Agricultural crop production is largely dependent on inputs of mineral fertilizers. Mineral fertilizer prices are expected to rise as the competition for fossil energy increases. In order to increase the sustainability of crop production, alternatives to commercial mineral fertilizers are needed. Organic residues and fresh biomass are potentially important sources of nitrogen (N) in crop production. A study was conducted to investigate the impact of temperature on the release rate and profile of amino acids from soybean seeds (Glycine max L.) and fresh young shoots of chicory (Cichorium intybus), alfalfa (Medicago sativa) and red clover (Trifolium pratense) under anaerobic conditions. Plant material was incubated in water at either 15 °C or 35 °C, and the temporal release of total N, inorganic N, organic N and amino acids was measured during the 240 h incubation period. Amino acids and other organic N sources constituted significant proportions (20–84%) of the soluble N that was released. The contribution of organic N compounds to total N released decreased when the incubation temperature was raised from 15 °C to 35 °C, whereas the increase in temperature resulted in a higher total N release for only alfalfa and soybean. Amino acid profiles differed with plant material, incubation temperature and incubation length, with release patterns that only in a few cases could be described by first-order kinetics. Irrespective of the source, short-term liquid composting solutions will contain a low amino acid:inorganic N ratio with multiple amino acids present, which has to be taken into account when calculating the fertilizer effect. To conclude, this study reports, to our knowledge for the first time, the multiple amino acid release profiles from four different plant materials incubated under anaerobic conditions. The findings demonstrate that amino acids constitute significant proportions of the total N released and that the release patterns only for some amino acids followed first-order kinetics.