Soil organic phosphorus in tropical forests:: an assessment of the NaOH-EDTA extraction procedure for quantitative analysis by solution 31P NMR spectroscopy

The extraction of soil organic phosphorus by the NaOH-EDTA procedure was assessed in detail for a tropical forest soil (clay-loam, pH 4.3, total carbon 2.7%). Optimum conditions for the quantification of soil organic phosphorus and characterization of its composition by solution P-31 NMR spectroscopy were extraction in a solution containing 0.25 M NaOH and 50 mM Na(2)EDTA in a 1:20 solid to solution ratio for 4 hours at ambient laboratory temperature. Replicate analyses yielded a coefficient of variation of 3% for organic phosphorus as a proportion of the spectral area. There was no significant difference in total phosphorus extraction from fresh and air-dried soil, although slightly more organic phosphorus and less paramagnetic ions were extracted from dried soil. The procedure was not improved by changing the concentration of NaOH or EDTA, extraction time, or solid to solution ratio. Pre-extraction with HCl or Na(2)EDTA did not increase subsequent organic phosphorus extraction in NaOH-EDTA or improve spectral resolution in solution P-31 NMR spectroscopy. Post-extraction treatment with Chelex resin did not improve spectral resolution, but removed small concentrations of phosphorus from the extracts. Increasing the pH of NaOH-EDTA extracts (up to 1.0 M NaOH) increased the concentration of phosphate monoesters, but decreased DNA to an undetectable level, indicating its hydrolysis in strong alkali. The standardized NaOH-EDTA extraction procedure is therefore recommended for the analysis of organic phosphorus in tropical forest soils.