Effects of CO2 and nitrogen enrichment on production, standing crop, and survivorship of mycorrhizal root tips in a loblolly pine FACE experiment over 12 years

Background/Question/Methods

Rising atmospheric CO₂ will influence productivity and function of forests.  Greater C  assimilation by forests will likely be accompanied by higher nutrient demands and greater absorptive surface area either of fine roots or mycorrhizal fungi.  Increased atmospheric CO₂ may also affect the dynamics of mycorrhizal structures - the major absorptive surface of the fine roots of trees.  We tracked the production, standing crop, and survivorship of mycorrhizal root tips over 12 years in a loblolly pine forest at the Duke FACTS-1 FACE site in Durham, NC to assess their response to free-air CO₂ enrichment as well as nitrogen fertilization.  Main plots were exposed to either ambient or elevated (ambient +200 ppm) CO₂ and were split into fertilized and unfertilized halves.  Mycorrhizal root tips were quantified using digital images obtained from minirhizotron tubes.

Results/Conclusions

We found significant effects of CO₂ enrichment on mycorrhizal root tip standing crop and production.  Survivorship of these mycorrhizal root tips was affected by CO₂, N fertilization, and soil depth.  Elevated atmospheric CO₂ increased longevity of mycorrhizal root tips in unfertilized plots, but did not significantly affect longevity in the presence of nitrogen fertilization.  Effects of CO₂ enrichment on mycorrhizal root tips were largely confined to deeper soils (15-30cm).  Given the importance of mycorrhizal root tips in the absorption of water and nutrients from the soil and thus in the overall function of forests, it is essential that we gain a better understanding of how these root tips will be affected by the changes in atmospheric CO₂.