Carbon use efficiency of mycorrhizal fungal mycelium increases during the growing season but decreases with forest age across a Pinus sylvestris chronosequence

1. In boreal forest soils, mycelium of mycorrhizal fungi is pivotal for regulating soil carbon (C) cycling and storage. The carbon use efficiency (CUE), a key parameter in C cycling models, can inform on the partitioning of C between microbial biomass, and potential soil storage, and respiration. Here we test the dependency of mycorrhizal mycelial CUE on stand age and seasonality in managed boreal forest stands. 2. Based on mycelial production and respiration estimates, derived from sequentially incubated ingrowth mesh bags, we estimated CUE on an ecosystem-scale during a seasonal cycle and across a chronosequence of eight, 12- to 158-years-old, managed Pinus sylvestris forest stands characterised by decreasing pH and nitrogen (N) availability with increasing age. Mycelial respiration was related to total soil respiration, and by using eddy covariance flux measurements, primary production (GPP) was estimated in the 12- and 100-years-old forests, and related to mycelial respiration and C...