Assessing organic phosphorus status of Cerrado oxisols (Brazil) using 31P-NMR spectroscopy and phosphomonoesterase activity measurement

Plant production in the Brazilian savannas, also known as the Cerrados, is limited mainly by low P availability in soils. Little is known about the P status in the Cerrados region, despite its increasing significance for the country’s agriculture. 31P-NMR spectroscopy of alkali extracts and phosphomonoesterase (PME) activity measurements were used to study forms of P and their stability in oxisols of natural and pasture systems. Total P content (Pt) in topsoils ranged from 301 to 456 mg kg-1 and organic P content (Po) from 84 to 194 mg kg-1 with the highest values under natural vegetation. The estimation of forms of soil P with different lability (provided by NaHCO3 and NaOH extractions) showed little difference between natural vegetation and pastures, but the proportions of Po extracted suggest the importance of organically bound P as a source of plant-available P. All NMR spectra showed signals of organic P (monoesters and diesters) and inorganic P forms (orthophosphate and pyrophosphate), with little influence of land use. Organic P appears to be mainly in the form of stable phosphomonoesters. The most marked effect of land use was a decrease in PME activity under recent pastures, indicating an increase of PME stability in topsoils after the maize-grass ley plantation. The importance of the pH of the PME activity measurement is also discussed.