Eight-year effect of biochar amendment on soil properties, extracellular enzyme activity, N-cycling genes and microbiome structure in two Danish fallow soils

Biochar improves agricultural soil properties and short-term microbial diversity. However, biochar's long-term effects on microbiomes and soil health remain poorly understood. This study assessed the effects of 8-year field-aged biochar on microbiomes from two contrasting soils: a sandy clay soil and a coarse sandy soil, under temperate climate conditions. We hypothesize that even after 8 years, biochar amendment persistently alters soil physicochemical properties, stimulates extracellular enzyme activity, increases the abundance of N-cycling genes, and shifts the prokaryotic and fungal community structures. In June 2015, the topsoil in field lysimeters was amended with 2 % w/w straw biochar, and by August 2023, this biochar amendment resulted in a significant increased activity of five key extracellular enzymes (α-glucosidase, β-galactosidase, cellobiosidase, phosphomonoesterase, and arylsulfatase) involved in C, P, and S cycling in both soils. In the coarse sandy soil, biochar boosted the abundance of prokaryotes (16S rRNA gene), key nitrification genes (AOA-amoA and AOB-amoA), and the denitrification gene nosZ Clade I. In both soils, biochar caused an increase in the abundance of the nitrite reductase (nirS) gene, indicating a sustained impact on the N cycle, and an enrichment of an ammonia-oxidizing archaeon of the family Nitrosophaeraceae. Finally, a persistent shift in prokaryotic community structure was observed in both soils. The study clearly demonstrates that the effects of biochar persist after eight years, providing insights into the long-term impact of biochar on soil health.