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 (P-ox) associated with poorly crystalline iron (Fe) and aluminium (Al) (oxy)hydroxides (Fe-ox and Al-ox) were desorbable and 25% of that fraction (i.e. 11% of P-ox) was labile. That labile P pool matched well with the 24 h isotopically exchangeable P (E value) in these soils (R-2 = 0.74). Both the fast and total desorbable fraction of P-ox were larger at higher degrees of phosphorus saturation (DPS). In soils with a low DPS (<0.30), the labile fraction of P-ox increased as the ratio of soil organic carbon to Fe-ox + Al-ox increased (R-2 = 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