Rapid allocation of freshly added organic nitrogen to particulate organic matter in ectomycorrhiza- but not arbuscular mycorrhiza-dominated forests

Abstract

Mycorrhizal fungi mediate the partitioning and transformation of organic matter in forest soils worldwide. Ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) fungi differently influence the formation of soil organic matter (SOM) pools, such as decomposable particulate (POM) and persistent mineral-associated organic matter (MaOM). While plant-derived effects were studied extensively, the role of other organic resources in SOM formation across different mycorrhizal systems remains understudied. To address this, we investigated the incorporation of carbon and nitrogen derived from more bioavailable (bacterial and fungal biomass) and less bioavailable (litter, artificial MaOM; a-MaOM) source organic matter into soil POM and MaOM pools in ECM- and AM-dominated soils, using a dual stable isotope (¹³C and ¹⁵N) labelling approach in a temperate experimental forest (MyDiv experiment). We detected ¹⁵N, but not ¹³C label in mycelium and POM across treatments (except a-MaOM) and mycorrhizal types, while no label was detected in soil MaOM. ECM mycelium showed higher ¹⁵N uptake from litter, while AM mycelium accumulated more ¹⁵N after the addition of fungal biomass. POM in ECM systems had a higher ¹⁵N label compared to AM systems in all bacteria, fungi and litter treatments, indicating the role of ECM fungi as important drivers of fresh organic nitrogen allocation in temperate forest soils. The absence of any label in MaOM indicates its slow turnover in Chernozem soil across forests dominated by a single mycorrhizal type. Overall, our study highlights the fast incorporation of freshly added organic nitrogen into the mycelium of mycorrhizal fungi and POM, emphasizing its fast turnover in forest soils.