Increasing soil organic carbon content can enhance the long‐term availability of phosphorus in agricultural soils

The stocks of phosphorus (P) in soil resulting from decades of over‐fertilisation can be used as a long‐term source of P provided that crop P bioavailability is ensured. This study was set up to identify to what extent soil organic matter (SOM) affects the long‐term availability of these stocks, the premise being that OM may limit irreversible P fixation in soil by blocking P sorption sites on sesquioxides. An ensemble of 42 agricultural soils, composed from experimentally amended soils (field and incubation trials) and soils with contrasting properties, was subjected to 288 days of P depletion with anion exchange membranes as a P sink; this method was previously shown to yield P pools with agronomic significance. Cumulative P desorption data were fitted with a two‐pool kinetic desorption model, yielding estimates for a fast (labile) and total desorbable P pool. On average, 42% of oxalate extractable P (Pₒₓ) associated with poorly crystalline iron (Fe) and aluminium (Al) (oxy)hydroxides (Feₒₓ and Alₒₓ) were desorbable and 25% of that fraction (i.e. 11% of Pₒₓ) was labile. That labile P pool matched well with the 24 h isotopically exchangeable P (E value) in these soils (R² = 0.74). Both the fast and total desorbable fraction of Pₒₓ were larger at higher degrees of phosphorus saturation (DPS). In soils with a low DPS (<0.30), the labile fraction of Pₒₓ increased as the ratio of soil organic carbon to Feₒₓ + Alₒₓ increased (R² = 0.70; p < 0.001), but soils with a higher DPS did not exhibit that trend. These results adhere to soil chemical views that enhanced SOM contents reduce fixation of P by competitive sorption and prevention of P diffusion into micropores. No such effects occur in more P saturated soils, probably because orthophosphate sorption and electrostatic effects outcompete effective SOM sorption. The findings suggest that simultaneous application of OM with P fertilisers could keep P better available in the long term, but that this OM application does not affect P fixation when soils are excessively dosed with P. HIGHLIGHTS: Long‐term availability of P in soil was assessed in contrasting soils in a P depletion experiment. A high ratio of organic C to Fe and Al in soil enhances the P fraction that is readily available. The positive effect of organic matter on soil P availability is only found at low soil P saturation. Residual P in European soils can sustain adequate P for crops for 5–55 years depending on the soil.