Use of black soldier fly larvae (BSFL) to process large volumes of organic waste is an emerging industry to produce protein. A co-product of this industry, the larval faeces (frass), has potential to be used as an organic fertiliser in a circular economy. However, BSFL frass has a high ammonium (N-NH4+) content which could result in nitrogen (N) loss following its application to land. One solution is to process the frass by combining it with solid fatty acids (FA) that have previously been used to manufacture slow-release inorganic fertilisers. We investigated the slow-releasing effect of N after combining BSFL frass with three FAs - lauric, myristic and stearic acid. Soil was amended with the three forms of FA processed (FA-P) frass, unprocessed frass or a control and incubated for 28 days. The impact of treatments on soil properties and soil bacterial communities were characterised during the incubation. Lower N-NH4+ concentrations occurred in soil treated with FA-P frass compared to unprocessed frass, and N-NH4+ release was slowest for lauric acid processed frass. Initially, all frass treatments caused a large shift in the soil bacterial community towards a dominance of fast-growing r-strategists that were correlated with increased organic carbon levels. FA-P frass appeared to enhance the immobilisation of N-NH4+ (from frass) by diverting it into microbial biomass. Unprocessed and stearic acid processed frass became enriched by slow-growing K-strategist bacteria at the latter stages of the incubation. Consequently, when frass was combined with FAs, FA chain length played an important role in regulating the composition of r-/K- strategists in soil and N and carbon cycling. Modifying frass with FAs could be developed into a slow release fertiliser leading to reduced soil N loss, improved fertiliser use efficiency, increased profitability and lower production costs.