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TECHNICAL REPORTS

Landscape and Watershed Processes

Carbon Storage of Different Soil-Size Fractions in Florida Silvopastoral Systems

^a Center for Subtropical Agroforestry, School of Forest Resources and Conservation, P.O. Box 110410
^b Soil and Water Science Dep., P.O. Box 110510; Inst. of Food and Agricultural Sciences, Univ. of Florida, Gainesville, FL 32611

^* Corresponding author (pknair{at}ufl.edu).

Received for publication September 25, 2007. Compared with open (treeless) pasture systems, silvopastoral agroforestry systems that integrate trees into pasture production systems are likely to enhance soil carbon (C) sequestration in deeper soil layers. To test this hypothesis, total soil C contents at six soil depths (0–5, 5–15, 15–30, 30–50, 50–75, and 75–125 cm) were determined in silvopastoral systems with slash pine (Pinus elliottii) + bahiagrass (Paspalum notatum) and an adjacent open pasture (OP) with bahiagrass at four sites, representing Spodosols and Ultisols, in Florida. Soil samples from each layer were fractionated into three classes (250–2000, 53–250, and <53 µm), and the C contents in each were determined. Averaged across four sites and all depths, the total soil organic carbon (SOC) content was higher by 33% in silvopastures near trees (SP-T) and by 28% in the alleys between tree rows (SP-A) than in adjacent open pastures. It was higher by 39% in SP-A and 20% in SP-T than in open pastures in the largest fraction size (250–2000 µm) and by 12.3 and 18.8%, respectively, in the intermediate size fraction (53–250 µm). The highest SOC increase (up to 45 kg m^–2) in whole soil of silvopasture compared with OP was at the 75- to 125-cm depth at the Spodosol sites. The results support the hypothesis that, compared with open pastures, silvopastures contain more C in deeper soil layers under similar ecological settings, possibly as a consequence of a major input to soil organic matter from decomposition of dead tree-roots.

Abbreviations: MRT, mean residence times • NPP, net primary productivity • OP, open pasture • SOC, soil organic carbon • SP, silvopasture • SP-A, center of the alley in a silvopasture • SP-T, in-between trees in a row of a silvopasture