Progressive Decomposition of Algal Organic Matter Decouples Nitrogen Transformations in Lake Sediments: Evidence from Short-Term Incubation

Against the backdrop of global lake
eutrophication, algal bloom
decay is increasingly affecting ecosystems. Algal organic matter (AOM),
a natural complex mixture, undergoes multiple release and transformation
stages, yet its composition and pathways remain unclear. This study
used spectroscopic, mass spectrometric, and metagenomic analyses to
monitor a time-compressed algal decay experiment. Results showed that
AOM release and transformation can be divided into three stages. Within
1 day, labile AOM consisting mainly of proteins (8.36%), lipids (8.22%),
and unsaturated carbohydrates (7.72%) was rapidly released, reshaping
nitrogen (N) cycling. Its high bioavailability promoted sediment mineralization
and a positive priming effect, while anaerobic conditions reduced
nitrification and denitrification rates by 88.7% and 34.5%. Within
3–7 days, semilabile AOM rich in tannins (19.2%) and carbohydrates
(9.41%) was gradually decomposed, maintaining anaerobic conditions.
The imbalance of excessive NH4+ and depleted
NO3– led to the decoupling of nitrification–denitrification.
After 7 days, humic-like AOM dominated by lignins (56.8%) prevailed,
reducing oxygen consumption and enabling rapid recovery of nitrification
and slow rebound of denitrification. These findings clarify the phased
transformations of AOM and their microbial interactions, providing
mechanistic insights into the short-term fluctuations of lake water
quality and microbial processes during bloom decay.