Partitioning root and heterotrophic respiration from soil CO2 efflux in clonal loblolly pine plantations that differ in growth efficiency and carbon allocation

Background/Question/Methods

In forest ecosystems, soil CO₂ efflux (S_f) accounts for >50% of ecosystem respiration.  Quantifying root (R_a) and heterotrophic (R_h) respiration components of S_f is important for predicting how forest carbon cycling will respond to changes in climate and management.  Small changes in R_a and R_h can greatly alter net ecosystem productivity and soil carbon storage.  We measured S_f in an eight-year-old loblolly pine clone by silviculture study to determine how genotype and soil organic matter influence R_a (root and symbiotic mycorrhiza) and R_h.  Two fast growing clones were employed.  Both clones had similar stem growth rates; however, the broad-crown ideotype (BC) had greater leaf (15-20%) and fine-root biomass than the narrow crown ideotype (NC).  Soil organic matter treatments consisted of no addition (-LR) and addition of 25 Mg ha^-1 comminuted logging debris (+LR) incorporated into the soil at planting.  A 10.7cm diameter by 20cm root exclusion pipe was used to halt R_a.  R_a was estimated as the difference in a S_f measured prior to pipe installation and again after when S_f had stabilized (80-95 days).  Measurements were made quarterly.  We hypothesized that 1) because of greater leaf and fine-root biomass, the BC clone would have greater S_f, R_a, and R_h compared to the NC clone, and 2) incorporation of LR would increase S_f and R_h.

Results/Conclusions

S_f ranged from 3.4 umol m^-2 s^-1 in March to greater than 15 umol m^-2 s^-1 in July.  Incorporation of LR (+LR) increased S_f by 15% (p=0.04) compared to –LR treatments.  Clone had no effect on S_f and there was no clone x LR interaction; however, there was a strong clone x season interaction (p=0.008), where S_f in the BC clone had 17% greater S_f than the NC clone during the growing season.  The root exclusion pipe significantly reduced S_f and the magnitude of the response varied with season and treatment.  R_a as a proportion of S_f was greater in the summer (32%) than in the spring (13%) or fall (22%).  There were no clone, LR, or clone x LR effects on R_a.  The clone treatment had no effect on R_h; however, +LR treatments increased R_h 25% compared to –LR (p=0.0001).  These early results suggest that genotype and management can have significant effects on S_f, R_a, and R_h. Establishing the seasonal variation in the relationship between S_f, R_a, and R_h will help define carbon partitioning rules and improve our ability to  accurately model carbon sequestration.