Application of nitrogen fertilizers to reach high crop production is common practice. However, this can come with an environmental cost, irrespectively of the synthetic or organic origin of the nitrogen fertilizer. Intensively managed soils often fail to retain excess nitrogen, which leads to contamination of ground- and surface water. Next to abiotic factors like soil texture, limited nitrogen retention is ascribed to low activity of saprotrophic fungi. It has been shown that amendment of arable soils with cellulose-rich materials can effectively stimulate resident saprotrophic fungi. The current study investigated the relationship between fungal dynamics (biomass, composition) and nitrogen immobilization-remobilization dynamics upon soil amendment with woody materials Mineral  nitrogen pools, ergosterol and ITS2 amplicon sequences were analyzed during a 6-months pot experiment. Carbon-rich amendments included sawdusts of deciduous (beech, willow) and coniferous (Douglas fir, larch) tree species, beech wood chips, wheat straw and combinations of these materials. Excess nitrogen derived from addition of either mineral or organic fertilizer.

Deciduous wood sawdust resulted in rapid stimulation of fungal biomass, mainly consisting of saprotrophic Sordariomycetes. This was accompanied by a reduction in the mineral N pool equivalent to  24–60 kg N ha^-1 for four to over six months. The intensity of nitrogen immobilization depended on sawdust application rate and the type of fertilizer. Single amendments of coniferous sawdust and beech wood chips had minor effects, but led to prolonged nitrogen retention when combined with beech sawdust.

Our conclusion is that, fungus-stimulating woody soil amendments  have great potential to increase nitrogen use efficiency in arable soils.