Organic waste and their respective hydrochars: Characteristics, carbon stability and nutrient release dynamics in soil

Hydrothermal carbonization (HTC) is a suitable treatment to valourize the organic matter and nutrient content of organic waste. However, the characteristics of the resulting hydrochars and their potential to act as organic fertilizers and soil amendments depend on nutrient transformations during HTC. Hydrochars were produced at 210 °C from five different feedstocks (pulp bio-sludge, digested and undigested sewage sludge, food digestate and cattle manure) and characterized. Hydrochars showed lower hydrogen-to-carbon and oxygen-to-carbon ratios than their respective feedstocks. Total nitrogen content decreased after HTC, with associated reductions ranging between 44 % and 67 % for manure-based and sewage sludge hydrochars, respectively. In a 120-day soil incubation with feedstock and hydrochar treatments, carbon dioxide (CO 2 –C) emissions, net nitrogen mineralization and water-extractable phosphorus (WEP) were monitored. Hydrochars were applied to soil either alone or in combination with NH 4 NO 3 fertilizer. Hydrochars led to reduced CO 2 –C emissions compared with the feedstocks, while the addition of mineral nitrogen did not systematically affect cumulative CO 2 –C emissions from hydrochar-treated soils. Net N mineralization of the materials decreased in the order feedstock > hydrochar > hydrochar + mineral N. Tested hydrochars did not negatively affect radish seed germination, but plant growth decreased during the first three weeks of a pot trial, indicating N limitation in sewage sludge- and manure-based hydrochar treatments. Hydrochars may therefore serve as suitable amendments where C retention and soil conditioning are prioritized but may be less effective as short-term N and P sources than the raw feedstocks. • Hydrothermal carbonization concentrates carbon and phosphorus in hydrochars. • Dried hydrochars show lower CO 2 –C emissions from soil compared with their feedstocks. • Co-application of hydrochars and NH 4 NO 3 may reduce CO 2 –C emissions in certain cases. • The potential of hydrochars to act as organic N fertilizers is limited. • Phytotoxic effects of hydrochars should be evaluated on a case-by-case basis.