Effects of Hydrochars Derived from Woody Residues on Soil Enzyme Activities and Plant Response

The growing need for biomass waste recycling has led to increased interest in hydrothermal carbonization, which produces hydrochar as a by-product. This carbonaceous material can be applied in agriculture; however, its role in soil health is still not fully understood. Data on the effects of hydrochar on enzymatic activity, one of the key indicators of biological soil health, remain limited and contradictory. This study demonstrates the effects of hydrochars derived from alder wood chips, produced at three temperatures (180, 200, and 220 °C) and residence times (30, 60, and 90 min), on soil enzyme activity and the early seedling response of winter wheat (Triticum aestivum). Dehydrogenase and urease activities increased by 29–45% and 70–169% (p < 0.05), respectively, in hydrochar-amended soil compared with control soil. Hydrochars had no effect on catalase activity and, conversely, negatively affected invertase activity, reducing it by 19–47% relative to the control. Phosphatase activity showed an ambiguous response. It was found that hydrochars H180_90, H200_90, and H220_90 increased the geometric mean of enzyme activities by 17%, 22%, and 18%, respectively, which apparently indicates the advantages of these production conditions in terms of achieving the most positive effects on enzymatic activity. These findings highlight the importance of hydrothermal carbonization conditions in determining the enzyme-specific effects of hydrochar on soil biochemical functioning.