Soil CO2, CH4, and N2O Fluxes in a Switchgrass and Loblolly Pine Intercropping System.

Soil respiration (R_S) recycles CO₂ fixed by terrestrial plants to the atmosphere and is an important and large part of the global carbon cycle. R_S is the result of root respiration (R_A) and microbial decomposition of organic matter (R_H) releasing CO₂ as a by-product. Accurate estimates of the two components are required as they vary greatly spatially and temporally with species composition, temperature, moisture, NPP, soil type and management activities. Therefore, we measured soil CO₂ efflux every six weeks in 2012 from an intercropped system of loblolly pine (Pinus taeda L.) and switchgrass (Panicum virgatum L.), both of which are promising feedstock for bioenergy production, in the Lower Coastal Plain in North Carolina. We quantified R_A and R_H components of soil respiration by using root exclusion cores. The technique is based on root carbohydrate depletion ideally eliminating root respiration within the cores over time.  We separated roots into species and analyzed root length (cm), area (cm²) and biomass (kg/ha). As trace greenhouse gases (CH₄ and N₂O) have a higher warming potential than CO₂, fluxes of CH₄ and N₂O were monitored quarterly using static chambers.  Since pine (C₃ plant) and switchgrass (C₄ plant) have varying photosynthetic pathways, we analyzed the stable isotopic composition of soil CO₂ efflux (δ¹³CO₂) to predict the role of switchgrass and pine in driving R_S. The relationship between CO₂ and trace gas fluxes and root biomass were determined. We predict that switchgrass by way of competition for water and nutrients and their additional organic matter inputs will significantly influence R_S of intercropped plots.