Fertilidade e contaminação por metais pesados e microrganismos fecais de um solo sob pastagem pela aplicação de lama residual urbana Soil fertility and contamination by heavy metals and faecal microorganisms as affected by biosolids application in pasture

Abstract

Em vastas áreas do Alentejo, os solos sob pastagem natural apresentam baixa fertilidade. A aplicação de lama residual urbana (LRU) veicula matéria orgânica (M.O.) e nutrientes para o solo, mas também pode introduzir metais pesados e bactérias de origem fecal, pelo que é conveniente monitorizar o solo após a adição destes resíduos. Comparam-se as fertilizações orgânica com LRU e a mineral, nos efeitos em alguns índices de fertilidade e contaminação metálica e fecal de um solo derivado de xistos e grauvaques, no Alentejo, no 1º ano de um campo experimental com pastagens. O campo, com um esquema experimentalem “split-plot”, foi constituído por seis talhões de 0,5 ha, correspondentes a três tratamentos de fertilização (nula, mineral e orgânica, com LRU), em dois tipos de pastagem, natural e semeada. Aplicaram-se cerca de 13 t/ha de uma mistura de LRU das ETARs de tratamento secundário de Alvito e de Vila Nova de Baronia, com teores apreciáveis de M.O., N e Ca. A adubação incluiu N, P, K, Zn e Mo. Determinaram-se os valores de pH (H2O) e os teores de M.O. total, N total, P e K “assimiláveis”, catiões de troca e de Cd, Cr, Cu, Ni, Pb e Zn extraíveis por água régia, em amostras de terra (fracção In wide areas of Alentejo, soils under natural pasture have low fertility. Urban biosolids (UB) application introduce organic matter (O.M.) and nutrients in the soil, but it can also add heavy metals and bacteria of faecal origin. Thus, soil monitoring after the application of these residues is required. Organic with UB and mineral fertilisations were compared regarding their effects on some fertility and metallic and faecal pollution indicators of a soil derived from schists and grauwacks, in the 1st year of a field experiment with pastures. The experimental layout was a split-plot design, with six plots of 0.5 ha, referring to three fertilisation treatments (“nil”, mineral, and organic, with UB), in natural and sown pastures. The UB application was about 13 t/ha of a UB mixture from the Alvito and Vila Nova de Baronia plants, rich in O.M., N, and Ca. Mineral fertilisation included N, P, K, Zn, and Mo. Soil samples (fraction <2 mm) collected at the 0-15/20 cm depth, before the fertilisers application and around one year after the establishment of the experiment were analysed for pH (H2O) value and O.M., total N, available P and K, exchangeable cations, and extractable aqua regia Cd, Cr, Cu, Ni, Pb, and Zn. Microbial indicators of faecal contamination (coliform bacteria and enterococci) were also evaluated from topsoil samples (0-5/10 cm), collected immediately after the UB application, and 6 and 13 months afterwards, in the treatments with and without UB application. For both types of pasture, the UB application significantly improved the O.M. content in the soil surface layer, one year after the fertiliser application, and did not change the initial soil levels of heavy metals and bacteria populations of faecal origin.