Application of nitrogen fertilizers to reach high crop production is common practice. However, this has a high environmental cost, irrespectively of the synthetic or organic origin of the fertilizer. In particular, intensively managed arable 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-month 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 the 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 up to 17 kg N t−1 wood, followed by a gradual remobilization. The intensity of nitrogen immobilization depended on the type of woody materials and of fertilizer. Nitrogen immobilization by single amendments of coniferous sawdust was the lowest, but these materials resulted in a prolonged nitrogen retention when combined with beech sawdust. Our conclusion is that fungus-stimulating woody soil amendments have great potential to reduce nitrogen losses in arable soils.
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