N-fixing tree species introduced in Eucalyptus forest modify soil organic P and low molecular weight organic acid pools

Eucalypts, fast-growing trees, are planted in nutrient-poor soils such as sandy savannas in Congo, and deplete highly soil macronutrients such as N and P. To increase N inputs, the N-fixing tree species acacia was intercropped in eucalypt plantations. Increasing N availability could also change organic P (Po) cycling via different litter decomposition rate and increased contents of low molecular weight organic acids (LMWOAs) released by microbes. To investigate the effects of acacia on P cycling, we used first high performance ion chromatography to quantify Po forms and LMWOAs in soil from different plantations: pure stand of acacia, ofeucalyptus and the mixture of both species. We separated and identified six Po forms (glucose-6-P, AMP, ATP, phytate, fructose bis-P, uridine-diP) and four LMWOAs (malate, malonate, oxalate, citrate). Glucose-6-P and AMP were the dominant forms of Po in all stands, but their concentrations decreased in the mixture compared to the pure stands. Phytate was present in all soil samples and was much higher in eucalypt than in acacia stands, with intermediate values in the mixture. Oxalate and malate were the dominant LMWOAs, especially in eucalypt pure stands. Po mineralization was estimated in Olsen extract using two recombinant fungal enzymes, a phytase (from Aspergillus niger) and an acid phosphatase (from an ectomycorrhizal species). Both enzymes were able to release inorganic P from Po present in the soil extracts. To go further into the effect of LMWOAs on P bioavailability, two concentrations of organic acids were added to soil before running enzyme measurements. In conclusion, our results show that intercropping of acacia modified both organic P forms and LMWOAs concentrations in pure and mixed stands and presumably P cycling