Hydrothermal carbonization allows rapid conversion of biomass into a carbon-rich, lignite-like product (hydrochar). It is assumed to have benefi cial eff ects on soil properties and plant growth, but detailed studies are lacking, especially in the fi eld. Th e objective of our study was to investigate the eff ect of hydrochar incorporated into arable soils on soil mineral nitrogen (Nmin) content and sugar beet growth. In 2010–2011, a fi eld and a pot trial were conducted. Hydrochars (fi eld: 10 Mg ha−1; pot: equivalent to 30 Mg ha−1) processed from sugar beet pulp (HSP) and beer draff (HBD) were tested against an untreated control. As a second factor, mineral nitrogen (N) fertilizer level (fi eld: 0, 50, 100, 150 kg N ha−1; pot: 0, 100, 200 mg N kg−1 soil) was varied. In both trials, hydrochars reduced initial sugar beet growth, especially when hydrochar with a high C/N ratio (38, HSP) was combined with a low N fertilizer level; high N supply partly compensated for the reduced seedling growth. Without N fertilization, no extractable Nmin was present at the end of the pot trial in the HSP treatment, whereas in HBD even more Nmin was extracted than in the control. Th is suggests remineralization of previously immobilized N when hydrochar with a low C/N ratio was applied (16, HBD). In the fi eld, beet yield was equal at the high N fertilizer level in HSP and at all N levels in HBD treatment. Our results suggest that hydrochar can decrease plant-available N due to N immobilization. Other potential causes for the observed early growth reduction need to be studied more in detail.