Low-temperature hay biochar and acid treatment increases fertiliser value of slurry digestate

This study aimed to evaluate the effect of low-temperature-produced (torrefied at 300°C) hay biochar (BC) on the fertilisation effectiveness of digestate (SD) and the loss of nitrogen (N) and carbon (C). We also examined whether acidification could increase the effect of BC on SD effectiveness and compared the effects of untreated and acidified BCs with that of sulphuric acid (H₂SO₄). The treatments included unamended SD (SD_UT) and SD amended with untreated BC (BC_UT), H₃PO₄-acidified BC, (BC_H3PO4) H₂SO₄-acidified BC (BC_H2SO4), or pure H₂SO_4. These treatments were compared in a pot experiment with ryegrass (<i>Lolium perenne</i>, L.) and evaluated based on N, phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) taken up by plants and remaining in the soil. The N input to the soil was doubled for all applied amendments compared with SD_UT. However, the fertilisation effectiveness of SD amended with BC_UT alone was comparable to that of SD treated with pure H₂SO₄, offering the additional benefits of higher C input and reduced C loss from the soil. We concluded that the effect of BC on the effectiveness of SD as a fertiliser is similar to, or even better than, that of H₂SO₄.