I. H. Salcedo

Nutrient and soil organic matter dynamics under shifting cultivation in semi-arid northeastern Brazil

Abstract In subsistence farming systems using shifting cultivation, crop production depends on the natural fertility of the soil and depletion of fertility during cultivation is the cause for land abandonment. During the subsequent bush fallow, fertility levels are improved and the land may be available for further cultivation cycles. In the present study, the effects of cultivation and the regrowth of bush fallow on soil organic matter and phosphorus fertility were evaluated, using adjacent plots with different histories of cultivation and lengths of bush fallow in semi-arid northeastern Brazil. Six years of cultivation with minimal fertilisation resulted in reductions of C, N and organic P by 30%, or about 10 t ha −1 of C. Available N and P were greatly reduced under these conditions. In a fertilised and limed field, this trend was to some extent avoided, but such inputs are usually not an option for local farmers. Eight to 10 years of bush fallow were sufficient to re-establish fertility levels similar to the original uncultivated site. Phosphorus fractionation indicated that the decline in P fertility was not a result of net export of P in the crop, but arises from the mineralisation of organic P and subsequent transformation of the surplus inorganic P to unavailable forms.

Mineralização de N após incorporação de adubos orgânicos em um Neossolo Regolítico cultivado com milho

Farms within the Agreste region of Paraiba often do not have sufficient animal manure to maintain the fertility of the soil and green manures, such as prunings of Gliricidia sepium, are an alternative to increase nutrient supply. The objectives of this work were to compare N mineralization dynamics following additions of cattle manure or Gliricidia sepium prunings to a Entisol, under field and laboratory conditions. In the field experiment, three treatments were established: addition of 20 t ha-1 of cattle manure; addition of 15 t ha-1 of green branches of Gliricidia, or a control without any addition. Maize was sown immediately after the additions. Soil N mineralization in the field was measured in situ during 120 days, in the 0-15 cm layer. Nitrogen uptake by corn was also recorded. Soil N mineralized in the laboratory was followed during 56 days in samples from the 0-15, 15-30 e 30-60 cm layers. Samples from the 0-15 cm layer received manure and Gliricidia at rates equivalent to those in the field. An average of 74 kg ha-1 of N were mineralized in the field until harvest (120 days). In the laboratory, Gliricidia additions hastened net N mineralization while cattle manure induced net N immobilization during 14 days.

Nutrient availability in soil samples from shifting cultivation sites in the semi-arid Caatinga of NE Brazil

Abstract Soil fertility differed substantially among three neighboring sites representing different stages of a shifting cultivation cycle on a deeply 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 abandoned after five years of manual cultivation to cassava (Abandoned), were compared using subtractive and additive fertilization greenhouse trials and incubations for C and N mineralization. The effect of burning was to increase soil nutrient content relative to the Caatinga area samples, 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 increased by a factor of 5. In all three areas, the most severe nutrient limitation was of P, followed by N. The N deficiency somewhat increased relative to P in the Abandoned samples. Addition of 25 mg P kg−1 soil produced 4 to 5-fold dry matter increases in the Caatinga and 7 to 8-fold increases in the Burnt area samples. Further increasing P levels improved yields only marginally and only at high additions of N and K. In 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 followed 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 ratio. In addition to the nutrient limitations of this Oxisol, samples from the Abandoned area exhibited further limitations resulting in yield 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 predict soil quality.

Acumulação de nutrientes em solos arenosos adubados com esterco bovino

The objective of this work was to quantify changes on carbon and nutrient contents in soils under continuous manure additions. Eighteen cropped fields that had received annual additions of manure for at least two years and four fields under pasture with no history of manure addition were sampled to assess if this practice results in the accumulation or losses of added nutrients. Soils samples from the 0-20, 20-40 and 40-60 cm layers were taken from each field, and analyzed with regard to their physical (particle size and soil density) and chemical (pH, total C, N and P, extractable P by Mehlich-1 and water, and exchangeable cations) properties. Manure samples used in nine fields were also analyzed for chemical composition. Manure additions resulted in a build up of nutrient stocks near to 20 Mg ha-1 of C, 2 Mg ha-1 of N and Ca, and 0.5 to 1 Mg ha-1 of P, K and Mg (0-60 cm). Water-soluble P concentrations of 7 mg kg-1 in the 40-60 cm layer indicated losses of this nutrient, the less mobile in soils. Accumulation of exchangeable cations in the 0-60 cm layer, in comparison with control areas, indicates an even greater potential for losses of these nutrients.

Produtividade de milho solteiro ou em aléias de gliricídia adubado com duas fontes orgânicas

The objective of this work was to evaluate biomass productivity of maize, intercropped or not with gliricidia (Gliricidia sepium), and to evaluate the effect of manure and gliricidia prunnings fertilization on maize and spontaneous vegetation biomass productivity. The work was conducted from 2003 to 2005 within an area with gliricidia rows, spaced 6 m between each other and 1 m between trees, in a Typic Usthorthent, in Esperanca, PB, Brazil. The experimental design was a randomized split plot with two main treatments (with and without gliricidia), and three secondary treatments (control, fertilization with manure, fertilization with gliricidia prunnings), with four replications. During the 1 st , 2 nd and 3 rd years of the study, maize grain productivity in the plots without gliricidia were 268, 129 and 116% greater than yield of plots with trees, respectively. However, total biomass productivity (sum of the biomass produced by maize, gliricidia and spontaneous vegetation) was 86, 120 and 37% greater in the plots with gliricidia in the 1 st , 2 nd and 3 rd years of the study, respectively. Maize and spontaneous vegetation biomass productivity did not differ between plots fertilized with manure or gliricidia prunnings, but these two organic fertilizers led to significantly greater biomass productivity compared to the control plots.

Water budget in a soil with food crops in the semi-arid region of northeast Brazil.

Considering that water is a limiting factor and there are few studies on water balance in the semi-arid Northeastern Brazil, an experiment was established at Coxixola, PB, Brazil, with four treatments (corn and beans crops, bare soil and soil covered with mulch), in plots of 7.7 x 10 m and plant spacing of 1.1 x 1.0 m. Rainfall, evaporation (Class A tank) and soil water (neutron probe) were monitored. Low (212 mm) and irregular rainfall resulted in water deficit towards the final period of the experiment and was responsible for the low water use efficiencies and low productivity of corn (grain and total biomass, 282 and 1141 kg ha-1) and beans (166 and 558 kg ha-1). Bare soil and mulch had similar results, with high water losses, 1.57 and 1.48 mm day-1, respectively, and only small increases in stored soil water (28 and 35 mm, respectively) were observed at the end of experiment. Mulching does not seem to be a promising practice in the area due to this small increase and the difficulty in obtaining plant residues.

Formas preferenciais de acumulação de fósforo em solos cultivados com cana- de-açúcar na região nordeste

The objective of this work was to quantify the main forms of P derived from fertilizer in soils of varying physical and chemical properties, cultivated with sugar-cane, in Pernambuco. Soils from 11 commercial plantations were sampled following the harvest of the plant-cane (Jan. 1994), that had been fertilized in the furrow 18 months before. To find samples with fertilizer-derived P, a pit was opened perpendicular to the plant line and 28 soil samples were taken with the aid of a metal grid measuring 70 cm long by 40 cm high, with 10x10 cm cells. The grid was fixed 10 cm below the soil surface and centered on the residues of the seedpieces. Phosphorus in all these samples was extracted with 0.1 mol L-1 NaOH. In all cases the largest P concentrations were found in the 10-20 cm layer. Two samples were then selected from this layer for additional work, those with the highest and lowest NaOH extractable-P content, for each soil. Phosphorus in these selected samples was sequentially extracted with resin (P-res), NaHCO3 (inorganic and organic P-bic), NaOH (inorganic and organic P-OH), H2SO4 (P-ac.), and followed by a final digestion with H2SO4 and H2O2 (P-rdu). Most of the difference between samples with high and low P contents (DP), attributed to the fertilization 18 months before, was found in inorganic (P-res, Pi-bic, Pi-OH and P-ac)) forms. In five of the soils, fine textured and with high dithionite-extractable Fe and Al, more than 50% of DP was found as Pi-OH. In the other six soils, P was similarly distributed among P-res, P-bic and P-OH or even favored the resin and bicarbonate-extractable forms (more sandy soils). The presence of large amounts of labile P-res and P-bic forms, even after 18 months of soil-fertilizer contact, suggests the possibility of obtaining residual fertilizer effects in some of these soils.

Influence of tree species on the herbaceous understory and soil chemical characteristics in a silvopastoral system in semi-arid northeastern Brazil

Studies from some semi-arid regions of the world have shown the beneficial effect of trees in silvopastoral systems, by promoting the formation of resource islands and increasing the sustainability of the system. No data are available in this respect for tree species of common occurrence in semi-arid Northeastern Brazil. In the present study, conducted in the summer of 1996, three tree species (Zyziphus joazeiro, Spondias tuberosa and Prosopis juliflora: ) found within Cenchrus ciliaris pastures were selected to evaluate differences on herbaceous understory and soil chemical characteristics between samples taken under the tree canopy and in open grass areas. Transects extending from the tree trunk to open grass areas were established, and soil (0-15 cm) and herbaceous understory (standing live biomass in 1 m2 plots) samples were taken at 0, 25, 50, 100, 150 and 200% of the average canopy radius (average radius was 6.6 ± 0.5, 4.5 ± 0.5, and 5.3 ± 0.8 m for Z. joazeiro, P. juliflora, and S. tuberosa , respectively). Higher levels of soil C, N, P, Ca, Mg, K, and Na were found under the canopies of Z. joazeiro and P. juliflora: trees, as compared to open grass areas. Only soil Mg organic P were higher under the canopies of S. tuberosa trees, as compared to open grass areas. Herbaceous understory biomass was significantly lower under the canopy of S. tuberosa and P. juliflora trees (107 and 96 g m-2, respectively) relatively to open grass areas (145 and 194 g m-2). No herbaceous biomass differences were found between Z. joazeiro canopies and open grass areas (107 and 87 g m-2, respectively). Among the three tree species studied, Z. joazeiro was the one that presented the greatest potential for use in a silvopastoral system at the study site, since it had a larger nutrient stock in the soil without negatively affecting herbaceous understory biomass, relatively to open grass areas.

Residual N and P fertilizer effect and fertilizer recovery on intercropped and sole-cropped corn and bean in semiarid northeast Brazil

The effects of a single 15N and P fertilizer application (16 and 12 kg ha–1) on intercropped and sole-cropped corn and beans was followed over three consecutive years. Grain (0.1–0.9 ton ha–1 yr–1) and straw productions (0.2–2.5 ton ha–1 yr–1) were limited by rainfall and showed small responses to fertilizer. In the first year, plant N uptake was more than twice the fertilizer amounts, while P uptake was less than half the fertilizer amounts. Plant N derived from fertilizer was low (9–19%). Sole corn took up more (34%) than beans (16%) and the combined intercrop (26%) and also had higher recovery of fertilizer in the soil than single beans (50% against 28%). The distribution of fertilizer N and P in the soil showed a similar pattern in all treatments, with a high concentration around the application spot and decreasing concentrations at greater distances and above and below this point. Total P increases in a soil volume 10 cm around the application spot corresponded to 60% of the amount applied. Fertilizer contributions to the second crop were < 3% of total plant N and represented <6% of the applied amount. Therefore, the residual fertilizer effect on production was attributable to P. The patterns of fertilizer N and P distribution in the soil remained similar but N recoveries decreased 14–18%. Despite low rainfall, low productivities and reasonable proportions of fertilizer N remaining in the soil, the residual effects of the applied fertilizer N were too low to justify a fertilizer recommendation based on economic returns on the investment.

Phosphorus and Global Change

Phosphorus (P) is both an agent of global change, with P loads increasing in most global environments due to the loss of mined phosphate from agricultural, industrial, and urban environments, and is affected by global change processes such as land degradation or the need for P in biofuel production. P plays a fundamental role in food security and, because the only source for new P inputs to agriculture are phosphate rock deposits, P is a strategic, limited resource. Increasing the food supply for a growing world population requires additional P while sources are slowly being depleted. Sustainability of food, fiber, and fuel demands efforts towards maximizing the efficient use of this nutrient and defining priorities for its use. However, P is being used in production systems in such a way that large amounts of P leak into down-stream ecosystems. The negative effects of eutrophication are well known and occur now at a global scale. P use will have to be accompanied by greater efforts towards re-use, recycling, and strategic targeted applications.