Environmental amelioration potential of silvopastoral agroforestry systems of Spain: Soil carbon sequestration and phosphorus retention

This study investigates the environmental amelioration potential for silvopasture agroforestry systems of Spain to store and retain soil carbon (C) and phosphorus (P). Interest in C has grown due to its role in affecting global climate. Fertilizer P from can become an environmental pollutant when applied in excess of a soil's storage capacity. To assess soil C and P retention in Spain, two study sites with were selected: a "Dehesa" silvopasture planted with cork oak (Quercus suber) and a silvopasture experiment planted with radiata pine (Pinus radiata) and birch (Betula alba). Soils underneath trees and in adjacent open pastures were sampled to 100 cm, wet sieved into four size classes (<53 microm, 53--250 microm, 250--2000 microm, and > 2000 microm), and combusted for C determination. Potential for P contamination was assessed using the Phosphorus Saturation Ratio (PSR) and Soil Phosphorus Storage Capacity (SPSC), indicators of soil P saturation and storage capacities. Results from the cork oak site indicated that soils closer to the tree, as compared to away, had C greater in the 250--2000 microm soil fraction and that overall C was higher in the 50--100 cm depths. Soil C to 100 cm increased from 20.01 to 41.22 Mg C ha-1, from away from the tree canopy to underneath the canopy. In the simulated silvopasture, birch had more C in the 250--2000 microm size class from 50--100 cm, as compared to the pasture, and radiata pine exceeded pasture C storage in the 250--2000 microm size class in the 75--100 cm depth. More C was found in the <53 microm size class in the pasture, as compared to silvopastures from 0--25 cm depth. Maintenance and expansion of silvopastures in Spain will increase C storage in macroaggregate-sized soil fractions in the deeper soils, and improve long term storage potential. In the PSR and SPSC studies, extraction of P from soils by deeply-rooted trees in silvopasture was believed to reduce soil P in the deeper soils examined, reducing potential P loss as compared to a pasture under similar conditions.