Climate Change and Silvopasture: The Potential of the Tree and Weather to Modify Soil Carbon Balance

Silvopastoral systems play an important role in climate change mitigation, considering their effect on soil carbon sequestration. In silvopastoral systems, sewage sludge can be used as fertiliser, which is promoted by the Circular Economy Package of the European Commission. This study evaluates the soil chemical properties (pH, carbon), tree growth (top height, canopy cover), and their interactions from 1998 to 2012 in a Pinus radiata D. Don silvopastoral system in northwest Spain. Nine fertilisation treatments were applied: three doses of sewage sludge (160, 320, and 480 kg total N ha−1) or no fertilisation, all with or without liming, and mineral fertiliser with no liming. Soil pH decreased over time due to cations extraction by trees and pine needles deposited in the understory. Tree growth increased light interception, decreasing soil carbon incorporation. The interannual variation of carbon also depended on weather conditions. Initially, fertilisation increased soil pH and carbon, but without compensating cations extraction over time. Therefore, it is advisable to apply amendments in the middle years of the plantation. Tree management is also needed to decrease competitiveness and enhance carbon incorporation. Moreover, control plots should be linked to the next CAP 2023–2027 eco-schemes accounting for soil carbon levels.