Resumo

SERRAO, M. G. et al. Contribution to the improvement of degraded soils under pastures through sewage sludge application, without environmental risks. Rev. de Ciências Agrárias [online]. 2009, vol.32, n.1, pp.258-272. ISSN 0871-018X.

Sewage sludge (SS) application to soils reserved for pastures, still scarcely used in the country, often contributes to improve organic matter (O.M.) and some plant nutrient contents and to reduce the erosion risk, by increasing the soil vegetation cover. However, the occasional occurrence in SS of high levels of heavy metals, organic pollutant compounds, and pathogenic organisms restrict the SS rate to apply and makes indispensable their control in the soils to which they were applied. Also, the high nitrogen concentration often present in SS can inhibit the symbiotic rhizobium activity, with sequent damage in the leguminous species survival in grassland. In this work, dry matter yield, floristic composition, and copper (Cu) concentration in the plant biomass were evaluated, in two successive years of a field experiment with a sown pasture mixture, established in a poor Haplic Luvisol in the Mértola region. A biologically treated SS from Évora, rich in Cu, was applied. The evolution in the topsoil of the O.M., some plant nutrients, and aqua regia extractable Cu concentrations, was also examined for the same period. The magnitudes of the Rhizobium population (Rhizobium leguminosarum biovar trifolii) and of some indicators of faecal contamination (coliform bacteria and enterococcus) were evaluated too. Moreover, the evolution in the soil superficial layer of 11 polychlorinated biphenyls (PCBs), 13 organ chlorine pesticides, and 16 polycyclic aromatic hydrocarbons (PAHs) concentrations was examined for the 1^st year following the SS application. The experiment, with a randomised block design, had three SS application rates (L0 = 0, L1 = 12, and L2 = 24 t/ha) as treatments and two replicates. The sown mixture consisted of Italian ryegrass, cocksfoot, five clover species, bird’s foot, and biserrula. Besides much higher biomass production, induced by higher soil available phosphorus concentration, the added SS neither polluted the soil with the analysed organic compounds, one year after application, nor increased the pathogenic microbial flora in two consecutive years. However, at the highest SS rate, soil aqua regia extractable Cu (0-10 cm depth) exceeded the maximum Portuguese legislated level in soil (100 mg kg^-1), the rhizobium population was also reduced at the beginning of the 1^st growing season, and legume percentage decreased at the 2^nd growing season. The plant Cu concentrations were much lower than the maximum tolerable levels (25 mg kg^-1) for the small ruminant’s diet, strongly suggesting that the SS application will not have damaging effects on the animal nutrition. Due to the undesirable effects of the L2 application rate, the L1 rate (12 t/ha) would be recommended for fertilizer purposes.

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