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 (R-A), net primary productivity (NPP) and soil CO2 evolution (S-ff), 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 R-A 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 R-A in C and I treatments and 38%, 12% and 50% of R-A in F and IF treatments. Annual gross primary production (GPP=NPP+R-A) 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, R-A, and carbon allocation, carbon use efficiency (CUE=NPP/GPP) averaged 0.42 and was unaffected by manipulating site resources. Ecosystem respiration (R-E), the sum of S-ff, and above ground R-A, ranged from 12.8 to 20.2 Mg C ha(-1) yr(-1). S-ff contributed the largest proportion of R-E, but the relative importance of S-ff decreased from 0.63 in C treatments to 0.47 in IF treatments because of increased aboveground R-A. Aboveground woody tissue R-A was 15% of R-E in C and I treatments compared to 25% of R-E in F and IF treatments. Net ecosystem productivity (NEP=GPP-R-E) 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.