Organic and inorganic phosphorus as characterized by phosphorus-31 nuclear magnetic resonance in subtropical soils under management systems

Abstract

The area under no tillage system largely increased in the last decade in southern Brazil. The increase in total phosphorus (P), mainly in the topsoil layer in no-tillage management, is distributed in different inorganic and organic forms. The ³¹P nuclear magnetic resonance (NMR) technique can improve the understanding of the dynamics of phosphorus by identifying the structure of the major groups of this element in soils. This research determined the organic and inorganic phosphorus groups, identified by NMR and by chemical analysis in three subtropical soils under conventional and no-tillage systems. Samples of two soil layers from three long-term experiments located in the state of Rio Grande do Sul, southern Brazil, were used. The first experiment was set up in 1979, in a very clayey Rhodic Hapludox; the second, in 1983, in a clayey Rhodic Hapludox, and the third, in 1985, in a sandy clay loam Rhodic Paleudult. Total, inorganic, and organic P, biomass-associated and NaHCO₃ extracted P were determined by chemical analysis and the NaOH+EDTA extract was submitted to ³¹P NMR for organic and inorganic determination. There was an overestimation of the organic phosphate content by the ignition method when compared with the NMR in surface-active soils (Hapludoxes). The NMR method allowed a good characterization of organic and inorganic phosphate groups in soils. Inorganic orthophosphate was the dominant P form; the organic P portion was higher in the low surface-active (Paleudult) soil, with a dominance of monoester over diester P, independently of soil type, tillage, or depth. A significant positive correlation was found between diester and microbial biomass P content.

Keywords:

• Tillage systems
• Nuclear magnetic resonance
• Phosphorus fractions

Acknowledgments

To PRONEX-FINEP and FAPERGS for the financial support, and to the researchers João Mielniczuk (UFRGS), Rainoldo Kochhann (EMBRAPA-Trigo), and Amando Dala-Rosa and Fernando Becker (COTRISA) for making the experimental areas available for collecting the soil samples.