Recycling of phosphorus (P) from feed input in aquaculture systems gains increasing importance, especially relating to sustainable agriculture and food production. In order to find possible areas of application of African catfish solid waste, the purpose of this study was to characterize the elemental and organic matter composition and P speciation in the aquaculture fish waste. Pyrolysis-field ionization mass spectrometry (Py-FIMS) was used to investigate the composition of organic matter and P K-edge X-ray absorption near edge structure (XANES) spectroscopy to describe the occurring P-containing compounds in African catfish solid waste from an intensive recirculation aquaculture system (RAS). The solid fish waste was mainly composed of sterols, free fatty acids and alkylaromatics, as it is common for digestive systems of animals. Ingredients such as the phytosterol beta-sitosterin confirm plant-based feed ingredients and some recalcitrance against digestion in the African catfish gut. The P in the solid fish waste was exclusively bound as calcium-phosphates. These calcium-phosphate minerals as major constituents of African catfish waste may have beneficial effects when applied to soils, suggesting the use of this waste as possible soil amendment in the future.

Sequential phosphorus (P) fractionation procedures are one of the most widely used wet chemical methods for characterizing P pools in soils and sediments, but have also been criticized repeatedly for their lack of accuracy to measure chemically specified phosphate fractions. In the recent investigation, sediments from two different sample locations with the same pretreatments were analyzed with sequential P fractionation. To verify traditional assignments of P fractionation results, P K-edge X-ray absorption near edge structure (XANES) spectroscopy was applied on the sediments and especially on the residues after the sequential extraction steps. Results of both methods indicated that the influence of sample pretreatment on the distribution of P pools was much lower compared to the effects of different sample origins. Kettle hole sediments were dominated by moderately labile iron (Fe) and aluminum (Al) associated P species, whereas Bodden sediments contained more stable calcium (Ca)–P species. Sample pretreatment of sediments can be similar to traditional soil sample pretreatment without causing fundamental changes in P speciation. The P K-edge XANES spectroscopy confirmed most assumptions of sequential P fractionation.