In line with the principles of a circular and sustainable economy, food waste is currently a highly solved problem. Ways are still sought to valorise and reuse waste most efficiently. Grape pomace is one of the potentially highly valuable bio-wastes. Current processing of this waste is inadequate and problematic, as it cannot be stored due to subsequent fermentation. In the Mediterranean, waste is usually distilled to produce a widely consumed national spirit (Oliveira and Duarte, 2016). However, in other wine-producing regions, this practice is not as widespread, leading to a lack of sustainable solutions for the management of grape pomace waste. Feeding the waste to animals is not a viable option as it is indigestible (Baumgärtel a kol., 2007). Incorporating the fresh pomace into the soil poses significant environmental risks due to the presence of phytotoxic phenolic compounds (Olszewska et al., 2020Composting is not an ideal method as it undergoes a lengthy process of microbial decomposition. Additionally, compost made solely from pomace has a poor carbon-to-nitrogen ratio (Flavel et al., 2005) and a lower pH, which may lead to undesirable soil acidification (Karak et al., 2013). Pyrolysis is a modern method of recycling and valorising bio-waste to create value-added products (Ghodake et al., 2021). The resulting biochar, produced from various raw materials, is frequently applied as a soil amendment (Peng et al., 2020). Soil application of biochar aligns with the principles of circular economy and sustainable agriculture. It involves valorising waste by applying it back to its place of origin, which helps to close the cycle of consumed nutrients by returning them to the soil. This practice also reduces the need to apply industrially produced fertilisers (De Corato et al., 2018). The addition of biochar to soil improves aeration and provides a biological method of carbon sequestration (Yu et al., 2019). Biochar's porosity enhances soil water retention and significantly reduces nutrient leaching (El-Naggar et al., 2019). Biochar serves as substrate for the growth of microorganisms, also thanks to its high content of carbon and other nutrients, provides the microorganisms with the necessary nutrition (Ghezzehei et al., 2014). The resulting soil improvement from biochar application subsequently improves plant growth, leading to increased biomass yield (Abhishek et al., 2022). Grape pomace biochar has a high potassium content (30 ± 3 g/kg) and other nutrients compared to other biochars tested in various studies. This makes it a suitable soil amendment. The effect of different doses of grape pomace biochar on the yield and prosperity of lettuce (Lactuca sativa L.) was tested by applying it to the soil. As part of the experiment, we evaluated the effect of its addition on the soil composition and monitored the activity and amount of soil microorganisms.

The research was partly supported by NATO SPS MYP G6094 "Mitigation of climate change through advanced phytotechnology for military landsʺ.