Tree Integration Into Land-Use Systems as a Strategy for Soil Carbon Sequestration.

Land-use change or intensification can influence the dynamics and storage of soil organic matter (SOM) and the extent of carbon (C) sequestration in soils because of changes in ecosystem dynamics such as plant productivity and plant-litter inputs. In this study, we compared silvopastures (pastures into which trees had been integrated) and adjacent treeless pastures for their C3- and C4-derived soil organic C (SOC) content. Soil samples were collected from silvopastures of slash pine (Pinus elliottii) + bahiagrass (Paspalum notatum), and adjacent open (bahiagrass) pasture, at six depths down to 125 cm, at four locations in Florida. The C content in each soil sample and the δ¹³C values for soils and composite samples of plant parts of slash pine (C3 plants) and bahiagrass (C4 plants), were determined. Based on δ¹³C values for composite of plant-part samples (-28.5‰ for slash pine and -12.9‰ for bahiagrass), we calculated the SOC contribution of C3 and C4 plants. With a knowledge of the times of tree establishment in the pastures (8, 12, 14 or 40 years at the 4 sites), the C3-derived SOC in soils under the silvopasture was shown to increase by 6−31.8 and 0−5 Mg ha^-1 yr^-1 at the top (0−5 cm) and deep (75−125 cm) soil layers, respectively, compared to corresponding soil layers under treeless pasture. The C4-derived SOC values in silvopasture were 0.9−3.5 Mg ha^-1 yr^-1 lower at the top soil layer than in the corresponding treeless pasture, whereas, in the deep (75−125 cm) soil layer, the values (Mg ha^-1 yr^-1) varied within a narrow range of   - 0.1  (decrease) to 0.2 (increase). The average SOC content across four sites and all depths increased 30% in silvopastures compared to the adjacent treeless pastures during the period after tree incorporation. This possibly represents the input to SOM from decomposition of dead tree-roots.