Mycelia‐derived C contributes more to nitrogen cycling than root‐derived C in ectomycorrhizal alpine forests

Abstract

Abstract Plant roots and their associated microbial symbionts impact carbon (C) and nutrient cycling in ecosystems, but estimates of the relative contributions of root‐ versus microbe‐derived dynamic inputs are highly uncertain. Roots release C into soil via exudation and turnover (i.e., root‐derived C), but also by allocating C to mycorrhizal fungal mycelia, which exude C and undergo turnover (i.e., mycelia‐derived C). Given that the relative contributions of root‐ and mycelia‐derived C inputs are unknown, a key knowledge gap lies in understanding not only the relative contributions of root‐ versus mycelia‐derived C inputs, but also the consequences of these fluxes on nutrient cycling. Using ingrowth cores and stable isotope analyses, we quantified root‐ and mycelia‐derived C inputs into the soil and their relative contributions to nitrogen (N) cycling in two ectomycorrhizal alpine forests, a 70‐year‐old spruce plantation and a 200‐year‐old spruce‐fir dominated forest, in western Sichuan, China. Across the two forests, extramatrical mycelia of ectomycorrhizal fungi accounted for up to two‐thirds of the new root C inputs into soil and ~80% of the stimulated N mineralization. Moreover, flux‐specific (per gram) mycelia‐derived C inputs stimulated multiple indices of soil N cycling to a greater degree than the flux‐specific root‐derived C inputs, accounting for ~70% of the stimulated N mineralization in both forests. Collectively, our findings indicate that the effects of mycorrhizal fungi on soil C and N cycling may exceed those of roots in alpine coniferous forests dominated by ectomycorrhizal fungi, highlighting the need to incorporate mycorrhizal fungal inputs into biogeochemical models for ecosystems. A plain language summary is available for this article.