The effect of soil organic matter on long-term availability of phosphorus in soil: evaluation in a biological P mining experiment

The plant uptake of legacy phosphorus (P) from over-fertilised agricultural soils could offer a solution to decrease dependency on finite mineral P resources. This study evaluated the long-term availability of legacy P in soils with an accelerated biological mining assay, thereby testing to what extent this availability is affected by soil organic carbon (SOC). A 15-months long pot trial was set-up, in which 25 soils with 1.2-24% SOC were mined for P by continuous cropping and harvesting of ryegrass (Lolium Perenne) in a plant growth cabinet. The cumulative uptake of P was, on average, 19% of the P associated with poorly crystalline iron (Fe) and aluminium (Al) (oxy)hydroxides (oxalate-extractable P; Pox), and half of this uptake occurred fast enough to maintain crop production at an adequate level of > 90% of its potential. This P available for adequate uptake (PUA) strikingly matched with the isotopically exchangeable P or “E value” of a soil (median PUA/E24h = 0.81), whereas it was markedly underestimated by Olsen P (median PUA/POlsen = 1.51). The fractions of plant-available Pox increased at increasing ratios of P and SOC to Feox and Alox, showing that enhanced SOC contents reduce ageing of P by preventing its diffusion into micropores. That effect of SOC on P availability was more pronounced in soils with a low initial P saturation status. The comparison of the results from biological mining with available soil P pools determined in a (sterile) P desorption experiment could not confirm a significant contribution of organic P to plant P supply. Our findings suggest that legacy P in well-fertilised agricultural soils could act as a sufficient P source for plants for 12-175 years, and that this long-term availability is positively affected by SOC as long as the soil is not too highly saturated with P.