Plant nitrogen acquisition from inorganic and organic sources via root and mycelia pathways in ectomycorrhizal alpine forests

Abstract Ectomycorrhizal (ECM) fungi associated with plant roots play important roles in facilitating plant nutrient uptake. Ectomycorrhizal trees obtain soil nitrogen (N) through the direct root uptake from the rhizosphere (i.e., root pathway), but also via the extramatrical mycelia of ECM fungi followed by translocation to fine roots (i.e., mycelia pathway). However, estimates of the relative contributions of the root pathway versus the mycelia pathway to plant acquisition of different soil N sources are highly uncertain, leaving a key gap in understanding plant N acquisition strategies and plant-mycorrhiza interactions in forest ecosystems. In this study, by combining an ingrowth core method and in situ 15 N labelling techniques, we quantified the plant N uptake (inorganic vs. organic N) from root and mycelia pathways in two ectomycorrhizal alpine forests: a 70-year-old spruce plantation and a 200-year-old spruce-fir dominated forest in western Sichuan, China. We found that, across the two forests and seasons, plants obtained inorganic N in the soil primarily through the root pathway (81.9%), while acquiring ∼ 44% of organic N via the extramatrical mycelia of ECM fungi. The contribution of the mycelia pathway to plant organic N uptake was significantly enhanced during the non-growing season (52.6%) compared to that during the growing season (35.5%). Our study suggests that the mycelia of ECM fungi are of critical importance in the plant organic N economy in alpine coniferous forests. In addition, our findings highlight that plants associated with ECM fungi can adjust the dominance of the root pathway vs. the mycelia pathway in plant N acquisition dependent on their N demand and soil N availability. This study may provide new insight into understanding the ectomycorrhizal effects on N cycling and the structure and function of forest ecosystems.