Phosphorus speciation in the sediment profile of Lake Erhai, southwestern China: Fractionation and 31P NMR

The distribution characteristics of phosphorus (P) forms in the sediment profile of Lake Erhai, in southwestern China has been investigated by sequential extraction and P-31 nuclear magnetic resonance spectroscopy (NMR) of NaOH extracts to understand P dynamics and its potential contribution to lake eutrophication. Contents of P fractions varied in the order of NH4Cl extracted P (H4Cl-P) < bicarbonate-dithionite extracted P (BD-P) < HCl-P, Residual-P < NaOH extracted P (NaOH-P). The highly available NH4Cl-P represented less than 1% of total P (TP). BD-P and NaOH extracted reactive P (NaOH-rP) averaged 39%, while the ratio of Fe/P was higher than 15, indicating low P release from the sediments under permanent oxic condition. The less bio-available HCl-P, NaOH extracted nonreactive P (NaOH-nrP) and residual-P contributed 61% of TP. Regression analysis revealed that BD-P, NaOH-rP and HCl-P were positively correlated with the contents of Fe and Mn, Al and Fe, and Ca, respectively. The investigation of P compound groups in NaOH extracts by 31P NMR showed that ortho-P and monoester-P were the largest two constituents of the P pool, followed by diester-P, phosphonate and pyro-P. A comparison of vertical variations of P groups in the sediment profile suggested that these compounds were involved in the P recycling to different extents in Lake Erhai. In particular, the lake exhibits high potential for labile P release from the surface sediments, which should be taken into consideration even after the outsourced P runoff ceased.