Finely ground biochar has been reported to enhance water retention of coarse sandy subsoils and mitigate drought-induced yield reductions, while its impact on indigenous microbiomes remains unclear. This study aimed to assess the effects of biochar amendment on the microbiomes of coarse sandy subsoils from two Danish sites. We hypothesized that amendment of high doses of straw biochar would increase water-extractable soil P, affect phosphatases, and modify microbiomes. To test this, subsoils were amended with straw biochar (1–4 % wt) and incubated for 430 days in a mesocosm column experiment with spring barley grown in the topsoil. The activity of three extracellular enzymes and the structure and diversity of the prokaryotic and fungal communities were analyzed at 50 cm depth using 4-methylumbelliferone (MUF)-labeled substrates and 16S rRNA and ITS2 amplicon sequencing, respectively. The biochar amendment affected the extracellular enzyme activity of both subsoils, which exhibited distinct enzyme activity profiles. The biochar amendment led to increased water-extractable phosphate, which, in turn, decreased extracellular phosphomonoesterase activity. Both subsoils harbored different microbiomes, with biochar modifying the prokaryotic community structure while the fungal community structure was almost unchanged. The highest amount of biochar amendment (4 % wt) caused the strongest effects on the microbiomes. We found a significant rise in the relative abundance of the bacterial genus Iamia and an unknown archaeon genus member of Marine Group II. Overall, our findings highlight that the prokaryotic microbiomes in the subsoil are influenced by biochar amendment, with the changes depending on concentration and incubation time. This knowledge is useful for risk assessment of biochar's effects on soil health.
Iturbe-Espinoza et al. (Wed,) studied this question.