Elevated atmospheric CO2 changes phosphorus fractions in soils under a short rotation poplar plantation (EuroFACE)

Future high levels of atmospheric carbon dioxide (CO₂) may increase biomass production of terrestrial plants and hence plant requirements for soil mineral nutrients to sustain a greater biomass production. Phosphorus (P), an element essential for plant growth, is found in soils both in inorganic and in organic forms. In this work, three genotypes of Populus were grown under ambient and elevated atmospheric CO₂ concentrations (FACE) for 5 years. An N fertilisation treatment was added in years 4 and 5 after planting. Using a fractionation scheme, total P was sequentially extracted using H₂O, NaOH, HCl and HNO₃, and P determined as both molybdate (Mo) reactive and total P. Molybdate-reactive P is defined as mainly inorganic but also some labile organic P which is determined by Vanado-molybdophosphoric acid colorimetric methods. Organic P was also measured to assess all plant available and weatherable P pools. We tested the hypotheses that higher P demand due to increased growth is met by a depletion of easily weatherable soil P pools, and that increased biomass inputs increases the amount of organic P in the soil. The concentration of organic P increased under FACE, but was associated with a decrease in total soil organic matter. The greatest increase in the soil P due to elevated CO₂ was found in the HCl-extractable P fraction in the non-fertilised treatment. In the NaOH-extractable fraction the Mo-reactive P increased under FACE, but total P did not differ between ambient and FACE. The increase in both the NaOH- and HCl-extractable fractions was smaller after N addition. The results showed that elevated atmospheric CO₂ has a positive effect on soil P availability rather than leading to depletion. We suggest that the increase in the NaOH- and HCl-extractable fractions is biologically driven by organic matter mineralization, weathering and mycorrhizal hyphal turnover.