Acacia and eucalypt change P, N and C concentrations in POM of Arenosols in the Congolese coastal plains

As an active part of soil organic matter (SOM), particulate organic matter (POM; 4000-50 mu m) quickly reveals changes occurring in SOM status after land-use change. To evaluate the impact of planting eucalypts and acacias in the tropical savannas of Congolese coastal plains on SOM quality, we determined P, N and C concentrations in POM in the 0-10 cm layer in afforested stands (pure or in combination) and savannas. Soil available P in the coarse fraction of POM (cPOM; 4000-250 mu m) in afforested stands (> 60 mg kg(-1)) was higher than in savannas (11 mg kg(-1)), probably due to both high P content and high decomposition rates of organic residues that have accumulated over a 30-year period. However, only in the afforested stands containing acacias was N concentration (> 1.50%) in cPOM higher than in savannas (< 1%), while the whole soil C content of afforested stands (> 1.25%) was significantly higher than in savannas (< 0.60%). Low C: N ratios of whole soil and cPOM in afforested stands containing only acacia confirmed the improvement of N status in these stands compared with afforested stands of pure eucalypt and mixed-species stands. Planting acacias and eucalypts in the savannas of coastal Congolese plains improved SOM quality of inherently infertile soils. This practice may be used for this purpose in other areas of savanna of surrounding countries of the central Africa.