There is growing evidence of the importance of extramatrical mycelium (EMM) of mycorrhizal fungi in carbon (C) cycling in ecosystems. However, our understanding has until recently been mainly based on laboratory experiments, and knowledge of such basic parameters as variations in mycelial production, standing biomass and turnover as well as the regulatory mechanisms behind such variations in forest soils is limited. Presently, the production of EMM by ectomycorrhizal (EM) fungi has been estimated at ~140 different forest sites to be up to several hundreds of kg per ha per year, but the published data are biased towards Picea abies in Scandinavia. Little is known about the standing biomass and turnover of EMM in other systems, and its influence on the C stored or lost from soils. Here, focussing on ectomycorrhizas, we discuss the factors that regulate the production and turnover of EMM and its role in soil C dynamics, identifying important gaps in this knowledge. C availability seems to be the key factor determining EMM production and possibly its standing biomass in forests but direct effects of mineral nutrient availability on the EMM can be important. There is great uncertainty about the rate of turnover of EMM. There is increasing evidence that residues of EM fungi play a major role in the formation of stable N and C in SOM, which highlights the need to include mycorrhizal effects in models of global soil C stores.

The effectiveness of biochar amendment for remediation purposes depends on many factors related to the biochar and the contaminated site. Therefore, each application must be evaluated site-specifically. To facilitate full-scale implementation, more information from field studies on biochar-amended contaminated sites, as well as cost-effective approaches to evaluate the remediation efficacy of specific biochar materials are needed. We studied the effects of biochar and peat on metal solubility and bioavailability in a contaminated soil in a fully factorial field trial. The biochar was produced from wood via gasification in a floating fixed-bed reactor at 750 °C. Soil solutions from field-installed lysimeters, grass (Lolium perenne L), and earthworms (Eisenia fetida) were analyzed. In addition, a standardized batch leaching test (ISO 21268-2:2019) was performed to evaluate its feasibility to mimic soil solution concentrations. The results showed that biochar generally reduced the solubility and uptake of cationic metals. In situ solubility of Cu and Hg was reduced more than 80%, and Zn up to 70%. Soil solution concentrations of Cr increased in biochar-amended soils, but this effect was reduced by peat. Peat had small effects on in situ solubility of other metals. For cations, the batch test showed the same trends as the soil solution, with biochar decreasing solubility. However, mobilization of colloids during shaking in the batch test induced artefacts, leading to an overestimation of the solubility of some metals, especially Pb and Hg, an effect that was enhanced by peat applications.