Dynamics of phosphorus fractions in a savanna Alfisol under continuous cultivation

We examined the dynamics of inorganic P (P-i) and organic P (P-o) pools of a savanna Alfisol under continuous cultivation complemented with nitrogen (N) and phosphorus (P) fertilizers with or without cow manure (D), using a modified Hedley fractionation. Continuous cultivation without P fertilizer decreased the concentration of P-i and P-o pools including the residual P fraction compared with an unfertilized treatment with natural vegetation. Adding P fertilizers alone or in combination with D (P, NPK, DS-P, D + NP and D + NPK) increased the concentration of labile P-i pools, but decreased the concentration of P-o pools and the residual P fraction. The tendency of the residual P fraction to decrease along with P, pools suggested that the residual P fraction was largely P-o. This is consistent with the correlation between the residual P fraction and total P-o (r = 0.74**) and the residual P fraction and organic carbon content (r = 0.47**). Analysis of the relationships between plant available P, estimated by resin-extractable P, and the more stable P-i and P-o pools indicated that 85% of the variation in resin-P was attributable to the hydroxide extractable P-i (OH-P-i) and HCl-P-i which acted as sinks for fertilizer-P. The contribution of the residual P fraction to resin-P was indirect via bicarbonate-extractable P-o (HCO3-P-o) and OH-P-o. These results also clarify why attempts to find P extraction methods which correlate well with response to P and to P uptake in the savanna have given inconsistent results.