Plant, mycorrhizal and saprotrophic fungal contributions to soil organic matter fractions differ between tree-mycorrhizal types

Determining the biotic factors most responsible for the formation and stability of soil organic matter (SOM) is critical for understanding soil carbon storage in the wake of shifts in forest composition. We investigated the relative contributions of plant and microbial residues to SOM fractions in temperate forest stands dominated by arbuscular or ectomycorrhizal trees, and exposed to simulated N deposition. Using a Bayesian mixing model informed by stable isotope data, we partitioned contributions of plants, saprotrophic fungi and mycorrhizal fungi to particulate and mineral-associated organic matter fractions under contrasting tree-mycorrhizal dominance. Additionally, we explored bacterial and fungal contributions to both fractions using amino sugar biomarkers extracted from the same soils. Overall, ectomycorrhizal-dominated stands differed from arbuscular mycorrhizal-dominated stands in their particulate and mineral-associated organic matter distributions and sources. Carbon and nitrogen isotope analysis revealed that particulate organic matter was dominated by plant residues in both ectomycorrhizal (76%) and arbuscular mycorrhizal stands (59%), with greater plant contributions occurring in topsoil (0-15 cm) relative to subsoil (15-30 cm). In contrast, mineral-associated organic matter was dominated by fungal residues, with mycorrhizal residues contributing most in ectomycorrhizal stands (58%) and saprotrophic residues in arbuscular mycorrhizal stands (55%). Amino sugar analyses showed that contributions of fungi exceeded those of bacteria by several-fold in both SOM fractions. Under nitrogen deposition, microbial contributions slightly increased in arbuscular mycorrhizal stands, suggesting enhanced SOM turnover, whereas plant inputs increased in the particulate fraction in ectomycorrhizal stands. Collectively, our results highlight the key influence of tree-mycorrhizal dominance on the distribution and composition of SOM fractions, with consequences for soil carbon storage in temperate forests.