Appliqués seuls, les engrais minéraux azotés et phosphatés accélèrent la dégradation de la matière organique d’un sol acide d’altitude du Mugamba sud (Burundi)
Two concomittant studies were conducted, one under laboratory conditions during 56 days (8 weeks) at ambient temperature
(25±1°C), and a pot study conducted during 60 days with an amaranth crop test. The laboratory investigation aimed at
evaluating the soil CO2 evolution following N and P mineral fertilizers, whereas the pot study measured the residual effect
of the same fertilizers on amaranth aerian dry biomass. The global objective of the experiments was to verify the hypothesis
stating that N and P mineral fertilizers, when solely applied decrease the soil organic matter content (priming effect).
Nitrogen and Phosphorus mineral fertilizers used in the studies were NH4NO3 and K2HPO4. They were applied at equivalent
rates of 0, 40 and 80 kg N or P2O5 per ha, in a completely randomized design with 3 replicates. Obtained results indicate an
increasing amaranth aboveground dry biomass in the following order: N80P80 ≥ N0P80 = N40P80 > N80P40 = N40P40 = N0P40 >
N80P0 = N40P0 = N0P0. Thisis a clear indication of highly marked effects of P (p < 0.0001), in comparison with N (p < 0.007).
Such observation confirms the fact that P constitutes the most limiting nutritive element of soil productivity in Burundi.
Similarly, application of higher P rates (N0P80, N80P80) is characterized by higher soil organic matter decomposition (k=
and 0.000146±(0.000009) year-1
), with shorter half-live values oscillating around 13 years. The
present investigation fully highlights risks associated with over-fertilization with mineral fertilizers on the sustainability of
soil organic matter. It demonstrates once again the rationality associated with the organo-mineral fertilization approach of
Burundi soils, combining mineral fertilizers, lime and organic matter, in what is considered an integrated soil fertility