Respiratory carbon use and carbon storage in mid-rotation loblolly pine (Pinus taeda L.) plantations: the effect of site resources on the stand carbon balance

We used estimates of autotrophic respiration (RA), net primary productivity (NPP) and soil CO2 evolution (Sff), to develop component carbon budgets for 12-year-old loblolly pine plantations during the fifth year of a fertilization and irrigation experiment. Annual carbon use in RA was 7.5, 9.0, 15.0, and 15.1 Mg C ha-1 in control (C), irrigated (I), fertilized (F) and irrigated and fertilized (IF) treatments, respectively. Foliage, fine root and perennial woody tissue (stem, branch, coarse and taproot) respiration accounted for, respectively, 37%, 24%, and 39% of RA in C and I treatments and 38%, 12% and 50% of RA in F and IF treatments. Annual gross primary production (GPP=NPP+RA) ranged from 13.1 to 26.6 Mg C ha-1. The I, F, and IF treatments resulted in a 21, 94, and 103% increase in GPP, respectively, compared to the C treatment. Despite large treatment differences in NPP, RA, and carbon allocation, carbon use efficiency (CUE=NPP/GPP) averaged 0.42 and was unaffected by manipulating site resources. Ecosystem respiration (RE), the sum of Sff, and above ground RA, ranged from 12.8 to 20.2 Mg C ha-1 yr-1. Sff contributed the largest proportion of RE, but the relative importance of Sff decreased from 0.63 in C treatments to 0.47 in IF treatments because of increased aboveground RA. Aboveground woody tissue RA was 15% of RE in C and I treatments compared to 25% of RE in F and IF treatments. Net ecosystem productivity (NEP=GPP-RE) was roughly 0 in the C and I treatments and 6.4 Mg C ha-1 yr-1 in F and IF treatments, indicating that non-fertilized treatments were neither a source nor a sink for atmospheric carbon while fertilized treatments were carbon sinks. In these young stands, NEP is tightly linked to NPP; increased ecosystem carbon storage results mainly from an increase in foliage and perennial woody biomass.