Boosting CO2 sequestration potential of a degraded soil by hydrochar from sewage sludge

A dramatic loss of healthy soils is occurring worldwide due to degradation and desertification by both natural and anthropogenic processes. Such actions accelerate the depletion of soil organic carbon (SOC), turning soil from a major CO₂ sink to a source of CO₂ emissions.

On the other hand, global overpopulation is causing an unprecedented generation of sewage sludge. The high water content of sewage sludge makes it ideal for hydrothermal carbonization (HTC), a novel thermochemical conversion technology in which water acts as a reagent and catalyst to obtain a C-enriched solid known as hydrochar.

In this study, the impact of a hydrochar (H) obtained from sewage sludge (HTC at 195 °C for 3 hours) and a biochar (B) produced by standard carbonization of holm oak (500 °C) on the capacity of a degraded soil for capturing carbon is evaluated. The soil under study comes from a landfill of industrial origin that over time was mixed with natural sandy soil, resulting in a technosoil with a low SOC content and incapable of supporting stable vegetation.

The experiment was carried out in 60-l IBC containers, thus constituting a larger scale variation of the classic pot tests. Containers with only original soil (S) and those amended with 10 wt.% hydrochar (SH) and biochar (SB), after 15 days of stabilisation of the amendments, were vegetated with Lolium perenne and left for 12 months outdoors.

At a depth of 0-10 cm, both treatments increased the concentration of labile fractions of SOC, extracted respectively with cold water and hot water. In contrast, in the 10-20 cm layer, this effect was also relevant in the container SH. This could be attributed to the migration of the hydrochar along the profile, facilitated by its finer particle size. Both materials, B and H, contributed positively to enhancing the amount of recalcitrant organic carbon (R) in the soil, although the impact was greater with the use of biochar. Average values of R~30 g·kg^-1 have been detected for both 0-10 and 10-20 cm depths in the container with SB. In the case of SH, the migration of hydrochar led to R concentration of 13.08 g·kg^-1 in the upper 10 cm and 27.40 g·kg^-1 at 10-20 cm layer.

Biochar and hydrochar efficiently managed to store organic carbon in soil, but the former caused a higher increase in reserves due to a greater contribution of recalcitrant carbon (Pearson correlation between R and SOC stock of 0.96, P < 0.01). The stock in SB reaches 48.13 and 32.10 tC·ha^-1 at 0-10 and 10-20 cm, respectively, whereas the corresponding values in SH were 20.53 and 32.13 tC·ha^-1.

On the other hand, it was found that the 197 g of biomass generated in SC was reduced to 165 g when biochar was added, suggesting the capacity of B to fix certain nutrients and make them less accessible to vegetation in the short term. In contrast, hydrochar application increased the amount of vegetation to 478 g, thus favouring greater carbon sequestration.