Determining Media and Species Effects On Soil Carbon Dynamics in the Landscape

Increased atmospheric carbon dioxide (CO₂) concentration is an important factor in global change research. While agriculture is a major contributor to greenhouse gas (GHG) emissions, it has great potential to offset emissions by altering management practices. Much of the work on reducing GHG emissions and enhancing carbon (C) sequestration has been conducted in row crop and forest systems. Horticulture is a multi-billion dollar industry, but no research has focused on the industry’s impact on global change. Baseline estimates of C emissions and the ability of growers/landscapers to sequester C using current production practices must be evaluated. This work investigated the effects of growth media and species on soil C dynamics. Two commonly grown nursery crops [crape myrtle (Lagerstroemia x ‘Acoma’) and magnolia (Magnolia grandiflora’D.D. Blanchard’)] were transplanted (from 3 in. and 4 in. liners, respectively) into 3 gallon containers on March 25, 2008. Three growth media mixed with standard amendments were used: 1) pinebark (PB) (industry standard); 2) WholeTree (WT); and 3) clean chip residual (CCR). WholeTree and CCR are forestry industry byproducts currently being evaluated as alternative potting media due to decreasing PB supplies. Plants were outplanted to the field in the winter of 2008. In the summer of 2009, plots were sampled for soil C and an Automated Carbon Dioxide Efflux System (ACES) was installed to continuously monitor C lost via soil respiration (24 hr d^-1) from each potting media/species combination; non-plant (bare soil) areas were also monitored. The main effect of species showed that crape myrtle CO₂ efflux was higher than magnolia. The main effect of media indicated that while PB and CCR were similar, both had higher CO₂ efflux than WT. The interaction of treatment variables showed that crape myrtle had higher flux than magnolia in all media. Further, in crape myrtle all three media had significantly different CO₂ efflux values with CCR highest and WT lowest. In magnolia, PB had higher values than the other two media. Soil C analysis indicated that PB had higher C content than the other two media; however, all media dramatically increased soil C content compared to native soil. It is clear that placing container media into the landscape will increase soil C; the length of time this C remains in the soil requires further investigation, but may vary by media type.