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 31P 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 (NH4Cl-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.