Arbuscular mycorrhizal fungal necromass: Rapid decomposition and texture-dependent formation of mineral-associated organic matter

Fungal necromass represents a major source of soil organic carbon (C), partly stabilized as mineral-associated organic matter (MAOM). Mycorrhizal fungi, particularly arbuscular mycorrhizal fungi (AMF), are key contributors to soil C input, yet their necromass decomposition and role in MAOM formation remain poorly understood. We investigated, using a range of unsterile soils, (I) the fate of AMF necromass-C from Rhizophagus irregularis into MAOM, particulate organic matter (POM), and CO 2 , (II) the influence of abiotic soil properties, and (III) effects on microbial communities, particularly the fungal-to-bacterial (F/B) ratio. Highly 13 C-labelled ( 97 atom% ) AMF necromass was incubated in six grasslands soils with contrasting textures and chemical properties. 13 CO 2 efflux was measured five times. After 12 and 83 days, soils were fractionated to quantify 13 C in MAOM and POM. Initially, 36.2 ± 12.5 % of necromass-C was found in MAOM, but this declined over time in most soils indicating the dynamic nature of MAOM. Total MAOM-C correlated positively, whereas necromass-derived 13 C in MAOM correlated negatively with clay content, possibly due to varying sorption mechanisms. Soils with more clay showed less 13 C loss from MAOM with time, suggesting higher stability. Necromass-derived CO 2 reached 33.1 ± 4.4 % after 12 days and 60.0 ± 8.5 % after 83 days, indicating rapid microbial utilization. Necromass addition altered microbial communities: Acidic soils showed increased fungal dominance, while alkaline soils showed little to no change. These microbial shifts coincided with altered SOM decomposition. In summary, AMF necromass contributes rapidly to MAOM but decomposes with time simultaneously fuelling respiration and altering microbial dynamics, thus influencing soil C in complex, texture-dependent ways.