Nitrogen-removed organic matters from cyanobacterial decomposition promote the release of nitrogen from lake sediment.

Cyanobacterial blooms, which carry a lot of nitrogen (N) and phosphorus (P), have emerged as one of the most severe environmental issues in freshwater ecosystems. However, there are few studies on the effect of organic matters released during cyanobacterial decomposition in promoting N release from lake sediments that remain underexplored. An essential step is to eliminate the impact of the N contributions from cyanobacteria when evaluating sedimentary N release. The response surface methodology (RSM) was developed to optimize the struvite precipitation model, and the results indicated that 1.3 of Mg/N, 1.0 of P/N, and pH 9.5 were the optimum conditions for N removal from cyanobacterial pyrolysis liquid. Following this, calcium phosphate crystallization (at pH 10 and Ca/P = 4.98) removed residual P, and zeolite adsorption (at pH 8 and 10 g/L zeolite dosage) eliminated the remaining N. Ultimately, 99.3% of N was removed with the two methods in cyanobacterial pyrolysis liquid. The cyanobacterial pyrolysis liquid, stripped of N, was found to significantly enhance the release of N from lake sediment under anaerobic conditions, which can then be reutilized by cyanobacteria. These findings reveal that organic matter derived from cyanobacterial decomposition promotes sedimentary N release, creating a feedback loop that sustains cyanobacterial blooms in freshwater ecosystems.