Soil CO2 Efflux and Root Productivity in a Switchgrass and Loblolly Pine Intercropping System

Abstract

Switchgrass intercropped with loblolly pine plantations can provide valuable feedstock for bioenergy production while providing ancillary benefits like controlling competing vegetation and enhancing soil C. Better understanding of the impact of intercropping on pine and switchgrass productivity is required for evaluating the long-term sustainability of this agroforestry system, along with the impacts on soil C dynamics (soil CO₂ efflux; R_S). R_S is the result of root respiration (R_A) and heterotrophic respiration (R_H), which are used to estimate net C ecosystem exchange. We measured R_S in intercropped and monoculture stands of loblolly pine (<i>Pinus taeda</i> L.) and switchgrass (<i>Panicum virgatum</i> L.). The root exclusion core technique was used to estimate R_A and R_H. The results showed pure switchgrass had significantly higher R_S rates (July, August and September), root biomass and length relative to intercropped switchgrass, while there were no significant changes in R_S and roots between intercropped and monoculture loblolly pine stands. A significant decrease in switchgrass root productivity in the intercropped stands versus monoculture stands could account for differences in the observed R_S. The proportions of R_S attributed to R_A in the intercropped stand were 31% and 22% in the summer and fall respectively, indicating that the majority of the R_S was heterotrophic-driven. Ancillary benefits provided by planting switchgrass between unutilized pine rows can be considered unless the goal is to increase switchgrass production.