present study shows that elevated carbon storage at high plant diversity is a direct function of the soil microbial community, indicating that the increase in carbon storage is mainly limited by the integration of new carbon into soil and less by the decomposition of existing soil carbon.

• Because the phenology of trees is strongly driven by environmental factors such as temperature, climate change has already altered the vegetative and repro-ductive phenology of many species, especially in the temperate zone. Here, we aimed to determine whether projected levels of warming for the upcoming decades will lead to linear changes in the phenology of trees or to more complex responses. • We report the results of a 3-yr common garden experiment designed to study the phenological response to artificial climate change, obtained through experimental warming and reduced precipitation, of several populations of three European oaks, two deciduous species (Quercus robur, Quercus pubescens) and one evergreen species (Quercus ilex), in a Mediterranean site. • Experimental warming advanced the seedlings’ vegetative phenology, causing a longer growing season and higher mortality. However, the rate of advancement of leaf unfolding date was decreased with increasing temperature. Conversely, soil water content did not affect the phenology of the seedlings or their survival. • Our results show that the phenological response of trees to climate change may be nonlinear, and suggest that predictions of phenological changes in the future should not be built on extrapolations of current observed trends.

Significance: Ecosystems are responding to climate change and increasing atmospheric CO₂concentrations. Interactions between these factors have rarely been assessed experimentally during and after extreme climate events despite their predicted increase in intensity and frequency and their negative impact on primary productivity and soil carbon stocks. Here, we document how a grassland exposed to a forecasted 2050s climate shows a remarkable recovery of ecosystem carbon uptake after a severe drought and heat wave, this recovery being amplified under elevated CO₂. Over the growing season, elevated CO₂entirely compensated for the negative impact of extreme heat and drought on net carbon uptake. This study highlights the importance of incorporating all interacting factors in the predictions of climate change impacts.