Abstract:
Several s tudies in the savanna continue to justify the benefits of inorganic ferti lizers. Parallel studies on the long – term effects of fertilizers on the soils a r e scanty. The objective of this study was to determine t h e impact of inorganic f e rtilizers with or without farmyard manure on carbon (C), nitrogen (N) and p h o sphorus (P) in a savanna alfisol after 45 years of continuous cultivation. Soils for t he study were fertilized with N;N+P. N+P+K. farmyard manure (FYM) FYM+N. FYM+Pr FYM+NP and FYM+NPK. Continuous cultivation caused significant losses of C.N and P. The N+P or N+P+K fertilization could not mitigate t h e s e losses. Losses of N and Poc curred mainly in organic forms. However, combination of farmyard manure with N+P and N+P+K fertilization maintained C.N and P at par with or greaterth an the native site soil. Inorganic fertilizersstressed microbial activity , evidenced by the low microbial biomass C.N and P compared to t h e soil under native vegetation and soils fertilized with FYM+N a n d FYM+NPK. The study further examined the dynamics of inorganic P(P,) and organic (P0) pools of the soils using a modified Medley Fractionation. Continuous cultivation without P, fertilization decreased the concentration of P1 and P0pools including residual P fraction. Fertilization with P, or in combination with FYM (P.N+P+K. FYMP+P, FYM+NP a n d FYM+NPK) increased the concentration of labile P1 pools, but decreased the concentration of P0 pools and the residual P fraction. The tendency of th e residual P fraction to decrease along with P0 pools suggested that the residual P fraction was largely P0, consistent with the correlation between residual P fraction and total P0 (r = 0.74) and with organic carbon (r = 0.47). Path analysis of the relationships between resin extractable P and the more stable Pt and P0 pools indicated that 85% of the variations in resin-P were explained by hydroxide extractable P, (OH-P1) and HC1-P, which acted as sinks for fertilizer-P. The contribution of residual P fraction to resin-P was indirect via bicarbonate extractable P„ (HC03-Po) and 0H-Po suported by the significant path coefficients between them. Thus, the hypothesis that considers residual P fraction stable and inactive needs to be revised. The study thus concluded that from the perspective of soil quality maintenance and sustainable landuse systems a rational fertilization strategy in the savanna is one that combines inorganic fertilizers with FYM or organic/crop residue management to maintain SOM.
CARBON. NITROGEN AND PHOSPHORUS DYNAMICS INDUCED BY LONG-TERM CULTIVATION AND MANAGEMENT
