Extractability and Transformation of Copper and Zinc Added to Tropical Savanna Soil under Long-Term Pasture

The extractability and slow reactions of copper (Cu) and zinc (Zn) in a weathered savanna soil under Brachiaria decumbens, Digitaria smutsii, and Stylosanthes guianensis pastures were determined in a laboratory incubation study using a sequential extraction to remove operationally defined fractions of the metals, consisting of exchangeable, organically bound, precipitate [occluded in aluminum (Al) and iron (Fe) oxides], and residual metal fractions. The soils from the pasture fields were spiked with 100 mg Cu kg(-1) soil and 200 mg Zn kg(-1) soil for 24 weeks. Copper and Zn extractable with 1 N potassium nitrate (KNO3) solution decreased exponentially with time but reached a steady state after 2-3 weeks. The concentrations of Cu and Zn exchangeable with potassium (K) were greater in the Digitaria smutsii field soil than Brachiaria decumbens and Stylosanthes guianensis field soils. The exchangeability of added Cu and Zn (indexed M-(exch)(n+)) with time was described by a simple exponential decay equation: M-(exch)(n+) = alpha e(beta t), where alpha is a constant, beta is a coefficient that defines the rate of transformation of added Cu and Zn from the exchangeable to nonexchangeable pools, and t is time. The beta values for Cu (0.040-0.076 mg kg(-1) d(-1)) were almost 10 times greater than those of Zn (0.005-0.007 mg kg(-1) d(-1)). Sequential extraction of added Cu and Zn indicated that between 26 and 30% of the total Cu and between 19 and 30% of the total Zn were associated with organic matter. Similarly, between 35 and 38% of total Cu and between 47 and 60% of total Zn were associated with Fe, Al, and manganese (Mn) oxides. The differential capacity of the pasture fields to transform added Cu and Zn from exchangeable and labile form to nonlabile and nonexchangeable form appears to be governed by organic matter (OM), pH, and active Fe ratio in the pasture field soils.