Background
The authors write that "after nearly two decades of research on forest ecosystem responses to global change, uncertainty about the role of roots and rhizosphere processes in soil C [carbon] and N [nitrogen] retention and loss has limited our ability to predict biogeochemical feedbacks to long-term forest productivity." But now ...

What was done
Working at the Duke Forest FACE site in Orange County, North Carolina (USA), where eight 30-meter-diameter plots of loblolly pine (Pinus taeda L.) trees were enriched with an extra 200 ppm of CO2 from 1996 to 2010, while four similar plots were maintained under then-current ambient-air conditions, Phillips et al. measured root-induced changes in soil C dynamics of trees exposed to CO2 and nitrogen enrichment by combining stable isotope analyses, molecular characterizations of soil organic matter, and microbial assays.

What was learned
When all was said and done, the six scientists concluded that the CO2-enriched trees "may be both enhancing the availability of N by stimulating microbial decomposition of soil organic matter via priming and increasing the rate at which N cycles through the microbial pools owing to the rapid turnover of N-rich fungal tissues," noting that "the accelerated turnover of hyphal tissues under elevated CO2 may represent an important source of N to plants and microbes."

What it means
Referring to this CO2-induced phenomenon as the Rhizo-Accelerated Mineralization and Priming or RAMP hypothesis, Phillips et al. say that it may have "important consequences for N availability and forest productivity," which consequences could logically be expected to sustain CO2-enhanced tree growth over the lifetime of the trees.