Long-term impact of elevated CO2 on phosphorus fractions varies in three contrasting cropping soils

The long-term effect of elevated CO2 (eCO2) on P biogeochemistry in farming systems is largely unknown. This study compared the effects of eCO2 on P fractions in three contrasting soils after growing crops for seven years. An experiment of free-air-CO2-enrichment (FACE) was conducted with a rotation of wheat, field pea and canola grown in intact cores of Chromosol, Vertosol and Calcarosol under ambient CO2 (aCO2) (390 ± 10 ppm) or eCO2 (550 ± 30 ppm). Crop P removal, soil P fractions and biochemical properties were determined. Elevated CO2 resulted in extra 134, 91 and 93 mg P core−1 removed as grains, compared to aCO2, for Chromosol, Vertosol and Calcarosol, respectively. It decreased the concentration of NaHCO3-extractable inorganic P (by 17–36%), and decreased NaOH-extractable inorganic P by 24% in Chromosol, and 77% in Vertosol but did not affect it in Calcarosol. Elevated CO2 also decreased NaOH-extractable organic P by 20, 12 and 7 mg kg−1 in the three soils, respectively. Furthermore, eCO2 decreased soil organic carbon (by 8.2%) and increased microbial biomass carbon and respiration in Chromosol but not in other two soils. Long-term eCO2 favoured microbial mineralization of organic P in Chromosol and chemical mobilization of non-labile inorganic P in all three soils.