The Role of AMF in Living Mulch Systems: The Potential for AMF as a Bridge in Root-Hyphae-Root Nitrogen Exchange

Cover crops in farming systems are traditionally integrated into crop rotations to enhance root access to resources in top- and subsoils, particularly nutrients and water. Living mulch systems, functioning as cover crop, also referred to as strip-tillage systems, involve low-growing, often perennial, ground cover (like clover) beneath or between cash crop rows, creating a living soil cover during the growing season – a strategy to diversify crop management to agricultural polycultures. Living mulch systems not only improve soil structure at the physical level, but also create opportunities for diverse rhizosphere microbial interactions. However, current research provides only limited insight into the role of soil microorganisms in living mulch systems, particularly under field conditions where system complexity is substantially higher. Besides the implications of direct root-root or root-bacteria-root interactions, plants can also interact belowground via their mycorrhizal partners. As one of the most effective endosymbiotic fungi, arbuscular mycorrhizal fungi (AMF) are obligate symbionts that rely on carbon supply provided by their plant host in exchange to nutrients. As this symbiosis is often not species-specific, plant-plant interactions can occur via a shared hyphal network.

We evaluated how living mulch system with white clover affects AMF colonization in maize and hyphal network formation. We hypothesize that a living ground cover of white clover would enhance AMF abundance, diversity, and maize root colonization. To investigate the potential role of AMF in nitrogen exchange between white clover and the maize plants, we designed a sandwich-structured mesh tube system that allows control of AMF hyphae as the only way for isotopic nitrogen transport under field conditions. This setup enables testing whether ¹⁵N-labeled in the white clover can be transferred via the hyphal network of AMF to maize roots. By EA-IRMS we could quantify the one-directional ¹⁵N transfer from white clover to the maize via the hyphal pathway, an observation that was supported by using a combination of PLFA analysis, MBC and MBN measurements, and high-throughput Illumina sequencing. This provides clear evidence that AMF hyphae function as a bridging network facilitating connectivity and N transport between the two plant species of the living mulch system.

In conclusion, our study specifically investigated the role of AMF in living mulch systems, aiming to provide guidance for optimizing plant partner selection for this sustainable agricultural practice.