Comparison of methods for analysis of organic and inorganic phosphorus in river sediment

Abstract

During a study of the dynamics of sediment nitrogen, phosphorus (P) and organic matter in rivers, a common method for analysis of soil P differentiating between total phosphorus (TP), inorganic phosphorus (P i) and organic phosphorus (P o = TP - P i) revealed serious errors. Negative values of P o were calculated in approx. 40% of 420 river sediment samples and the reproducibility was low. The method involved a mild extraction from 2 mm wet-sieved samples, respectively ignited (550°C for 2 h) and unignited, by shaking for 2 h at room temperature with 0.1 M H 2SO 4. The poor reproducibility of the initial method was greatly improved by grinding the dried sediment to <250 μm. Even better results were obtained by also changing to stronger extractants (1 M HCl and 7.7 M HCl) and by boiling for 20 min. The combined effect of grinding and changing extractant yielded approx. 30% more P i and up to 44% more TP, and good reproducibility was achieved. Further, the number of negative P o values has reduced to 8%. Analysing sediment P in 1094 sub-samples, in 282 iron-poor samples only 5% of the P o values were negative, and in 812 iron-rich samples 10% of the P o values became negative. About 75% of the negative P o-values were associated with very low organic contents (<0.6% loss on ignition). We conclude, that extraction involving 1 M HCl can be recommended for routine determinations of TP and P i in river sediments, and to determine P o when TP - P i > 10 mg P kg −1 dw. For very iron-rich sediment, however, some reservation still must be made due to some interference, possibly arising during the ignition phase.