Carbon sequestration potential of hydrothermal carbonization char (hydrochar) in two contrasting soils; results of a 1-year field study

Soil amendment with hydrochar produced by hydrothermal carbonization of biomass is suggested as a simple, cheap, and effective method for increasing soil C. We traced C derived from corn silage hydrochar (δ13C of −13 ‰) added to “coarse” and “fine” textured soils (δ13C of −27 ‰ for native soil C (SOC)) over two cropping seasons. Respiration rates increased in both soils (p < 0.001) following hydrochar addition, and most of this extra respiration was derived from hydrochar C. Dissolved losses accounted for ~5 % of added hydrochar C (p < 0.001). After 1 year, 33 ± 8 % of the added hydrochar C was lost from both soils. Decomposition rates for the roughly two thirds of hydrochar that remained were very low, with half-life for less estimated at 19 years. In addition, hydrochar-amended soils preserved 15 ± 4 % more native SOC compared to controls (negative priming). Hydrochar negatively affected plant height (p < 0.01) and biomass (p < 0.05) in the first but not the second crop grown on both soils. Our results confirm previous laboratory studies showing that initially, hydrochar decomposes rapidly and limits plant growth. However, the negative priming effect and persistence of added hydrochar C after 1 year highlight its soil C sequestration potential, at least on decadal timescales.