Speciation, spatial distribution and bioavailability of phosphorus in a tropical soil cultivated with soybeans and wheat as affected by soil tillage in a long-term field experiment

Abstract

Globally, phosphorus (P) is one of the most limiting macronutrients for agricultural production. Humid tropical soils have historically low natural P contents, with most P forming high-binding energy compounds with mineral colloids. No-tillage (NT) systems, where fertilizers are positioned in the furrow, can influence plant nutrient uptake and fertilizer use efficiency. In this study, ³¹P nuclear magnetic resonance (³¹P–NMR) and X-ray absorption near-edge structure (XANES) were used to evaluate P species in the soil solid phase and solution, focusing on different positions: crop row, between crop rows, and rhizosphere of soybean [Glycine max (L.) Merr.] and wheat (Triticum spp.) cultivated for 36 years in an Oxisol under NT and conventional tillage (CT, with disc plowing and harrowing). Labile P levels were, on average, 25 % higher in NT compared to CT, mainly due to increases in moderately labile and non-labile fractions (p < 0.05). Total P did not differ between systems, with inorganic P representing 65–69 % of total P. In both crops, P was enriched in the rhizosphere and crop row relative to the between-row position, with orthophosphate accounting for 72–85 % of Na-EDTA-extracted P. XANES and chemical fractionation consistently indicated a predominance of P associated with Fe and Al oxyhydroxides. Additionally, XANES detected phytic acid accumulation in the rhizosphere, suggesting a role for root and microbial processes in shaping organic P dynamics. These results highlight the importance of long-term soil management in enhancing P bioavailability and fertilizer use efficiency in tropical agroecosystems.