Fertilization effects on soil ecology strongly depend on the genotype in a willow (Salix spp.) plantation

Willows (Salix spp.) grown in short rotation coppice (SRC) for biomass production offer diverse soil ecological benefits, however fertilization might interfere these impacts. To specify the significance of demand-based fertilization (NPK 21:3:10) on genotype-specific soil ecological impacts, the upper topsoil (0-10 cm soil depth) under six willow genotypes was tested. Activities of dehydrogenases as indicator of soil microbial activity, the concentration of glomalin-related proteins (GRSP), as an indicator of arbuscular mycorrhizal fungal biomass, soil organic carbon (SOC) stock, content of plant-available phosphorus (P), and the soil aggregate stability were investigated in a Vertic Cambisol in central Sweden after a growth period of 17 years in 3-year cutting cycles. Significantly higher microbial activity in the soil after fertilization was restricted to two genotypes ('Tora' and 'Tordis') out of six. Generally, the abundance of arbuscular mycorrhizal fungi indicated by GRSP was increased after fertilization, however significantly only under the genotypes 'Jorr' and 'Loden'. Non-fertilized genotype 'Jorr' was most effective to increase the soil SOC stock in 0-10 cm soil depth from initially 14 Mg ha(-1) in 2001 to 31 Mg ha(-1) in 2018. Under the genotype 'Tora' the highest and under 'Jorr' the lowest P plant-availability was revealed in the fertilized treatment. Site-specifically the aggregate-stability of soil was generally high without any significant effects of willow genotype or fertilization. We conclude that, from a soil ecological perspective, the genotype 'Jorr' is most promising at non-fertilized sites whilst 'Tora' and 'Tordis' are most promising for fertilized sites. In that context, the genotype 'Gudrun' was the least promising, independent of fertilization. Our results indicate large genotype-specific effects that might exceed the impact of demand-based fertilization on the soil ecological value of willow genotypes grown in SRC, suggesting that a genotype-specific response on fertilization should be considered.