Hydrothermal carbonization allows rapid conversion of biomass into a carbon (C) rich, lignite-alike product (hydrochar). It is assumed to have beneficial effects on soil properties and plant growth, but detailed studies are lacking, especially in the field. The objective of this study was to investigate the effect of hydrochar incorporated into arable soils on soil mineral nitrogen (Nmin) content and sugarbeet growth. In 2010-2011, a field (Luvisol) and a greenhouse trial (Cambisol, 1 kg pot-1) were conducted. Hydrochars (field: 10 Mg ha-1; greenhouse: equivalent to 30 Mg ha-1) processed from sugarbeet pulp (HSP) and beer draff (HBD) were tested against an untreated control. As a second factor, mineral nitrogen (N) fertilizer level (field: 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 sugarbeet growth, especially when hydrochar wide in 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 HSP treatment, while in HBD even more Nmin was extracted than in the control. This suggests re-mineralization of previously immobilized N when hydrochar with a low C:N ratio was applied (16, HBD). In the field, 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 reduced N availability and observed early growth reduction need to be studied more detailed.
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