MECHANISTIC PATHWAYS FOR MOBILISING LEGACY PHOSPHORUS IN CALCAREOUS SOILS: A COMPREHENSIVE REVIEW

Abstract

Calcareous soils contain substantial reserves of legacy phosphorus (P) accumulated from long-term fertilizer use, yet most of this P remains unavailable because of fixation by calcium carbonate minerals (Sharma et al.,2019). Under alkaline pH conditions, soluble phosphate reacts with Ca²⁺ and precipitates as stable calcium-phosphate compounds, limiting plant uptake despite high total soil P (Penn and Camberato 2019). Plants mobilize this Ca bound phosphorus through rhizosphere acidification caused by proton release during nutrient absorption (Gupta et al.,2022). Roots also release low-molecular-weight organic acids such as citrate, malate and oxalate into the rhizosphere (Gupta et al.,2022). These carboxylates chelate calcium and displace phosphate from mineral surfaces via ligand-exchange reactions, increasing phosphate concentration in soil solution (Penn and Camberato 2019). Soil microorganisms and their phosphatase enzymes further mineralize organic phosphorus into plant-available orthophosphate (Gupta et al.,2022). Enhanced root–microbe interactions improve phosphorus acquisition beyond the root-depletion zone (Sharma et al.,2019). Overall, integrated plant, microbial and soil chemical processes convert legacy phosphorus into available forms and improve phosphorus use efficiency in calcareous agroecosystems (Penn and Camberato 2019).