## 6 Conclusions  
Source-separated food waste at the household level (kitchen waste) is a clean and abundant source of nutrients and carbon that could constitute a feedstock for organic fertilizer manufacturing. In this paper, five scenarios of treated kitchen waste were tested towards agronomic performance in relation to N use via glasshouse studies, using anaerobic digestion, thermal sterilization, microbial addition, and combinations of these. In absolute terms, taking into account the whole growth period, the dominating processes that decided on the highest agronomic performance of kitchen waste were anaerobic digestion, coupling EM addition with sterilization, twice EM addition, sterilization solely, and EM addition solely. In absolute terms, CMG had the highest agronomic effectiveness (20.3%), followed by DKW ST-dig (17.9%), DKW 2 EM (13.8%), and DKW 1 EM ST (12.2%), and the weakest one for DKW ST and KW 1 EM (9.9%). Anaerobic digestion allows the bioenergy in the form of methane to be extracted and provides more NH3-N into the digestate; however, both 2 EM addition without sterilization has still a very high effect on N use. Sterilization which should be applied according to EU and PL regulations to utilize potential salmonella eggs, E. Coli, and other pathogens does not have as much of a substantial effect as EM addition. The calculated relative agronomic effectiveness (RAE) after 4 months (cumulative growth data) equalled 88%, 67.8%, 60.2%, 48.6%, and 48.6% for DKW ST-dig, DKW 1 EM ST, DKW 2 EM, DKW ST, and KW 1 EM, respectively, compared to cow manure and 36.3%, 27.9%, 24.8%, 20.0% and 20.0%, respectively, compared to the mineral NPK fertilizer. The final soil properties after 4 months of ryegrass growth on treated kitchen waste remain rather stable in terms of pH, EC, and redox potential. Residual soil N content increased with increasing application rate and varied from 0.5 to 1.1 g N/kg soil, with most of it retained after KW 1 EM and CMG for certain fertilizer application rates. Organic nitrogen-based fertilizers, including the ones examined in this study, release nitrogen more slowly than synthetic fertilizers due to their reliance on microbial mineralization processes. However, their effect on soluble nitrogen (primarily nitrate, NO\({}_{3}\)\({}^{-}\)) leaching and groundwater contamination varies depending on their composition, application rate, and timing. Animal-based organic fertilizers (e.g., poultry litter, manure) tend to have higher proportions of readily mineralizable nitrogen, increasing the risk of nitrate leaching, especially when applied in excess or just before heavy rainfall. Compost and stabilized organic materials release nitrogen more gradually, generally resulting in lower leaching risks. However, under conditions of high soil moisture and poor plant uptake (e.g., fall application or sandy soils), even these can contribute to nitrate losses. Plant-based organic fertilizers (e.g., alfalfa meal, cover crop residues) usually have higher carbon-to-nitrogen (C:N) ratios, promoting microbial immobilization of nitrogen and thus reducing short-term nitrate leaching. Nevertheless, as they decompose over time, they can contribute to delayed nitrate release, posing a risk to groundwater if not managed correctly. In summary, while organic nitrogen fertilizers can enhance soil health and reduce synthetic input reliance, their improper use can lead to significant nitrate leaching and groundwater contamination. Best management practices--such as appropriate timing, application rates, and use of cover crops--are crucial to minimizing these risks. Kitchen waste generally has a lower leaching risk than raw manure or poultry litter but can pose a greater risk than well-aged plant-based composts with high C:Nratios--especially if improperly processed. Kitchen waste-based fertilizers, when properly composted and applied at agronomically appropriate times and rates, pose a moderate risk of nitrogen leaching.