Production and architecture of fine root systems of loblolly pine after 15 years of FACE: Synthesis of minirhizotron and soil monolith studies

Background/Question/Methods

Elevated atmospheric CO₂ influences root growth and potential for belowground  carbon storage.  We used  minirhizotrons and soil monoliths (8000 cm³) to examine root systems in a loblolly pine plantation following 15 years of exposure to Free-Air-CO₂-Enrichment (FACE) and 5 years of nitrogen (N) addition.  We hypothesized that responses of fine roots to elevated CO₂ would be greatest in fertilized subplots and that CO2-enrichment would induce a shift in architecture of fine root systems from dichotomous to more herringbone branching patterns.  We predicted that N ferilization would decrease fine root production and cause a shift from herringbone to more dichotomous branching patterns.  

Results/Conclusions

We found significantly greater root length density in plots exposed to elevated CO₂  relative to ambient conditions (p=0.0296), but no effect of N fertilization on root length was detected.  Total root volume responded to CO₂ fumigation in opposite directions depending on N fertilization treatment, with elevated CO₂ increasing root volume in unfertilized plots but decreasing root volume under N fertilization (p=0.0379).  The number of root tips was generally greater under elevated CO₂ conditions. Neither CO₂ nor N treatments influenced average root diameter .  Specific root length of pine roots was not affected by N fertilization, but was marginally higher in CO₂ –enriched relative to ambient plots (p=0.084).  Altered architecture of fine root systems in CO₂-enriched compared to ambient plots was reflected by a significantly higher DBI (indicating a shift from a dichotomous to more herringbone branching pattern).   Finally, significant changes occurred in the distribution of resources to roots of different diameters in response to CO₂-enrichment and N.  Following 15 years of elevated CO₂ conditions, trees continue to show increased resource allocation to the production of root length in response to CO₂ and exhibited architectural changes that have been shown to increase efficiency of nutrient and water uptake.