An investigation of the distribution of phosphorus between free and mineral associated soil organic matter, using density fractionation

Aims We investigated whether density fractionation can be used to determine the distribution of organic phosphorus (OP) between free and mineral-associated soil organic matter (SOM). Methods We performed density fractionations using sodium polytungstate solution (specific gravity 1.6 g cm−3) on 20 soils from UK semi-natural and pasture ecosystems, to obtain a light fraction (LF) and a heavy fraction (HF) for each soil. The fractions were quantified by weight, and analysed for organic carbon (OC), total N (TN), total P (TP), inorganic P (IP), and OP (by difference). Results Good recoveries of soil mass (96%), OC and TN (both ∼ 90%) were obtained, but recovery of OP only averaged 56%. The average P:C ratio of HF SOM exceeded that of LF SOM by a factor of six, greater than the factor of two obtained for TN:OC. For the soils studied, the elements of SOM were predominantly in the HF, with averages of 75% for C, 82% for N, and 90% for P. Conclusions The incomplete recovery of OP demands further work. Nonetheless, the results show that HF SOM is much richer in P than LF SOM.