BayCEER Workshop 2025

Arbuscular mycorrhizal fungal necromass and soil carbon storage: Decomposition, priming effect, and formation of mineral-associated organic matter

Fungal necromass represents a major source of soil organic carbon (C), partly stabilised 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 (I) the fate of AMF necromass-C from Rhizophagus irregularis into MAOM, particulate organic matter (POM), and CO₂, (II) the influence of abiotic soil properties, and (III) effects on microbial communities and changes in the decomposition of soil organic matter. Highly ¹³C-labelled (97 atom%) AMF necromass was incubated in six grasslands soils with contrasting textures and chemical properties. ¹³CO₂ efflux was measured five times. After 11 and 82 days, soils were fractionated to quantify ¹³C in MAOM and POM. Initially, 11–56 % 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 ¹³C in MAOM correlated negatively with clay content, possibly due to varying sorption mechanisms. Soils with more clay showed less ¹³C loss from MAOM with time, suggesting higher stability. Necromass-derived CO₂ reached 23–37 % after 11 days and 43–71 % after 82 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.