Previsão dos fluxos de azoto a partir de resíduos orgânicos aplicados ao solo
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2019-08-08
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A adequada previsão da quantidade de azoto (N), mineralizado pelos resíduos orgânicos, é importante para o desenvolvimento de práticas que maximizem o uso eficiente do N e minimizem os impactos adversos deste nutriente, quer no ambiente, quer na produtividade das culturas. Permitirá, também, potenciar a valorização dos resíduos orgânicos e a sua integração nos princípios de uma bioeconomia sustentável e circular. Desta forma, é necessário estudar, desenvolver ou adaptar métodos simples e rápidos, que permitam quantificar em cada resíduo, a quantidade de N potencialmente mineralizável e consequentemente utilizável pelas culturas. O presente trabalho teve como objetivo estudar métodos expeditos para a previsão da mineralização de N orgânico (Norg) de resíduos, após a sua aplicação ao solo. Para o efeito, foram selecionados vinte e oito resíduos orgânicos, representativos da diversidade de resíduos disponíveis a nível nacional: estrumes tradicionais, resíduos de agropecuária não estabilizados, restolho de culturas, folhadas, material equivalente a diferentes siderações, resíduos de agroindústrias e compostados. Inicialmente, foi efetuada uma caraterização química, incluindo o fracionamento dos compostos de carbono dos resíduos. Procedeu-se também à quantificação do N potencialmente disponível pelo método de extração KCl 2M a frio e a 100ºC acidificado. Para avaliar o N potencialmente mineralizável, realizaram-se três incubações de curta duração: aeróbia, em suspensão aquosa e em suspensão de tampão fosfato. A incubação aeróbia foi conduzida a 35ºC durante 14 dias e constituiu o método de referência. Na mesma incubação, colocou-se in situ, em cada unidade experimental, uma membrana de troca aniónica para a determinação do N nítrico. A incubação em suspensão aquosa realizou-se em sistema fechado e aberto e a suspensão com tampão fosfato 1/15M em sistema aberto. Nas incubações em sistema aberto, colocou-se, em cada unidade experimental, uma membrana de retenção do N-NH3 com objetivo de melhorar a coerência e homogeneidade dos resultados. Com base nos resultados independentes da incubação aeróbia de 14 dias com os vinte e oito resíduos em estudo, validou-se uma árvore de decisão parametrizada e proposta por Lashermes et al. (2010). A caraterização dos resíduos confirmou a sua elevada diversidade, com valores de Norg compreendidos entre 5,5 e 86,3 g kg-1 MS. O acréscimo de N hidrolisado (ΔNhidr), pela acidificação e aquecimento a 100ºC do KCl 2M, apresentou boa correlação (r=0,611, p<0,01) com a mineralização líquida aparente (MLA) de N determinada com o método de referência, confirmando que se trata de uma metodologia promissora para a quantificação do azoto potencialmente mineralizável a partir dos resíduos orgânicos. Na incubação aeróbia, observou-se um efeito significativo do tempo de incubação, com a duração de 14 dias a mostrar-se mais eficaz na amplificação e discriminação do comportamento dos resíduos. A diversidade dos resíduos considerados permitiu validar este método para situações extremas, tendo-se observado quer elevados potenciais de mineralização, com uma mineralização líquida aparente máxima de 0,60 g g-1 (g N mineral por g N orgânico adicionado), quer elevados potenciais de imobilização, com uma imobilização máxima de -2,39 g g-1 . As melhores relações entre a MLA e as características dos resíduos foram obtidas com a razão Corg/Norg e o N orgânico. No entanto, a razão Corg/Norg no intervalo de valores 10-30 não se mostrou adequada à previsão da mineralização/imobilização do N dos resíduos. A nitrificação líquida aparente (NLA), avaliada pela colocação in situ de membranas de troca aniónica, apresentou correlação elevada com a MLA. As incubações em suspensão aquosa fechada e em suspensão de tampão fosfato não se apresentaram como métodos fiáveis para a previsão da mineralização do Norg, nem mais expeditos que a incubação aeróbia aos 14 dias (método de referência). A membrana de retenção de N-NH4 + não melhorou as correlações, das incubações em sistema aberto, com a MLA da incubação de referência. A validação da árvore de decisão, realizada com os resíduos em estudo, apresentou uma grande robustez, tratando-se de uma ferramenta eficaz na tipologia/classificação dos resíduos por potencial de mineralização. Apenas em dois resíduos, de todos os estudados, a aplicação da árvore conduziu a previsões não confirmadas pelos resultados experimentais. Os restantes resíduos foram distribuídos por quatro classes de potencial de mineralização do Norg. Uma das classes agrupou 89% dos resíduos e os restantes 11% foram distribuídos por outras três classes. A árvore de decisão, baseada nas caraterísticas iniciais dos RO, parece ser, das alternativas estudadas, o método mais fiável de previsão do potencial de mineralização. Os parâmetros teor de Norg e o fracionamento dos compostos de C, permitiram prever o sentido (imobilização/mineralização) e intensidade da MLA, e deverão ser incluídos em análises de rotina.
The correct prediction of nitrogen (N) mineralization from organic wastes is fundamental to set adequate practices that maximize the efficient use of N and decreasing the negative impacts of this element on both environment and crop productivity. Furthermore, it will also enable the exploitation of organic wastes within the principles of a sustainable and circular bioeconomy. Thus, it is necessary to study, develop or adapt simple and fast methods that will serve as an indicator of the amount of potential N mineralizable, and by consequence, easily available for crops. The main objective of the current work was to study feasible methods to be used in the prediction of N organic (Norg) mineralization, after wastes been applied to the soil. For this purpose, twenty-eight organic wastes, representative of the diversity of wastes available in the country, were selected, namely: farmyard manure, non-stabilized animal manure, crop residues, leaf litter, green manure, agroindustrial residues and composting. Initially, a chemical characterization was performed, including the fractionation of carbon compounds from wastes, and also a quantification of the potential N available by KCl extraction method at 2M acidified, at cold and 100ºC. To evaluate the potential N available from organic N mineralization, three short-term incubations were performed, as follows: aerobic, waterlogged with water and with phosphate buffer. The aerobic incubation was conducted at 35°C for 14 days and was considered the reference method. In the same incubation, an anion exchange membrane was placed in situ in each experimental unit for the determination of nitric N. The incubation in aqueous suspension was done in a closed and open system and the suspension in 1/15M phosphate buffer was made in an open system. In the open system incubations, an N-NH3 retention membrane, but permeable to other gases, was placed in each experimental unit. Based on the independent results from the 14 days aerobic incubation, a parametrized decision tree was proposed and validated according Lashermes et al. (2010) criteria. From our work, it was possible to establish important findings and conclusions. The characterization of wastes confirmed their high diversity, with Norg values between 5,5 and 86,3 g kg-1 dry weight (DW). The surplus N (ΔNhidr) hydrolyzed by acidification and heating at 100ºC, showed a good correlation (r=0,611, p<0,01) with the net mineralization of N (MLA), that confirm this methodology as promising to quantify the potential mineralization of N directly from wastes. The results from aerobic incubation showed a significant effect of the incubation time, and the duration of 14 days was more effective in the amplification or discrimination of the behavior of wastes. The diversity of the residues considered allowed to validate this method for extreme situations, with high potential of N mineralization with a maximum apparent liquid mineralization of 0,60 g g-1 and high potential of immobilization with a maximum immobilization of -2,39 g g-1 . The best relationships between the MLA and waste characteristics were obtained with the several forms of carbon and organic/total N. However, the Corg/Norg ratio in the range of 10-30 was not adequate for the prediction of N mineralization/immobilization. The net nitrification (NLA), evaluated by the in situ placement of anion exchange membranes, showed a high correlation with the MLA. Incubations in closed aqueous suspension and phosphate buffer suspension were not shown to be reliable predictors of Norg mineralization nor more expeditious than 14 days aerobic incubation (reference method). The N-NH3 retention membrane did not improve the correlations from incubations in open systems with the reference incubation MLA. In the validation of the decision tree, only in two wastes, the classification or not confirmed by experimental results. One potential mineralization classes grouped 89% of the wastes and the other three classes distributed the remaining 11%. The N content and the fractionation of the initial carbon compounds allowed the predicting of the direction (immobilization/mineralization) and intensity of the MLA. The validation of the decision tree presented a great robustness, being an effective tool to typify/classify the wastes by potential of mineralization. Finally, based in our results we can also conclude that the organic N mineralization is influenced by the quality parameters of the residues and, in addition to the organic N, the fractionation of C composts is important as it brings information about the quality of the RO and should be included in routine analyzes.
The correct prediction of nitrogen (N) mineralization from organic wastes is fundamental to set adequate practices that maximize the efficient use of N and decreasing the negative impacts of this element on both environment and crop productivity. Furthermore, it will also enable the exploitation of organic wastes within the principles of a sustainable and circular bioeconomy. Thus, it is necessary to study, develop or adapt simple and fast methods that will serve as an indicator of the amount of potential N mineralizable, and by consequence, easily available for crops. The main objective of the current work was to study feasible methods to be used in the prediction of N organic (Norg) mineralization, after wastes been applied to the soil. For this purpose, twenty-eight organic wastes, representative of the diversity of wastes available in the country, were selected, namely: farmyard manure, non-stabilized animal manure, crop residues, leaf litter, green manure, agroindustrial residues and composting. Initially, a chemical characterization was performed, including the fractionation of carbon compounds from wastes, and also a quantification of the potential N available by KCl extraction method at 2M acidified, at cold and 100ºC. To evaluate the potential N available from organic N mineralization, three short-term incubations were performed, as follows: aerobic, waterlogged with water and with phosphate buffer. The aerobic incubation was conducted at 35°C for 14 days and was considered the reference method. In the same incubation, an anion exchange membrane was placed in situ in each experimental unit for the determination of nitric N. The incubation in aqueous suspension was done in a closed and open system and the suspension in 1/15M phosphate buffer was made in an open system. In the open system incubations, an N-NH3 retention membrane, but permeable to other gases, was placed in each experimental unit. Based on the independent results from the 14 days aerobic incubation, a parametrized decision tree was proposed and validated according Lashermes et al. (2010) criteria. From our work, it was possible to establish important findings and conclusions. The characterization of wastes confirmed their high diversity, with Norg values between 5,5 and 86,3 g kg-1 dry weight (DW). The surplus N (ΔNhidr) hydrolyzed by acidification and heating at 100ºC, showed a good correlation (r=0,611, p<0,01) with the net mineralization of N (MLA), that confirm this methodology as promising to quantify the potential mineralization of N directly from wastes. The results from aerobic incubation showed a significant effect of the incubation time, and the duration of 14 days was more effective in the amplification or discrimination of the behavior of wastes. The diversity of the residues considered allowed to validate this method for extreme situations, with high potential of N mineralization with a maximum apparent liquid mineralization of 0,60 g g-1 and high potential of immobilization with a maximum immobilization of -2,39 g g-1 . The best relationships between the MLA and waste characteristics were obtained with the several forms of carbon and organic/total N. However, the Corg/Norg ratio in the range of 10-30 was not adequate for the prediction of N mineralization/immobilization. The net nitrification (NLA), evaluated by the in situ placement of anion exchange membranes, showed a high correlation with the MLA. Incubations in closed aqueous suspension and phosphate buffer suspension were not shown to be reliable predictors of Norg mineralization nor more expeditious than 14 days aerobic incubation (reference method). The N-NH3 retention membrane did not improve the correlations from incubations in open systems with the reference incubation MLA. In the validation of the decision tree, only in two wastes, the classification or not confirmed by experimental results. One potential mineralization classes grouped 89% of the wastes and the other three classes distributed the remaining 11%. The N content and the fractionation of the initial carbon compounds allowed the predicting of the direction (immobilization/mineralization) and intensity of the MLA. The validation of the decision tree presented a great robustness, being an effective tool to typify/classify the wastes by potential of mineralization. Finally, based in our results we can also conclude that the organic N mineralization is influenced by the quality parameters of the residues and, in addition to the organic N, the fractionation of C composts is important as it brings information about the quality of the RO and should be included in routine analyzes.
Descrição
Tese de Doutoramento em Ciências Agronómicas e Florestais
Palavras-chave
Incubação aeróbia , incubação em suspensão aquosa