Effect of garden and park waste hydrochar and biochar in soil application: a comparative study

Abstract The thermochemical treatments of biowaste yield alternative renewable carbon-rich materials, while achieving waste valorization. These technologies allow to reduce amount of biowaste and enhance its life span avoiding the landfill disposal. The agronomic application as a soil amendment strategy using fresh hydrochar (obtained via the hydrothermal treatment of garden and park waste (GPW) at 180 °C for 1 h), post-treated hydrochar (washed, aged, and thermally treated), and biochar (obtained via the pyrolysis of GPW at 900 °C for 90 min) has been studied and compared in order to stablish the best approach for its valorization. We evaluated the effects of mixing fresh hydrochar (1–5% on dry weight) with different peat-based growth substrates on the seed germination index as well as the fresh and dry weights of Arabidopsis thaliana , Chenopodium quinoa , and Solanum lycopersicum (tomato). We also performed a germination assay with marginal agricultural soil mixed with fresh and post-treated chars as well as biochar using the same doses. All carbonaceous materials complied with the European legal framework being categorized as a class A amendment and present a favorable chemical composition for their agronomic use, carbon sources with a low heavy-metal content, and a high mineral and organic matter content. Application of post-treated hydrochar to the agricultural marginal soil improved the germination index of tomato seeds (by 10–20%) at low dosages (< 3% on dry weight) when washed and thermally treated hydrochar was used. However, fresh hydrochar negatively affected seed germination and plant growth when applied to marginal soil and peat-based growth media, particularly sandy substrates. Washing improved the germination index (by approximately 18%), reduced 90% of the total volatile fatty acid content, and effectively removed furans, amines, amides, pyridines, pyrazines, benzoic compounds, and organic acids that can affect seed germination and plant growth. Because the use of hydrochar in soils for agricultural purposes requires post-treatment to alleviate germination and plant growth inhibition, washing is the most suitable option considering the energy and technological requirements.