Abstract: Microbes are increasingly recognized as important sources of soil organic matter (SOM), contributing to its accumulation through the production and deposition of dead cells as well as products of their metabolic reactions (i.e., necromass). Fungi can comprise over 90% of living microbial biomass in some systems, and are estimated to contribute ~70% of microbial necromass carbon (C) in soils, and may therefore be a significant source of microbial-derived SOM. Yet, we lack a framework for explaining their contributions to SOM, especially in relatively persistent pools, such as mineral-associated organic matter (MAOM). In this experiment, we characterized a suite of physiological, biochemical, and morphological traits of eight fungal isolates spanning three phyla (Basidiomycota, Ascomycota, Mucoromycotina) and assessed their contributions to functional pools of SOM (e.g., total SOM, particulate OM, MAOM, aggregates). We used artificial soils that were initially SOM-free, incubated with a simple C substrate (glucose), such that any SOM that formed could be directly linked to a fungal isolate and its corresponding traits. The eight fungal isolates span a gradient of traits, differing in their biomass chemistry (P = 0.012), extracellular enzyme profile (P = 0.001), respiration rate (P < 0.001), growth rate (P < 0.001), hyphal surface area (P < 0.001) and carbon use efficiency (CUE; P < 0.001).
Total soil C (%) was highest in soils grown with Basidiomycota and lowest in the Mucoromycotina soils (P = 0.002). Isolates differed in their contributions to MAOM-C, POM-C and water-stable aggregates, with the highest MAOM-C and aggregate values observed for Basidiomycota and Ascomycota isolates. Of the measured fungal trait variables, CUE best predicted variation in MAOM-C (PLSR; highest VIP). Interestingly, when sequential chemical extractions were conducted to acquire a relative metric of SOM chemical stability, the Basidiomycota isolates had the lowest remaining levels of chemically “stable” SOM, whereas the Ascomycota had the highest. Several Ascomycete taxa have emerged as the strongest contributors to multiple pools of relatively persistent SOM (MAOM, aggregates, chemically stable SOM). In addition, fungal isolates generated SOM with distinct chemistries (P < 0.001), including in the MAOM pool (P = 0.001). Variation in the chemical compounds produced by fungi, and their relative stabilities, may help to explain differences in isolate contributions to persistent pools of SOM. Our study provides critical insight into the relative importance of fungal traits to SOM development, with implications for understanding the mechanisms by which microbes contribute to SOM formation and persistence.
