Soil P availability under eucalypt and acacia on Ferralic Arenosols, republic of the Congo

• Soil fertility is improved by the introduction of nitrogen fixing species.
• Introducing nitrogen fixing species decreases soil available P.
• Nitrogen fixing species may induce the shift of from N-limitation to P-limitation.

Introducing nitrogen fixing species (NFS) in forest plantations reduces soil N-limitation, but also involves changes in phosphorus (P) availability in the Ferralitic Arenosols of the Congolese coastal plains or Batéké Plateaux in Central Africa. We evaluated soil-available P and total P in above-ground litters, leaves, bark and wood in pure (100A, 100E) and mixed-species (50A50E) stands of acacia (a NFS) and eucalypt plantations in the Congolese coastal plains at year 2 of the second rotation (Y2R2) compared to the end of the first 7-year rotation (EndR1). Soil available P was measured as resin P, bicarbonate-extractable inorganic (Pi-HCO3) and organic (Po-HCO3). Soil resin-P values (15–19 mg P kg− 1) in 100E were 80% higher relative to 100A (8–17 mg P kg− 1) at Y2R2 against no difference for both 100E and 100A (8–12 mg P kg− 1) at EndR1. Total P concentration was higher in acacia wood (0.61 g P kg− 1 of dry mass (DM)) than in eucalypt wood (0.57 g P kg− 1 of DM) in 50A50E at Y2R2, while higher stock of P and higher ratio of N:P ratios were found in the foliage of acacia than of eucalypt trees. Our data suggests that the risk of shifting from N-limitation to P-limitation system is minor. However, in the long term, P-limitation may eventually occur in pure acacia plantations, due to mining of soil available P by acacia's higher P uptake relative to eucalypt and additional requirement for symbiotic fixation of atmospheric N2.