The effects of aluminum (Al) treatment on sediment composition of carbon (C), nitrogen (N) and phosphorus (P) were investigated in sediment representing pre- and post-treatment years in the Danish Lake Sønderby. 31P NMR spectroscopy analysis of EDTA-NaOH extracts revealed six functional P groups. Direct effects of the Al treatment were reflected in the orthophosphate profile revealing increased amounts of Al-P in the sediment layers representing the post-treatment period, as well as changes in organic P groups due to precipitation of phytoplankton and bacteria at the time of Al addition. Furthermore, changes in phytoplankton community structure and lowered production due to the Al treatment resulted in decreased concentrations of sediment organic P groups and total C. Exponential regressions were used to describe the diagenesis of C, N, and P in the sediment. From these regressions, half-life degradation times and C, N, and P burial rates were determined.