Phosphorus fractions and its release in the sediments from the shallow lakes in the middle and lower reaches of Yangtze River area in China

Phosphorus (P) contents and P fractions before and after P release experiments were investigated in the sediments from the shallow lakes in the middle and lower reaches of Yangtze River area in China. The results indicated that the contents of different P fractions in the studied sediments varied greatly. The total P (TP) contents ranged from 221 to 2051 mg kg−1, whereas the loosely sorbed P (NH4Cl-P) ranged from 1 to 23 mg kg−1, the redox-sensitive P (BD-P) ranged from 48 to 112 mg kg−1, the calcium bound P (HCl-P) ranged from 1 to 129 mg kg−1, and the metal oxide bound P ranged from 180 to 426 mg kg−1. The contents of different P fractions in the sediments were in the order: NaOH-P > BD-P > HCl-P > NH4Cl-P. Both the contents of different P fractions and the amounts of P released from the sediments have a positive relationship with TP contents in the sediments. This indicated that the heavily polluted sediments (often have high P concentration) have the high potential for P release. It was calculated that over 50% P released was from NaOH-P, over 30% from BD-P, about 10% from HCl-P, and below 5% from NH4Cl-P; about 50% NH4Cl-P, 50% BD-P, 20% NaOH-P and 40% HCl-P may be released. It was suggested that NaOH-P and BD-P were the main P fractions (over 80%) that can be released from the sediments into overlying waters, and NH4Cl-P and BD-P were the main fractions that can be released easily.