Qitong Wang and Huajun Yin, Chengdu Institute of Biology of the Chinese Academy of Sciences, discuss their article: Rhizosphere as a hotspot for microbial necromass deposition into the soil carbon pool
When it comes to soil organic carbon (SOC) storage, the rhizosphere – a zone of soil around plant roots – plays a pivotal yet often underappreciated role. The microbes in the rhizosphere are fueled by the compounds released by roots, these compounds stimulate microbial activity and drive key processes in soil carbon cycling. Among these processes is the formation of microbial necromass – the organic remains of dead microorganisms. These microbial residues may form a stable pool of carbon, playing a critical role in long-term SOC sequestration.
At the heart of our understanding of SOC storage in the rhizosphere lies the soil Microbial Carbon Pump (MCP) concept, which explains how microbial anabolic processes contribute to soil organic matter (SOM) formation. This concept challenges the traditional views that plant-derived inputs dominate the SOC pool, with growing evidence that SOC is formed not only by plants but is also heavily reliant on microbial anabolism. This further inspires the idea that microbial necromass plays a more pronounced role in SOC accrual by iterative accumulation in microbial “hotspots”, like the rhizosphere. Naturally, this raises the question: do these microbial hotspots exhibit a more efficient MCP?
To answer this question, we engaged in extensive discussions with Dr. Chao Liang from the Shenyang Institute of Applied Ecology, Chinese Academy of Sciences, and explored whether parts of the rhizosphere with fast microbial growth might be particularly effective in building SOM. Professors Ziliang Zhang from Northwestern Polytechnical University, Biao Zhu from Peking University, and Hans Lambers from the University of Western Australia further helped refine this research plan. We aimed to understand how these microbial contributions to SOC may shift between the rhizosphere and bulk soil to provide critical insights for predicting SOC dynamics and informing strategies to manage soils.
To this end, we undertook a comprehensive field study in alpine coniferous forests on the Qinghai-Tibet Plateau. This unique region provides an ideal setting to study soils because it has varying nutrient levels and environmental conditions. We collected and analyzed samples from the rhizosphere and surrounding bulk soil at 39 sites, enabling a direct comparison of microbial contributions to SOC. By measuring the levels of SOC and amino sugars (specific biomarkers of microbial necromass), we revealed that rhizosphere soil indeed accumulates more microbial necromass, which further boosts SOC storage. This accumulation is closely tied to soil nutrient levels: higher nutrient availability increases microbial efficiency, which, in turn, enhances SOC formation.
Our work builds on and reinforces the MCP framework, providing solid empirical evidence at a regional scale to show how rhizosphere microbial anabolism significantly contributes to SOC storage. While plant-derived inputs remain important, the MCP can shift microbes in the rhizosphere to role of primary SOC contributor, particularly in microbial hotspots. Understanding these microbial contributions is crucial for addressing the global challenge of climate change. Soils are one of the largest carbon reservoirs on Earth, and their ability to act as carbon sinks depends on microbial metabolisms and the nutrient dynamics that support them. As environmental pressures intensify, ensuring efficient microbial anabolism and necromass production will be essential for SOC storage.
By integrating the MCP concept with our understanding of the rhizosphere, our research highlights the critical role of the rhizosphere in the global carbon cycle. The rhizosphere emerges as a dynamic interface where plants and microbes interact to secure carbon in the soil, with microbial necromass acts as a cornerstone in SOC formation and stabilization. As the world changes with rising temperatures and shifting ecosystems, leveraging the MCP and understanding the rhizosphere’s contributions could unlock new opportunities for enhancing SOC storage. This hidden hub beneath our feet has the large potential to shape more resilient ecosystems and mitigate climate change.