Droughts Legacy Effects on Phosphorus Transformation from Residues and Mineral Fertilizers in Calcareous and Carbonate-Free Soils: A ³³P Labeling Study

Background: While well-described for soil properties, we barely know how microbial traits determine the availability of various phosphorus (P) forms to crops. Aims: We traced the dynamics of mineral- versus residue-derived P applied to two contrasting soil types during wheat cultivation. Methods: The legacy effect of three pre-sowing moisture conditions was investigated: drought (30% water holding capacity, WHC), alternating cycles of drying (30% WHC) and wetting (70% WHC), and well-watered conditions (70% WHC). ³³P-labelled cowpea residues (Vigna unguiculate) and KH₂³³PO₄ were applied as fertilizers to calcareous and carbonate-free soils. Results: Under pre-sowing drought conditions, microbial incorporation of ³³P from residue P into polar lipids was four times higher than from mineral P. Calcareous soils showed double the microbial biomass than carbonate-free soils. However, when fertilized with residue P, carbonate-free soils exhibited twice the acid phosphatase activity and a 3- to 6-fold greater ³³P uptake into phospholipids normalized per unit microbial biomass C. Conclusion: Residue P enhances microbial growth, leading to increased P immobilization, especially in carbonate-free soils. Drought-triggered microorganisms efficiently acquire P from organic sources like residues. This increased microbial P immobilization under pre-sowing drought does not negatively affect plant growth, when a mineralizable organic P pool is consistently available. Regardless of the pre-sowing moisture conditions, residue P fertilization promotes a rapidly cycling microbial biomass-necromass pool, specifically in calcareous soils. This implies that residue P fertilization could be a sustainable, long-term strategy for continuous P supply in P-immobilizing calcareous soils, even under increasing drought conditions due to climate change.