Soil CO2 Efflux and Its Components Responded Differently to Throughfall Exclusion and Fertilization in a Loblolly Pine (Pinus taeda) Plantation

Abstract:

Soil CO₂ efflux (R_S) is an important component of forest ecosystem carbon budgets and net ecosystem CO₂ exchange, but little is known about how R_S and its components respond to decreasing soil moisture and changes in soil fertility. The experiment design was a 2 X 2 factorial combination of fertilization (2 levels) and precipitation (throughfall exclusion, 2 levels) replicated in four blocks. We measured R_S along with soil temperature (Ts) and soil moisture (W_S) from 2012 to 2014 in a loblolly pine plantation in Washington, GA. The autotrophic (R_A) and heterotrophic (R_H) components of soil CO₂ efflux were separated using trenched plots. Our objectives were to (1) quantify impacts of throughfall exclusion and fertilization on R_S and its components (R_A, R_H).and (2) determine soil CO₂efflux and its components individually response to environmental factors and biological factors in throughfall exclusion and fertilization treatments.

Annual mean R_S was 2.11, 1.73, 2.09 and 1.92 for treatments of control, fertilization, throughfall exclusion and combination of fertilization and throughfall exclusion, respectively, from 2012 to 2013. The apparent Q₁₀ for R_S was 2.26, 2.25, 2.12 and 2.35 in the four treatments, respectively. There were no significant differences in R_S among treatments except between the Ws treatments. However, there was slight reduction in R_S and R_A in fertilization and the fertilization plus throughfall exclusion treatment. In all treaments, Ts explained more than 80% of variation in R_S. The contribution of CO₂-derived from ectomycorrhizal hyphae was less than 15%. R_S and R_H was better predicted by T_S in the dormant season than the growing season, indicating that additional factors such as root growth and photosynthesis became more important contributors to R_S during the growing season. Fertilization slightly decreased R_S mainly from a decrease in R_H. Throughfall exclusion increased the contribution of R_A to R_S. We concluded that soil moisture had more effect on R_S and its components than changes in fertility.