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

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. 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-year-old, managed Pinus sylvestris forest stands characterized 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-year-old forests, and related to mycelial respiration and CUE. As hypothesized, mycelial CUE decreased significantly with increasing forest age by c. 65%, supposedly related to a shift in mycorrhizal community composition and a metabolic adjustment reducing their own biomass N demand with declining soil N availability. Furthermore, mycelial CUE increased by a factor of five over the growing season; from 0.03 in May to 0.15 in November, and we propose that the seasonal change in CUE is regulated by a decrease in photosynthate production and temperature. The respiratory contribution of mycorrhizal mycelium ranged from 14% to 26% of total soil respiration, and was on average 17% across all sites and occasions. Synthesis. Carbon is retained more efficiently in mycorrhizal mycelium late in the growing season, when fungi have access to a more balanced C and nutrient supplies. Earlier in the growing season, at maximum host plant photosynthesis, when below-ground C availability is high in relation to N, the fungi respire excess C resulting in lower mycelial carbon use efficiency (CUE). Additionally, C is retained less efficiently in mycorrhizal fungal biomass in older forest stands characterized by more nutrient depleted soils than younger forest stands.