Assessing organic phosphorus status of Cerrado oxisols (Brazil) using P-31-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. P-31-NMR spectroscopy of alkali extracts and phosphomonoesterase (PME) acti vity measurements were used to study forms of P and their stability in oxis ols 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 for ms of soil P with different lability (provided by NaHCO3 and NaOH extractio ns) showed little difference between natural vegetation and pastures, but t he proportions of Po extracted suggest the importance of organically bound P as a source of plant-available P. All NMR spectra showed signals of organ ic P (monoesters and diesters) and inorganic P forms (orthophosphate and py rophosphate), with little influence of land use. Organic P appears to be ma inly in the form of stable phosphomonoesters. The most marked effect of lan d use was a decrease in PME activity under recent pastures, indicating an i ncrease of PAM stability in topsoils after the maize-grass ley plantation. The importance of the pH of the PME activity measurement is also discussed.