15 th International Conference on Environmental Science and Technology Rhodes, Greece, 31 August to 2 September 2017 CEST2017_00672 Evolution of soil organic matter in Eucalyptus nitens Maiden silvopastoral systems fertilised with different types of sewage sludge Ferreiro-Domínguez N. 1, 2 , Rigueiro-Rodríguez A. 1 , Mosquera-Losada MR. 1,* 1 Crop Production and Project Engineering Departament. Escuela Politécnica Superior. Universidad de Santiago de Compostela. 27002-Lugo. 2 Forest Ecosystems Management under Global Change Research Group (ForChange), Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisbon, Portugal. *corresponding author: e-mail: mrosa.mosquera.losada@usc.es Abstract The silvopasture has been recognized as a possible greenhouse gas mitigation strategy under the Kyoto Protocol due to the potential for carbon storage in multiple plant species and in the soil. Soil organic matter (SOM) represents the most important pool of carbon storage in terrestrial ecosystems and can be modify among other factor by the fertilisation. The objective of this experiment was to evaluate during five years the evolution of SOM in a silvopastoral system established with Eucalyptus nitens Maiden in Galicia (NW Spain) and fertilised with sewage sludge that has been stabilised using anaerobic digestion, composting, and pelletisation. The results showed that the SOM gradually decreased from the second year of the study probably due to the fertilisation and the ploughing of the soil but also due to a negative effect of the trees on the biological activity in the soil. Therefore, the afforestation with Eucalyptus not served its task to soil carbon sequestration compared with other tree species established in silvopastoral systems in the same area and it is necessary to continue our study to properly evaluate the effect of Eucalyptus on the SOM. Keywords: agroforestry, afforestation, sowing, climate change, carbon sequestration 1. Introduction The climate change is one of the most important current environmental problems which affect the whole planet (Noaa, 2016). The agroforestry practices and therefore the silvopasture (combining woody vegetation with pasture and animal production) (Mosquera-Losada et al., 2016b) were mentioned in the Kyoto Protocol and recently in the framework of the EU strategy on adaptation to climate change (EU 2013) and in the last IPCC (Intergovernmental Panel on Climate Change) report (IPCC 2014) as a mechanism for mitigation to climate change due to the potential for carbon storage in multiple plant species and in the soil. Soil organic matter (SOM) represents the most important pool of carbon storage in terrestrial ecosystems, accounting for about 75% of total stored carbon (Mosquera-Losada et al., 2011a).In Galician silvopastoral systems, the productivity (of both the understory and the trees) can be limited by low soil fertility as a result of the increased acidity (Zas and Alonso, 2002). The fertilisation with sewage sludge may increases tree and pasture production as well as reduce or increase SOM content. Moreover, in Europe, it is compulsory to stabilise sewage sludge before it is used as a fertiliser in agricultural production systems. The stabilisation processes could cause differences in the mineralisation rates (EPA, 1994), and therefore, in the fertiliser's potential, but also in the levels of SOM. Anaerobic digestion and composting are the most important types of sludge stabilisation promoted by the EU (European Directive 86/278) (EU, 1986). However, both types of waste processing candeliver sewage sludge with high proportions of water which could be reduced by 98% through the pelletisation of anaerobic sludge via a thermic treatment. Consequently, pelletisation reduces storage, transport, and spreads costs, when compared with anaerobic or composted sludge (Mosquera- Losada et al., 2010).The objective of this experiment was to evaluate during five years the evolution of SOM in a silvopastoral system established with Eucalyptus nitens Maiden in Galicia (NW Spain) and fertilised with sewage sludge that has been stabilised using anaerobic digestion, composting, and pelletisation. 2. Materials and methods The experiment was carried out on an abandoned agricultural land in A Pastoriza (Lugo, Galicia, NW Spain, European Atlantic Biogeographic Region).At the beginning of the experiment, the soil was double ploughed to a depth of 50 cm, which is a traditional practice in this area, and the pasture was sown with a mixture of Dactylis glomerata L. var. Artabro (12.5 kg ha −1 ), Lolium perenne L. var. Brigantia (12.5 kg ha −1 ), and Trifolium repens L. var. Huia (4 kg ha −1 ), in the autumn of 2004. After pasture sowing in February 2005, plants of Eucalyptus nitens Maiden were planted at a density of 1667 trees ha −1 , with a distance between the rows of 3 × 2 m. The experimental