Long-term nitrogen-addition-induced shifts in the ectomycorrhizal fungal community are associated with changes in fine root traits and soil properties in a mixed Pinus koraiensis forest

Abstract

Nitrogen (N) deposition has marked effects on the structure and function of global forest ecosystems. Many forest tree species rely upon a symbiotic association with ectomycorrhizal (ECM) fungi to enhance nutrient acquisition, which affects nutrient and carbon (C) dynamics in forest ecosystems. A better understanding of the characteristics of ECM fungal diversity and composition and their primary determinants under N deposition will help us to identify the mechanisms underlying N-induced effects on forest nutrient and C cycling. In the present study, we explored the effects of long-term N addition (10 years) on the diversity and community composition of ECM fungi identified from fine roots of Corylus mandshurica, Pinus koraiensis and Tilia amurensis in a mixed temperate forest on Changbai Mountain in north-eastern China. We observed that N addition decreased the Shannon and Simpson indices of the ECM fungal community colonizing C. mandshurica and T. amurensis. Neither the Chao1 index nor the observed species richness of the ECM fungal community colonizing the three species was impacted by N addition. Nitrogen addition significantly decreased the fine root and stele diameter of C. mandshurica, and it significantly decreased the cortical thickness of fine roots of both P. koraiensis and T. amurensis. Nitrogen addition had a consistently negative effect on the mycorrhizal colonization of fine roots, but it did not affect the branching intensity of fine roots of the three tree species. Nitrogen addition altered the ECM fungal community composition, which was closely associated with fine root traits, such as mycorrhizal colonization, and edaphic factors, such as soil pH, soil total N concentration and soil CN ratio. An altered ECM fungal community composition was accompanied by shifts in the nutrient acquisition ability, as characterized by exploration types of ECM fungi. These findings have important implications for a better understanding of the effects of a changing ECM fungal community on the C and N cycling of forest ecosystems under increasing N deposition.