Assessment of soil carbon storage in three land use types of a semi-arid ecosystem in South Portugal.

• Litter and tree roots modify the proportion of soil aggregate fractions. • Extensive agriculture implies similar soil carbon storage than agroforestry. • Agroforestry increases the carbon associated with the small soil aggregate fractions. The land use sector currently represents ∼25% of the global greenhouse gas emissions, thus this human activity is having a huge impact on climate change. Semi-arid climate areas, which are widely distributed, are particularly influenced by such climate change. In these areas, land use changes have transformed the entire landscape and increased the greenhouse gas emissions. However, in semi-arid climate areas, the information on the effect of climate change is scarce and usually centred on one single type of land use. For the land uses (forest, agricultural and agroforestry) traditionally found in the semi-arid area in South Portugal, this study aimed to evaluate (i) the soil chemical (pH) and physical properties (bulk density and percentage of different soil aggregate fractions: 250–2000; 53–250 and <53 μm) and (ii) the carbon (C) storage in whole soil and each soil aggregate fraction in land with over 100 years of the same use. The results show that after >100 years with the same land use, C storage in the whole soil was similar in the plots with extensive agriculture based on the agroecology principles to that in the agroforestry plots. However, the concentration and storage of C in the microaggregates (53–250 μm) and the silt + clay soil fraction (<53 μm) were generally higher for the forest and agroforestry land uses than for the agricultural land use due to the tree effect. This C associated with the smallest soil fractions is very stable and is retained in the soil in the long term. In this context, agroforestry systems improve the soil C storage in the long term at the same time that agricultural production is allowed compared to the traditional forest systems. Therefore, the establishment and maintenance of agroforestry systems in the semi-arid climate areas is a sustainable land use for climate change mitigation. [ABSTRACT FROM AUTHOR]