Phosphorus speciation by P-XANES in an Oxisol under long-term no-till cultivation

In highly weathered soils with low phosphorus (P) content and high P fixing capacity, long-term application of P fertilizers may influence P speciation. The combined use of macro- and microscopic techniques can provide information on P distribution and reactivity in soils, which is an important key to determine its loss potential into the environment. In this study, we aimed to evaluate the effects of rates of P on solid-state P speciation in an Oxisol under no-till (NT) for 26 years in Brazil. Soil samples were collected from experimental plots that received 0 to 50 kg ha−1P annually. X-ray Fluorescence microanalysis (µ-XRF), X-ray Absorption Near-Edge Structure (XANES), and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies were used along with soil mineralogical and conventional chemical analyses. Phosphorus availability increased with the applied P rates. Based on the µ-XRF analysis, high Pearson correlation coefficients (r) were obtained for P vs Al, Si and Fe contents (r = 0.97, 0.81 and 0.84, respectively, with P < 0.0001); and partial correlation showed co-localized effects mainly for Al and Si. The Fe-EXAFS results hinted to the presence of non-crystalline iron (Fe) minerals being favored in the cultivated soils, likely as a consequence of the higher rates of P applied and utilization by the crops. P-XANES data revealed that after the fertilization, the proportion of P sorbed to goethite did not vary, but the proportion of P sorbed to hematite decreased, while P sorbed to kaolinite increased. The long-term continuous application of P fertilizers beyond plant uptake capacity favored P build-up in soils, resulting not only in poor P use efficiency but also in exacerbated risk of P loss to the environment.