The neutral rhizosphere – no role for priming effects in biosphere–atmosphere feedbacks?

Feng et al. challenge the generality of the link between plant-productivity responses to climate change and SOM decomposition by showing that, for several climate change factors…rhizosphere priming responses are statistically neutral. One established hypothesis regarding priming-mediated carbon feedbacks between ecosystems and the atmosphere is that increases in net primary productivity in response to elevated CO2 or warming are offset by accelerated SOM decomposition (Cheng et al., 2014). This negative plant effect on SOM stability may occur when greater plant growth is associated with increased root exudation, which stimulates microbial nutrient mobilization from SOM to support continued plant growth. Indeed, rhizosphere priming is controlled by photosynthesis and the supply of recent photo-assimilates to the soil (Kuzyakov Mueller et al., 2016; Huo et al., 2017), supporting the hypothesis that climate change driven increases in net primary productivity simultaneously promote mobilization of SOM stocks, thereby adding another level of complexity to predictions of global scale carbon feedbacks between the biosphere and atmosphere. However, in their meta-analysis, Feng et al. challenge the generality of the link between plant-productivity responses to climate change and SOM decomposition by showing that, for several climate change factors, including elevated CO2, warming, and changes in precipitation, rhizosphere priming responses are statistically neutral. Across studies, processes that stimulate and suppress SOM decomposition in rooted soils counterbalanced each other, reflecting divergent plant–microbe responses to climate change across the various studied systems. Feng and colleagues further show that rhizosphere priming responses and plant biomass responses to climate change are largely decoupled. Although climate change factors such as elevated CO2 and increased precipitation significantly stimulated plant growth, no significant relationships were detected between rhizosphere priming responses and plant growth responses. This unexpected pattern may reflect that enhanced plant growth and root exudation not only stimulate microbial nutrient mobilization from SOM but also intensify competition for nutrients between plants and soil microbes (Xu et al., 2023). In addition, factors beyond the quantity of labile carbon released from roots or plant nutrient uptake likely act as important but underappreciated mediators of plant–microbe interactions under climate change. These may include the composition and quality of root exudates, shifts in microbial community structure, consumer control within soil food webs, and physical or redox constraints within the soil matrix (Angst et al., 2024; Lacroix et al., 2025). Previous syntheses evaluating the direction and magnitude of rhizosphere priming across ecosystems found that positive priming (stimulated SOM decomposition) is much more common than negative priming (Cheng et al., 2014; Huo et al., 2017). Consequently, negative priming was often dismissed as a rare or exceptional outcome, with potentially little relevance for large-scale application. However, when focusing on changes in priming in response to environmental change rather than on baseline priming, it becomes evident that plant–microbe interactions that suppress SOM decomposition play a relevant role. Feng et al. show that stimulations and suppressions of rhizosphere priming in response to climate change are similarly frequent and strong. This indicates that plant–microbe interactions that suppress SOM decomposition are not rare exceptions but pervasive processes that, although only sometimes causing negative priming, frequently constrain the magnitude of positive priming. Feng and colleagues do not show that rhizosphere priming is unresponsive to climate change factors. Rather, their results indicate that the direction and magnitude of priming responses are highly variable and system-specific, suggesting that interactions among plant type and associated traits, physicochemical soil properties, soil microbial communities, and most likely also soil faunal communities, largely determine these responses. At the same time, the uneven representation of ecosystem types in their synthesis highlights persistent knowledge gaps in our understanding of the role of rhizosphere priming in biosphere–atmosphere feedbacks. The vast majority of studies have examined priming responses to climate change in temperate grasslands with mineral soils, whereas tropical, boreal, and arctic biomes – as well as forest ecosystems and organic soils – are poorly represented or entirely absent from current datasets. We argue that resolving rhizosphere priming responses to climate change is particularly urgent in ecosystems that are either highly sensitive to climate change, such as cold temperate and arctic regions, or those that store large amounts of soil carbon, including peatlands and other ecosystems harboring organic-rich soils (Mueller Friggens et al., 2025). In these systems, even regionally constrained or ecosystem-specific feedbacks are likely to exert disproportionately large influences on the global soil–atmosphere carbon exchange. With few exceptions, studies investigating rhizosphere priming have been limited to individual plant species or monocultures in controlled pot experiments. As a result, these studies primarily aim to capture how plant phenotypes or phenotypic plasticity relate to priming. However, in natural ecosystems, the responses of biota and biological communities to global change are not limited to phenotypic plasticity; they also involve mid- to long-term intraspecific adaptation through selection and evolution, and short- to mid-term community-level shifts in species composition and functional diversity through species sorting. This adaptive capacity of biological systems may induce much larger changes in rhizosphere plant–microbe interactions and carbon cycling than those observed in single-species studies. Future research should therefore prioritize multi-species assemblages and long-term field experiments to develop a more realistic mechanistic understanding of rhizosphere priming and to better capture ecosystem-level responses to climate change. Multi-site experiments along environmental gradients and global network experiments would be well-suited tools for achieving a high level of process understanding, enabling the parameterization of rhizosphere processes in Earth system models. Peter Mueller acknowledges funding by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) in the framework of the Emmy Noether program (502681570). Kai Jensen acknowledges funding by DFG in the framework of the Research Training Group 2530 (407270017). The New Phytologist Foundation remains neutral with regard to jurisdictional claims in maps and in any institutional affiliations.