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.
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