Recuperação de solos florestais ardidos com recurso a resíduos orgânicos e a sua influência na matéria orgânica do solo

Mestrado em Engenharia do Ambiente - Tecnologias Ambientais - Instituto Superior de Agronomia

Forest fires are common events in Mediterranean countries like Portugal, which contribute in a large way to desertification processes in these areas. In soil, surface is the most affect by fire, mainly the first 5 cm. This layer is often constituted by organic residues and decomposed organic matter, which are the major reservoir of nutrients, among them nitrogen (N), an essential element for plants and organisms. In a short-term, fire releases nutrients from organic matter and make them more available for plants. In a long-term their concentration may be reduced, considering that the replacement of these nutrients in soil is slow after fire. Depending on fire severity different organic matter pools are affected, the most thermolabile are lost even in low intensity fires, while the most resistant, as humin, are lost at higher temperatures. Organic residues application to soils may increase the nutrients content, as well as organic matter, which will affect positively the chemical and physical properties of soil and also the recovery of microbial life and the revegetation. In this work, the effects of fire on nutrient levels (N, P and K), organic matter pools (soil litter, fulvic and humic acids) and organic compounds (lipid, water-soluble compounds, primary cellulose and lignin) in natural burned and unburned soils at different temperatures in a muffle furnace (control, 65, 105 and 250 ºC) were studied. Mineralization after the application of digested pig slurry (P) and municipal organic waste compost (C) to soils was followed for 60 days, and the effects on the previous properties evaluated. Mineral N was analyzed by segmented flow spectrophotometry after 2M KCl extraction, and P and K were analysed by Riehm extraction. The most important results are related to the increased content of mineral N after this fire, and the contribution of pig slurry and mixture applications to increase these values. Simulated fires at the higher temperatures didn´t allow nitrification to take place, due to nitrifying populations death. P and K changes after fire, and after residues applications, were not conclusive, once the loss of these nutrients was small and the contribution of residues to increase their content was not evident. In order to evaluate humic substances content a modified lost on Ignition (LOI) method was used. Fire temperatures were able to reduce the litter and fulvic acids fractions, while the most resistant fraction, the humic acids fraction were not altered. Compost application seemed to contribute more to the increase of fulvic acid fraction, while pig slurry increased the litter fraction. Non-humic substances were determined after a sequence of extractions, non-polar extraction with methylene chloride and ultra-sound for lipids, polar extraction for sugars and acid extraction for primarily cellulose. Lignin was determined by ashing. In the burned andcontrol soils, pig slurry increased the cellulose content, while compost increased the lipid and sugar fractions, after 60 days of incubation.------------------------------------Os fogos florestais são eventos comuns em países mediterrâneos, como Portugal, contribuindo para a desertificação. A superfície do solo é a mais afectada pelo fogo, principalmente os 5 cm superficiais, sendo constituída por resíduos vegetais e matéria orgânica em decomposição, e constituindo um reservatório de nutrientes, como o azoto. A curto prazo, o fogo liberta e disponibiliza nutrientes para as plantas, mas a longo prazo o seu teor diminui, uma vez que a restituição destes ao solo pode ser lenta. Dependendo da intensidade do fogo, diferentes fracções da matéria orgânica são afectadas, as mais termolábeis são perdidas a temperaturas mais baixas, enquanto fracções mais resistentes a temperaturas mais elevadas. A aplicação de resíduos orgânicos ao solo pode contribuir para o aumento do teor de nutrientes e matéria orgânica, que irá afectar positivamente as propriedades químicas, físicas do solo, assim como a recuperação da vida microbiana e a revegetação. Neste trabalho foram estudados os efeitos do fogo sobre os nutrientes substâncias húmicas e não-húmicas, em solo naturalmente queimado e não queimado a diferentes temperaturas (controlo, 65, 105 e 250 ºC). A mineralização após a aplicação de chorume de suínos digerido, resíduos sólidos urbanos compostados e uma mistura dos dois, foi seguida durante 60 dias.