Phosphorus regulates ectomycorrhizal fungi biomass production in a Norway spruce forest

Ectomycorrhizal fungi (EMF) are important components of soil microbial communities, and EMF biomass can potentially increase carbon (C) stocks by accumulating in the soils as necromass and producing recalcitrant structures. EMF growth depends on the C allocated belowground by the host trees, and the nutrient limitation on tree growth is expected to influence this allocation. Therefore, studying EMF production and understanding the factors that regulates it in natural soils are important to understand C cycling in forests. Fungal mycelium collected from ingrowth mesh bags is commonly used to estimate EMF biomass, but these measurements might not reflect the total EMF production since turnover rates of the hyphae are not considered. Here we estimated EMF production and turnover in response to P fertilization (applied as superphosphate) in a Norway spruce forest where nitrogen (N) deposition has resulted in phosphorus (P) limitation of plant production by using a combination of mesh bags with different incubation periods and with Bayesian inferences. To test how localized patches of N and P influence EMF production and turnover we amended some bags with a nitrogen source (methylene urea) or P source (apatite). Additionally, the Bayesian model tested the effect of seasonality (time of mesh-bag harvesting) on EMF production and turnover. We found that turnover of EMF was not affected by P fertilization or mesh-bag amendment. P fertilization had a negative effect on EMF production in all the mesh-bag amendments, suggesting a reduced belowground C allocation to the EMF when P limitation is alleviated. Apatite amendment significantly increased EMF biomass production in comparison with the pure quartz bags in the control plots but not in the P-fertilized plots. This indicates that P-rich patches enhance EMF production in P-limited forests, but not when P is not limiting. Urea amendment had a generally positive effect on EMF production, but this was significantly reduced by P fertilization, suggesting that a decrease in EMF production due to the alleviated P limitation will affect N foraging. Seasonality had a significant effect on EMF production, and the differences registered between the treatments were higher during the warmer months and disappeared at the end of the growing season. Many studies highlight the importance of N for regulating belowground C allocation to EMF in northern coniferous forests, but here we show that the P status of the forest can be equally important for belowground carbon allocation to EMF production in areas with high N deposition.