NUTRIENT AVAILABILITY IN SOIL SAMPLES FROM SHIFTING CULTIVATION SITESIN THE SEMIARID CAATINGA OF NE BRAZIL

Soil fertility differed substantially among three neighboring sites re presenting different stages of a shifting cultivation cycle on a deepl y weathered Oxisol on the Chapada de Araripe in the semiarid region of NE Brazil. Samples taken from a native thorn forest area (Caatinga), from a recently slashed and burnt area (Burnt) and from a site abandon ed after five years of manual cultivation to cassava (Abandoned), were compared using subtractive and additive fertilization greenhouse tria ls and incubations for C and N mineralization. The effect of burning w as to increase soil nutrient content relative to the Caatinga area sam ples, followed by decreases of N and organic P by 20%: available P by 70%, and exchangeable bases by 55%, on average, after abandonment. The unfertilized control of the missing-element trial gave the lowest dry matter (DM) yields, but still those of the Burnt were twice as large as those of the Abandoned area samples. The relative yield ratios were not changed by nutrient addition, although total yields were increase d by a factor of 5. In all three areas, the most severe nutrient limit ation was of P, followed by N. The N deficiency somewhat increased rel ative to P in the Abandoned samples. Addition of 25 mg P kg(-1) soil p roduced 4 to 5-fold dry matter increases in the Caatinga and 7 to 8-fo ld increases in the Burnt area samples. Further increasing P levels im proved yields only marginally and only at high additions of N and K. I n samples from the abandoned area, 25 mg P kg(-1) not even doubled the dry matter yield, and no additional effects were observed for higher N and K rates. Carbon mineralization during the 12 week incubation fol lowed the order Caatinga > Abandoned > Burnt, whereas the order for N removed in 13 leachings was Caatinga > Burnt > Abandoned. Accordingly, organic matter mineralized from the abandoned soil had a higher C:N r atio. In addition to the nutrient limitations of this Oxisol, samples from the Abandoned area exhibited further limitations resulting in yie ld reductions, which were not explained by any of the factors examined . This indicates that alternatives to long-fallow shifting cultivation will require further refinement of methodologies to measure and predi ct soil quality. (C) 1997 Elsevier Science B.V.