Global climate change and the rising concentrations of atmospheric CO2 have increased
concerns about the future of planet Earth. Numerous studies have been carried out to
investigate and clarify the response of elevated atmospheric CO2 on different plant
species. The forests of the world play significant role in the global carbon budget.
Previous studies of elevated CO2 were mostly focused on single plant species grown in
controlled environments and for relatively short periods and often in small root
volumes. These systems do not allow investigation of the fate of phosphorus under
changing environmental conditions. In this study, a P budget has been estimated to
investigate the effect of elevated CO2 on P cycling. The experimental site used is in
central Italy (EUROF ACE), where three Populus species were grown under elevated
atmospheric CO2 conditions, using FACE (Free Air CO2 Enrichment). Phosphorus
fractionation and organic phosphorus were determined under ambient (350 ppm) and
elevated (550 ppm) atmospheric CO2 conditions in three Populus species; P. alba, P.
nigra and P. x euramericana in two fertilization treatments. P fractionation was carried
out by determining water, NaOH extractable, HCl extractable and HNO3 digestible P
fractions. Within the 0-60 cm soil depth investigated, growth of poplar under elevated
CO2 increases P stores in the soil. The values of total P in 0-60 cm layer for the three
species ranged between 130-175 mg kt1 in ambient and 205-230 mg kt1 in FACE.
Total P content decreased down the soil profile especially at the soil depth of more than
50 cm. Higher organic P was found under FACE (22-29 mg kt1
) as compared to
ambient (15-25 mg kg-1
). In all treatments organic P was highest at 50-60 cm soil depth.
There was no significant difference between species and between different fertilisation
treatments in the amount of inorganic P determined. The organic matter content was 4-
8% in the soils of experimental site, but was less under FACE. In leaves, roots and
wood samples, P was determined and an estimate of total biomass P was calculated.
General trends show that most of the species contained more phosphorus under FACE
in leaf and root samples, while in the wood samples the opposite was found. Using a
multi element analysis, an attempt was made to estimate the pools of P taken up by the
trees. Ca/Sr ratios have often been used for this purpose. In this work Ca/Sr ratios did
not change under FACE nor were species and treatments effects shown. Similarly no
clear insight was gained from the other element ratios about the source of P utilized by
the trees. By constructing a P budget, it was shown that P stores in the soil clearly
increased under FACE, and this was mainly in the HCI extractable fraction. As no
parallel decrease was seen in any other soil P pools, this suggests that P is being moved
into the 0-60 cm soil layer. The possible mechanisms of this movement are discussed.