Table 1 Road map for building basic knowledge of the forest plastic hub

Global data for MNP concentrations and characteristics in forest organic and mineral soils

Forests and their soils are very heterogenous and cover a large share of global land surface.

• We suggest researchers attain representative sample materials from poorly decomposed organic layers using spatially representative sampling and larger sample volumes, as well as subsequent sample deviation.

• We suggest developing a suitable extraction method to access MNPs concentrations and characteristics in forest soils and analyse MNPs in high organic little decomposed horizons via state-of-the-art analytical methods like µFTIR for > 20 μm MP, µRaman for < 20 μm MP and > 500 nm NP as well as py-GC-MS for < 500 nm NP.

• We suggest holistic MNP analysis and monitoring within different forest systems in different climate zones to develop global MNP budgets for forest soil systems.

MNP inputs, cycling and outputs in forest systems

Forest systems are complex; there is little known about the input and output of MNPs, as well as their incorporation into biological and geochemical turnover cycles. There is a lack of global data for dry and wet deposition in forests via throughfall, stem flow and transport with litter traps, soil mixing and leaching to groundwater systems, and plant-uptake.

• We suggest tracing MNP pathways in experimental field sites. Atmospheric deposition (via air and leave sampling), water samples (rainfall, stem flow, groundwater), and soil and plant material (leaves, litter) should be analysed to assess MNP inputs, cycling and budgets.

• We suggest investigating MNP uptake in common tree species under controlled laboratory and field conditions through state-of-the-art methods (e.g., metal, fluorescent or ¹³ C spiked MNPs in combination with imaging techniques).

The fate of MNPs in forests, organic soils and mineral soils

Within forests and forest soils, the environmental fate of MNPs (e.g., disaggregation, ageing) are different from those in agricultural soils, mainly due to variable UV radiation, microbial activity and the absence of physical disturbances like ploughing:

• We suggest performing MNP disaggregation, soil colloid/aggregate integration and ageing studies under controlled laboratory conditions via forest organic and mineral soil microcosm experimental setups.

• We suggest assessing the MNP degradation status within forest systems by analysing environmental MNP surface characteristics in forests via state-of-the-art methods (e.g., SEM).

Effects of MNPs on forest soil physiochemical properties

Studies on the effects of MNP on soil physiochemical properties have been conducted mainly for agricultural soils. As the soil management and structure is very different in agriculture and forests, the effects must be investigated for forest soils, separately.

• We suggest performing controlled laboratory and field experiments with realistic doses of MNPs in forest soils (derived from measurements campaigns; see above) to study the possible effects of MNPs on soil structure, soil water dynamics, and soil pH.

Effects of MNPs on forest (soil) biodiversity

Forests harbour organisms which are quite different from those in agroecosystems, partially as a direct consequence of different ecosystem properties. It is thus vital to examine any effects MNPs may have on soil biota (and perhaps beyond) in controlled experiments involving mesocosms or microcosm setups, as well as in the field.

• We suggest that experiments examine effects to the organic layer, as well as the mineral soil. Any effects should be tightly linked to changes in soil physicochemical properties. Given the role of fungi in the degradation of persistent organic matter, such studies should include and perhaps initially focus on fungi.

• We suggest that plant community effects be included as well, especially for forests featuring a pronounced understory layer.

Effects of MNPs on turnover cycles in forest systems

The presence of MNPs within forest turnover cycles could impact various biological cycles and plants and could therefore affect forest ecosystem functions.

• We suggest investigating interferences of forest soils biological activity and its influence on biogeochemical cycles in the presence of MNPs, with realistic doses under controlled conditions in micro- and mesocosm studies.

• We suggest performing controlled experiments that investigate the possible effects of MNPs on litter quality, litter decomposition, humus formation and material turnover processes (e.g., carbon, nutrients).