Feedstock optimization with low carbon to nitrogen ratio during algal sludge aerobic composting: Quality and gaseous emissions

[Display omitted] •Bacterial communities in high C/N conditions have stronger connectivity.•Compost products retain more inorganic nitrogen under low initial C/N ratio.•C/N ratio of 14.6 is a better choice for low C/N composting.•Pseudoxanthomonas and Chelicaria were associated with reduced nitrogen losses. This study investigated compost quality and gaseous emissions during the algal sludge composting. The experiment explored the feasibility of low initial carbon to nitrogen (C/N) ratio composting by using different volume ratios of algal sludge and spent mushroom substrates (1:1, 1:2, 1:3, and 1:4, corresponding to C/N ratios of 9.5, 12.3, 14.6, 16.0, respectively). The results showed that increasing the proportion of algal sludge in the initial material led to a longer maturation time and higher nitrogen losses but also enhanced the mineralization of organic nitrogen (converted to NH4+ and NO3-) and reduced carbon losses. The addition of carbon-rich bulking agents within a certain range improved the diversity and interactions of bacterial communities during algal sludge composting. In conclusion, considering the nitrogen and carbon lost, retained, and made available across the four treatments, treatment 3 (C/N = 14.6) appears to be the optimal choice for low C/N composting.