Fungi mediate long term sequestration of carbon and nitrogen in soil through their priming effect

It is increasingly recognized that soil microbes have the ability to decompose old recalcitrant soil organic matter (SOM) by using fresh carbon as a source of energy, a phenomena called priming effect (PE). However, efforts to determine the consequences of this PE for soil carbon and nitrogen dynamics are in their early stage. Moreover, little is known about the microbial populations involved. Here we explore the consequences of PE for SOM dynamics and mineral nitrogen availability in a soil incubation experiment (161 days), combining the supply of dual-labeled (¹³C and ¹⁴C) cellulose and mineral nutrients. The microbial groups involved in PE were investigated using molecular fingerprinting techniques (FAMEs and B- and F-ARISA). We show that mean residence time of SOM pool controlled by the PE decreased from 3130 years in the subsoil, where the availability of fresh carbon is very low, to 17-39 years in the surface layer. This result suggests that the decomposition of this recalcitrant soil C pool is strictly dependent on the presence of fresh C and is not an energetically viable mean of accessing C for soil microbes. We also suggest that fungi are the predominant actors of cellulose decomposition and induced PE and they adjust their degradation activity to nutrient availability. The predominant role of fungi can be explained by their ability to grow as mycelium which allows them to explore soil space and mine large reserve of SOM. Finally, our results support the existence of a bank mechanism that regulates nutrient and carbon sequestration in soil: PE is low when nutrient availability is high, allowing sequestration of nutrients and carbon; in contrast, microbes release nutrients from SOM when nutrient availability is low. This bank mechanism may help to synchronize the availability of soluble nutrients to plant requirement and contribute to long-term SOM accumulation in ecosystems.