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 Hedly 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, D+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 o 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 atiributable to the hydroxide extractable P i , (OH‐P i ) and HCI‐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 (HCO 3 ‐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.