Some effects of hydrochar, compost, and their mix on soil organic carbon and nutrients

The application of amendment guarantees the management of soil fertility and carbon sequestration due to their effect on soil organic carbon (SOC). The project “HYDRORG” aims to study these interactions, starting from the organic fertiliser characterization. The hydrochar (HC) is produced from the hydrothermal carbonisation of organic wet waste and the post-treatment can improve its agro-environmental properties. The HC (190° C for 2 h) was produced from winery and dairy agro-industrial sludges. The HC was compared to compost (COM) and MIX (obtained by mixing HC and compost, 1:1 w/w in an aerated pile). Besides the analysis of nutrients, pH, EC, TOC, the phytotoxicity and the effect on plant growth at increasing doses have been assessed. The effects were defined by incubating amended soil at 10 and 20 Mg/ha, 25°C, and 50% of WHC, for 1 and 12 months. The organic C was analysed by distinguished dissolved organic matter and the soil recalcitrant organic matter. The MIX pile showed a decrease in oxygen uptake (from 1789 to 421 mgO2/kgVSh). HC has a high content of both TOC and TKN (47 and 4.8 g/kg). P content was different among products (47 g/kg HC, 4.5 g/kg COM, 12.9 g/kg MIX). The MIX had a lower pH value compared to others. MIX showed a phytostimulant effect compared to COM, while HC is phytotoxic. HC contained a greater content of aliphatic compounds than aromatic ones. After 1 month, the soil amended with MIX and HC showed an increase of P available at higher doses. The MIX amended pots reached higher nitrate content (from 25.0 to 54.6 mg/kg), while HC determined higher ammonia content at the beginning of incubation (17.6 mg/kg). TOC increased in both, HC and COM-amended pots compared to control, after 12 months. The integrated knowledge of the amendments and their effects (chemical, on soil, GHG emission, etc.) can help to plan efficient and efficacy agronomic use and to better comprehend their environmental role.