A procedure for the simultaneous oxidation of total soluble nitrogen and phosphorus in extracts of fresh and fumigated soils and litters

Abstract

An autoclave method based on the oxidation of dissolved organic matter with alkaline persulphate for the analysis of total soluble nitrogen (N) and phosphorus (P) in 0.01M calcium chloride (CaCl2), 0.5M potassium sulfate (K2SO4), or 0.2M sulfuric acid (H2SO4) extracts of fresh and fumigated soils is described. Autoclaving at 110°C for 30 min oxidized N at 50 mg N/dm3 from urea, glucosamine, and the sodium salt of ethylenediaminetetra‐acetic acid (EDTA) to nitrate (NO3). Recoveries decreased with increasing concentrations of total organic carbon (TOC), hence glucosamine and EDTA required an increased concentration of persulphate. Tests with added glucose showed that recovery of urea N was complete up to a carbon (C) concentration of 100 mg/dm3. For P, it was important that solutions of 0.01M CaCl2 were acidic after oxidation to avoid the precipitation of appatite. Recoveries of P from orthphosphate and inositol hexaphosphate dodeca sodium salt in water or in 0.2M H2SO4 were complete after oxidation. Analysis of TOC in 0.01M CaCl2 and 0.5M K2SO4 extracts of highly organic soils and litter showed that only litter extracts contained more than 100 mg C, 50 mg N and 10 mg P/dm3. Brown precipitates and pink colorations, caused by managanese (Mn) compounds, appeared in oxidized extracts of coniferous litters, humus, and mineral soil. The precipitates were dissolved by the addition of approximately 0.5 mg ascorbic acid which had no significant effect on the colorimetric determination of N as NO3. When this addition had been made, total‐P (PT) values determined by oxidation were significantly correlated (PTOX = 3.3 + 0.94 PTICP) with those obtained by inductively coupled plasma emission spectrometry (ICP).