Fractionation of residual phosphorus in a highly weathered sludge‐treated soil: Organic phosphorus

Abstract

Numerous studies have shown that, through transformation processes, organic phosphorus (P) plays an important role in P cycling and plant nutrition in temperate and tropical soils. Field and laboratory studies were conducted to evaluate the transformation of residual organic P in the 0-15 cm surface layer of a sewage sludge-amended Decatur silty clay loam, six years after the final of five consecutive annual sludge applications to field plots of Huntsville and Chicago sludges at 20 Mg/ha and 10 years after a single application of 100 Mg/ha of the same sludges. Two indicator crops, corn (Zea mays L.) and sudangrass (Sorghum sudanenses L.), were grown on these plots for one season. Treatment plots-control, fertilizer, Chicago 20 Mg ha/yr, Huntsville 20 Mg ha/yr, Chicago 100 Mg/ha, (single application), and Huntsville 100 Mg/ha (single application)-were sampled and organic P fractions determined. The P fractions followed the order of: moderately labile P > moderately resistant P > highly resistant P > labile P, both before planting and after harvesting irrespective of the soil treatment. Generally, the Chicago and Huntsville 20 Mg/ha/yr treatments resulted in higher soil organic P fractions than the same sludges at the 100 Mg/ha (single application) rate. The Huntsville sludge treatments also resulted in higher P fractions than the Chicago sludge treatments at the corresponding application rate. Percentages of the two major P fractions to the total organic P, calculated both before planting and after harvesting, revealed that by the end of harvesting, the moderately labile P had decreased by 5.5% while the moderately resistant P had increased by 4.7%, suggesting that transformation of the organic P could have occurred during cropping.

Frações de fósforo acumuladas em Latossolo argiloso pela aplicação de fosfato no sistema plantio direto

“…Mesmo assim, o Po-NaOH 0,1 mol L -1 foi a fração predominante (Figura 4) do Po extraível, com 148 mg kg -1 contra 121 mg kg -1 de Pi-NaOH 0,1 mol L -1 , na média (Quadro 3). A fração orgânica moderadamente lábil (Po-NaOH) foi similar à respectiva fração inorgânica (Pi NaOH) (Quadro 3), embora os valores elevados de Pi-NaOH 0,1 mol L -1 obtidos pela adição de 980 kg ha -1 ou mais de P 2 O 5 (Figura 4), demonstrando, a exemplo de Lindo et al (1996), que essa fração também pode atuar como fonte de P. Os valores de Po encontrados nas extrações com NaHCO 3 e NaOH 0,1 mol L -1 (Quadro 3) evidenciaram a importância do Po como fonte de P às plantas, principalmente em solos intemperizados e, ou, em sistemas com baixas adições de fertilizantes fosfatados (Walker & Syers, 1976;Tiessen et al, 1984). Portanto, sistemas de produção que minimizem as perdas de Po do solo com o cultivo podem ser importantes para a manutenção da disponibilidade de P, garantindo a sua sustentabilidade.…”