Orthophosphate monoesters often constitute a significant fraction of total phosphorus in lake sediments. The knowledge on the specific composition and recalcitrance of these compounds is however limited. The main aim was therefore to identify and quantify specific orthophosphate monoesters in sediment from 15 Danish lakes by solution 31P NMR spectroscopy. The four most quantitatively important orthophosphate monoesters were myo-inositol hexakisphosphate (myo-IP6), scyllo-inositol hexakisphosphate (scyllo-IP6) α-glycerophosphate (α-GP) and β-glycerophosphate (β-GP). The compounds were identified in 9, 4, 8 and in all 15 lakes, respectively. In total these four components made up 46–100% of the orthophosphate monoester pool. The glycerophosphates (GPs) are most likely degradation products of phospholipids, created as an artifact by the alkaline extraction procedure used for 31P NMR spectroscopy, while the inositol hexakisphosphates (IPs) are naturally occurring compounds. There was a significant positive correlation between myo-IP6 and total aluminium in the sediment and a negative correlation between myo-IP6 and lake water pH, suggesting that myo-IP6 is stabilized in the sediment by adsorption at slightly acidic or neutral conditions. In three lakes, the depth distribution of the orthophosphate monoesters was investigated. The content of scyllo-IP6 and myo-IP6 was constant with sediment depth in two of the lakes while the content of myo-IP6 decreased with depth in one of the lakes. In all cases the IPs seem to be preserved with sediment depth to a higher extent than the orthophosphate diesters and especially the GPs suggesting that IPs can be a sink for phosphorus in the lake ecosystem or at least delay P-recycling for years.
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