Tree cover influences the soil C balance in Mediterranean cork oak-based silvopastoral systems

Mediterranean silvopastoral systems provide a wide range of ecosystem services, among which soil C sequestration is one of the most relevant. Although the role of trees in enhancing soil C content of silvopastoral systems is widely recognized, the impacts of the tree cover on the overall soil C balance and hence the soil C sequestration rates are not clear enough to identify feasible options and effective management pathways. This study aimed to assess the effect of trees on soil C balance in a Mediterranean cork-oak silvopastoral system under contrasting cover classes. A three-year study was conducted in a Long-Term Observatory in Sardinia (Italy). A hierarchical design was used to test the tree effect (below and outside the tree crown projection) and the tree cover (high tree cover – from 39% to 42%; low tree cover – from 15% to 36%) on the soil C balance. The C balance was calculated as the difference between the C input from the herbaceous and tree components of the ecosystem and the C output from soil heterotrophic respiration. The position with respect to the tree stem influenced the C input, the C output, and the overall C balance. The tree class influenced only the C output and C balance. The annual C balance was higher (more gains) in low-covered areas below the tree crown projection (8.30 ± 0.72 Mg ha−1 of C) than in open pasture and high-covered areas. At the field level, the overall weighted C balance was significantly influenced by tree cover; according to this last evidence, the maximum C balance was reached at about 24% tree cover. The results highlight the importance of management practices oriented to increase herbage production, tree cover, and regulating tree distribution, which in turn can provide ecosystem services such as SOC sequestration.