Oxide‐Occluded to Calcium‐Bound: Urbanization Increases Soil Phosphorus Stocks and Diversifies Pools in a Tropical West African Agroecosystem

Abstract Urban agriculture is widespread across tropical West African cities. Yet, the dynamics of phosphorus (P) pools in strongly weathered soils of these urban agroecosystems remain poorly constrained. We quantified, for the first time, the effect of urbanization on soil P stocks and partitioning in rainfed arable fields on Ferric Acrisols in Kumasi, Ghana. We hypothesized that inputs of P‐rich household and construction waste increase soil P stocks, and that accompanying calcium (Ca) enrichment weakens the soil organic matter (SOM) control on plant‐available P. We analyzed 225 topsoil samples (0–10 cm), classified into four groups representing a gradient of urbanization influence. We applied a tailored sequential P extraction, separating four operational fractions: plant‐available P (P Pa ), SOM‐bound P (P SOM ), Ca‐bound P (P Ca ), and P occluded in pedogenic oxides (P OCC ). Total P stocks increased from 28.0 ± 1.7 to 68.8 ± 6.7 g m −2 with urbanization influence. Stocks of all extracted soil P fractions consistently increased, with the greatest in P Ca (2.5 ± 0.6 to 14.2 ± 2.5 g m −2 ) and P Pa (1.4 ± 0.1 to 5.4 ± 0.8 g m −2 ). P OCC stocks also increased in absolute terms but declined in relative contribution from 79% to 53%. Urbanization did not weaken SOM control on P Pa as initially hypothesized. Instead, P Ca became increasingly relevant in explaining P Pa , suggesting an additional inorganic control on P Pa . We conclude that tropical urban arable soils can act as inadvertent sinks for P‐rich urban wastes, suggesting the potential for a deliberate waste management strategy that could enhance soil P supply.