Phytomining to re-establish phosphorus-poor soil conditions for nature restoration on former agricultural land

Aims: To restore species-rich grasslands on former agricultural land, typically phosphorus-poor soil conditions need to be re-established. Here we assess the potential of phosphorus extraction by biomass production, i.e. phytomining. We compare two techniques: (i) 'mowing', i.e. cutting and removing hay two or three times a year, and (ii) 'P-mining', i.e. mowing with yield maximization by adding growth-limiting nutrients other than phosphorus (i.e. nitrogen and potassium). Methods: In a five-year field experiment at three fields situated along a soil phosphorus gradient, we studied phosphorus removal through both biomass assessment and changes in two soil phosphorus pools: bioavailable phosphorus (P-Olsen) and slowly cycling phosphorus (P-Oxalate). Results: Phosphorus-mining doubled the phosphorus removal with biomass compared to mowing, and phosphorus removal with biomass was lower at fields with an initially lower concentration of P-Olsen in the soil. The P-Olsen concentrations decreased significantly during the experiment with the largest decreases in phosphorus-rich plots. Changes in the P-Olsen and P-Oxalate stocks were correlated with the amount of phosphorus removed with biomass. Conclusions: Phosphorus-mining effectively increases phosphorus removal compared to mowing, but becomes less efficient with decreasing soil phosphorus concentrations. Restoring phosphorus-poor soil conditions on formerly fertilized land remains a challenge: phytomining most often needs a long-term commitment