Disponibilidade de fósforo em solos avaliada por diferentes extratores Phosphorus availability in soils, determined by different extracting procedures

O objetivo deste trabalho foi o estudo comparativo dos extratores mais importantes do fósforo do solo. É apresentada uma revisão da literatura sobre métodos de avaliação da disponibilidade de fósforo em solos. Os métodos considerados foram: resina trocadora de ânions, Olsen, Bray 1, Bray 2, Mehlich 1, Truog, Morgan, Égner, Água, CaCl2 0,01M, papel de filtro com hidróxido férrico, e as técnicas de troca isotópica, expressos como valor E e valor L. Nos trabalhos considerados, as comparações de métodos foram feitas com base em correlações entre o P absorvido pelas plantas e o P extraído do solo pelas diferentes técnicas. Inicialmente, foi feita uma comparação conjunta dos resultados de todos os trabalhos, considerando os coeficientes de determinação (r²). Os valores médios obtidos e o número de artigos em que o método foi testado, indicados entre parênteses, foram os seguintes: resina trocadora de ânions, 70% (34); valor E, 68% (16); valor L, 65% (8); Olsen, 54% (48); Bray 1, 50% (42); Mehlich 1, 46% (25); Égner, 44% (9); Bray 2, 42% (19); Água, 42% (15); Truog, 38% (13); CaCl2, 36% (13), e Morgan, 32% (13). O confronto dos diversos métodos em duplas, considerando-se os pares de resultados (r²) obtidos pelos extratores que foram testados conjuntamente, através de correlações lineares e contraste de médias (teste t), levou à conclusão de que o método da resina foi estatisticamente superior aos demais. O método da resina tem os seguintes aspectos favoráveis: (a) apresenta valores de coeficientes de determinação, para a correlação entre P absorvido por plantas e P no solo, consistentemente superiores aos dos demais métodos na maior parte dos 72 trabalhos revisados; (b) pode ser usado tanto em solos ácidos como alcalinos, o que não é o caso para outros extratores importantes; (c) revela, adequadamente, o efeito da calagem em aumentar a disponibilidade de P para as plantas, o que não acontece com os métodos Mehlich 1, Bray 1 e Olsen; (d) não superestima, como os extratores ácidos, a disponibilidade de P em solos tratados com fosfatos naturais; (e) é o que apresenta o melhor embasamento teórico para a determinação do chamado "fator quantidade" de P em solos, que é o mais importante índice da disponibilidade do nutriente.<br>The objective of this paper was the comparative study of the most important extractors of soil phosphorus. It presents a literature review of papers that deal with methods of extraction for the evaluation of phosphorus availability in soils. The methods considered were: anion exchange resin, Olsen, Bray 1, Bray 2, Mehlich 1, Troug, Égner, water, 0,01M CaCl2, iron hydroxide impregnated filter paper (Pi) and the determinations of the E and L values. The comparison between results of soil analysis by different methods of phosphorus extraction in soils and phosphorus uptake by plants was initially made considering the coefficient of determination (r²) for several extractants. The following average values and corresponding numbers of articles in which the method was considered were obtained: resin, 70% (34); E value, 68% (16); L value, 65% (8); Olsen, 54% (48); Bray 1, 50% (42); Mehlich 1, 46% (25); Égner, 44% (9); Bray 2, 42% (19); Water, 42% (15); Truog, 38% (13); CaCl2, 36% (13), and Morgan, 32% (13). The comparison of several methods, considering the pairs of results (r²) obtained for the extractants, that were tested together through linear correlation and average contrasts (t student test), indicated that the resin method was statistically superior to the other methods. The resin method presents the following favorable features: (a) highest values for the determinations coefficients for the correlations between P uptake by plants and soil P by the different methods, as reviewed in 72 papers; (b) it can be used in both acid and alkaline soils, which is not the case for the other most commonly used methods; (c) it reveals adequately the effect of liming on the increase of P availability in soils for crops, which is not the case of Mehlich 1, Bray 1 and Olsen; (d) it does not overestimate P availability in soils treated with rock phosphates, as is the case of acid extractans; (e) it is the method with the best theoretical background

Interaction of contaminated sediment from a salt marsh with estuarine water: evaluation by leaching and ecotoxicity assays and salts from leachate evaporation

Purpose Wastes from a former Portuguese steel plant were deposited between 1961 and 2001 on the riverbank of a tributary of the Tagus River creating a landfill connected to the river, posing a potential contamination risk to the Tagus estuary ecosystem. This study aims to assess the transfer of chemical elements from contaminated sediments to the estuarine water from cycles of sediment leaching so as to evaluate the ecotoxicity of the leachates, and to analyze the solid phases crystallized from those leachates. Materials and methods Landfill sediment and estuarine water samples were collected during low tide. Sediment samples were analyzed for pH, electric conductivity (EC), C-org, NPK, and iron oxides. Leaching assays (four replicates) were done using estuarine water (200 cm(3)/replicate) and 1.5 kg of sediment per reactor. Each reactor was submitted to four leaching processes (0, 28, 49, and 77 days). The sediment was kept moist between leaching processes. Sediment (total (acid digestion) and available fraction (diluted organic acid extraction- Rhizo)) elemental concentrations were determined by inductively coupled plasma-instrumental neutron activation analysis (ICP/INAA). Leachates, and estuarine and sediment pore waters were analyzed for metals/metalloids by ICP/mass spectrometry (MS) and carbonates/sulfate/chloride by standard methodologies. Ecotoxicity assays were performed in leachates and estuarine and pore waters using Artemia franciscana and Brachionus plicatillis. Aliquots of the leachates were evaporated to complete dryness (23-25 degrees C) and crystals analyzed by X-ray powder diffraction (XRD). Results and discussion Sediment with pH=8 and high EC and Corg was contaminated with As, Cd, Cr, Cu, Pb, and Zn. The element concentrations in the available fraction of the sediment were low compared to the sediment total concentrations (<1 % for Rhizo extraction). The concentrations of potentially hazardous elements in the estuarine water were relatively low, except for Cd. Concentrations of hazardous elements in the leachates were very low. Calcium, K, Mg, Na, and chloride concentrations were high but did not vary significantly among the four leaching experiments. Total concentrations of carbonate were much higher in leachates than in estuarine water. Both estuarine water and leachates showed negligible toxicity. Crystals identified in the solids obtained from the leachates by evaporation were halite, anhydrite, epsomite, dolomite, and polyhalite. Conclusions The sediment showed the capacity to retain the majority of the potentially hazardous chemical elements. Remobilization of chemical elements from sediment by leaching was essentially negligible. The variation of total concentrations of Ca, carbonate, and sulfate in leachates indicates that the sediment contained reactive sulfides. Due to its composition, the sediment seems to be a dynamic system of pollution control, which should not be disturbed