The role of mycorrhizal type and plant dominance in regulating nitrogen cycling in Oroarctic soils

Abstract

Mycorrhizal fungi enhance plant access to nitrogen (N) in nutrient-poor environments like the Arctic tundra by depolymerizing N-rich organic compounds into forms available to plants and microbes. As climate change reshapes plant communities and mycorrhizal associations, shifting dominance from herbaceous species to shrubs, changes in mycorrhizal type and plant species dominance may differentially stimulate N cycling. Both dominant and rare species, along with mycorrhizal associations, contribute to ecosystem processes and stability, though the specific roles of these components in N cycling and overall ecosystem functioning remain uncertain. We investigated how mycorrhizal associations and plant diversity affect gross N mineralization and nitrification rates in an Oroarctic ecosystem. Four years after a plant removal treatment, we measured these rates using in situ 15N labelling and quantified a selection of nitrification genes. Treatment plots included (1) unmanipulated (Control); or the removal of: (2) ectomycorrhizal (EcM) and ericoid mycorrhizal (ErM) plants, letting arbuscular mycorrhizal (AM) and non-mycorrhizal (NM) plants dominate (AM/NM); (3) AM and NM plants, letting EcM and ErM plants dominate (EcM/ErM); (4) low-abundance species, leaving the most abundant species (Dominant); and (5) high-abundance species, leaving only the low-abundance species (Rare). Gross N mineralization rates were 73 % and 78 % higher in EcM/ErM and Dominant, respectively, compared to Control, while AM/NM and Rare showed more moderate increases of 30 % and 46 %. Gross nitrification was also highest in EcM/ErM, with a 26 % increase over Control. Gene abundances did not mirror nitrification patterns. Archaeal ammonia oxidizers (AOA), Nitrospira-type nitrite oxidizers (NIS), and comammox clade A (ComaA) were consistently more abundant than bacterial ammonia oxidizers (AOB), Nitrobacter-type nitrite oxidizers (NIB), and comammox clade B (ComaB), suggesting a stable site-level nitrifier community. Dominant had the lowest gene copy numbers overall, except for AOB, which was highest. In addition, AOA gene abundance was significantly lower in Dominant compared to Control, with a marginal reduction observed for NIS. Our findings highlight the key role of EcM/ErM fungi in accelerating N cycling in Oroarctic soils, challenging traditional assumptions that N transformation rates are slow in EcM/ErM dominated ecosystems. These insights underscore the need to consider mycorrhizal associations and plant community composition when predicting tundra ecosystem responses to environmental change.