Phosphorus accumulation fractions in a clayey oxisol in relation to phosphate doses under no-tillage

Increased phosphorus availability in the topsoil layer of no-tillage systems can lead to a higher transformation from inorganic into organic phosphorus fractions of different labilities. The preferential phosphorus accumulation forms and fractions under the application of soluble phosphate rates in a no-tillage system were determined. During five and a half years, the following accumulated P2O5 rates: 0, 130, 180, 260, 360, 540, 720, 980, and 1.240 kg ha(-1) of P2O5, were applied in the winter and/or summer, whereupon samples of the 0-10 cm layer of a clayey Rhodic Hapludox (Oxisol) were collected. Total, organic, inorganic, and microbial phosphorus, as well as six inorganic and three organic phosphorus fractions were determined in a decreasing lability sequence. The observed alterations in the total phosphorus content under phosphate addition were restricted to the inorganic fractions. The moderately labile fraction was the major sink for the applied phosphorus. The importance of labile phosphorus fractions increased in direct relation to the phosphate application rates. Labile and moderately labile organic phosphorus fractions in the soil were as important as the respective inorganic fractions, especially under low phosphate application rates.