2008 Joint Annual Meeting (5-9 Oct. 2008): Soil Carbon Sequestration under Silvopastoral Systems in Spain.

Soils represent a significant pool of carbon (C) in the global carbon cycle. Recently there has been considerable interest in understanding the mechanisms and extent of C sequestration in soils due to its purported role in regulating global climate. This study, based on the premise that deep roots of trees in agroforestry systems will help sequester more C in deeper soil layers compared to treeless pastoral or herbaceous systems, quantified the amount and type of C stored in soils under silvopastoral agroforestry systems developed in two regions of Spain (Mediterranean Extremadura and Atlantic Galicia provinces). Soil samples collected from various soil layers (up to 100 cm depth) were physically fractionated by wet sieving into three different size fractions (2000-250 µm, 250 -53 µm, and <53 µm) and their organic C content determined. Results indicated that the vast majority of C was found in the upper 25 cm of soil irrespective of the type of vegetative cover, with a reduction in total C in deeper soil layers. In the upper 25 cm of soils with a tree cover, more than half of the soil C at both sites was found in the 2000-250 µm fraction. A significantly greater amount of total soil C was found to be associated with the woody component of both silvopastures at deeper soil layers, as compared with treeless pastures. The results suggest that silvopasture helps sequester more C in soil than treeless pastures.