Partitioning Soil Respiration to Quantify Net Ecosystem Productivity of Loblolly Pine Plantations.

Quantification of the heterotrophic component of total soil respiration is important for estimating forest carbon pools and fluxes, and for understanding carbon dynamics associated with stand development and silvicultural management. We measured the proportion of heterotrophic respiration (R_H) to total soil respiration (R_S) in managed loblolly pine (Pinus taeda L.) stands of four age classes in the Piedmont physiographic province of Virginia. Our objectives were to evaluate the influence of stand age and seasonality on the proportion of R_H to R_S (R_H:R_S). R_H was partitioned using root exclusion cores, and both R_S and R_H were measured 90 days following installation of cores for five seasons. Repeated measures analysis revealed that stand age and measurement season each had a significant effect on RH:RS (P < 0.001), but that there were no interactive effects (P = 0.202). Mean R_H:R_S during the 12-month study declined with stand age, and were 0.82, 0.73, 0.59, and 0.50 for 3-year-old, 9-year-old, 18- year-old, and 25-year-old stands, respectively. Across all age classes, the winter season had the highest mean R_H:R_S of 0.85 while summer had the lowest of 0.55. This study provides estimates of R_H:R_S in managed loblolly pine systems, and demonstrates the need to consider the impact of stand age and seasonal patterns when estimating net annual carbon (C) budgets of forest ecosystems and to identify the point at which plantations switch from functioning as C sources to a C sinks.