Does the fertilisation strategy affect the long-term legacy phosphorus dynamic?

Phosphorus (P) imbalances in agricultural systems are a global problem because they have environmental and economic impacts, like eutrophication when there is a soil P surplus and low crop productivity in P deficient soils. Fertilisers are used to increase soil available P, but some fractions become less available P forms and reduce P use efficiency (PUE). Legacy P includes labile and moderately labile P fractions that could replenish soil available P and become available for crops under long-term P fertilisation and affects PUE. Our aim was to evaluate the long-term legacy P dynamic and PUE with different P fertilisation strategies and rates in soils of Entre Rios, Argentina. Long-term experiments in two contrasting soil types (Mollisol and Vertisol), were used to evaluate a combination of three initial (0, 100, and 200 kg P ha-1) and four annual P fertilisation rates (0, 12, 24, and 36 kg P ha-1 year-1). Soil samples were collected annually at 0-5 cm, 5-10 cm, and 10-20 cm soil depth. Bray1-P concentration was used as an index of soil available P. Grain P concentration and crops grain yield were measured. The effects of the initial P fertilisation on Bray1-P were observed for a longer time in the Mollisol than the Vertisol, which may be related to a lower P adsorption capacity of the Mollisol. Annual P fertilisation increased Bray1-P earlier in deep layers in the Vertisol, associated with the preferential flow of P fertiliser through soil cracks in the latter. After 10 years, Bray1-P stratification was higher in the Vertisol than in the Mollisol. In the treatment without initial P fertilisation (i.e., 0 kg P ha-1), the annual fertilisation rate to maintain Bray 1-P level was similar on both soils (on average 24 kg P ha-1). Our results showed that the annual P fertilisation strategy to feed crops' needs produces higher PUE compared with the use of residual legacy P after an initial and unique high P fertilisation rate. This information is relevant to design highly efficient P fertilisation strategies according to soil characteristics, thereby advancing the pursuit of sustainability in agricultural systems.