The primary role of phosphate-metal- natural organic matter (humic) ternary complexes as drivers of long-term phosphorus plant nutrition in acidic soils.

Several studies have shown that inorganic phosphate (Pi) can form stable complexes in soil and soil solution, with some fractions of natural organic matter (NOM), mainly humic substances, through metal bridges. However, the role of these types of complexes in plant phosphorus (P) nutrition remains unclear. On the other hand, Pi in acidic soils is rapidly absorbed and fixed, becoming unavailable for plants. In this framework, this study aims to unveil the role of Pi-metal-NOM (humic) complexes in plant P nutrition in acidic soils. To this end, we have selected nine acidic soils from Spain with different soil properties and nutrient availability. On these soils, we cultivated wheat (Triticum aestirum L., cv Fiel) for successive harvests until no new sprouts appeared. Pi fractions were characterized using two complementary Pi sequencing analyses adapted to acidic soils. The results showed that Pi-metal-NOM (humic) complexes are mainly extracted by oxalate-pH3 after extracting the more labile forms of Pi. Likewise, correlation and principal component analysis studies showed that the Pi extracted with water plus resin is linked to short-term wheat P-nutrition. On the contrary, the Pi extracted with oxalate (pH 3) (after extracting the more labile fraction of Pi) that contains the main fraction of Pi-metal-NOM (humic) complexes is mainly linked to long-term wheat P-nutrition. This fact indicates that the addition of humified organic matter to acidic soils can significantly improve the fraction of potentially available P for crops.