Effect of introducing a nitrogen-fixing tree species on the phosphorus cycle in tropical forest plantations
Thesis defended on 29 October 2015 at the Gaia Doctoral School
In the Congo and in Brazil, eucalyptus plantations have been set up in nutrient poor sandy soils. A nitrogen-fixing tree species (Acacia mangium) was introduced into these plantations to improve soil fertility. Previous studies showed that Acacia were more successful at fixing nitrogen in the Congo than in Brazil and that Acacia increased the eucalypt yield only in the Congo.
This thesis studied the P cycle in these two very different forest ecosystems to determine the interaction between N and P controlling the productivity of mixed plantations. An ionic chromatographic method was developed to separate, identify and quantify, simultaneously, 11 anionic forms (organic acids and organic P) in total soil extracts (NaOH 0.5 N). The mineralisation of the Po extracted using sodium bicarbonate and the P bioavailability for the eucalyptus were evaluated for both sites.
Adenosine monophosphate and glucose-6-phosphate were the main forms of organic P in the two ecosystems. However, in the Congo, a large part of the soil Po was not associated with microbial populations and the presence of the acacia significantly increased the P bioavailability for the eucalypts. In Brazil, however, the soil Po was very strongly associated with the microbial fraction with very low bioavailability for the eucalypts, even after planting acacia. However, the P bioavailability increased significantly with non-limiting N and P input.
In conclusion, the results from this thesis suggested that introducing acacia in a low nitrogen soil with optimum N2 fixation, as in the Congo, increased P availability through the recycling of organic P. In Brazil, the introduction of acacia might also increase P bioavailability if nitrogen-fixation were not limited, probably by the very low P availability of the soil.