What was done
Working at the EuroFACE short-rotation Poplar plantation in central Italy (POPFACE), which was established in early 1999 on former agricultural fields, the authors assessed soil carbon (C) and nitrogen (N) dynamics during the first two years of the experiment's second rotation (2001-2003), comparing their results with those obtained by Hoosbeek et al. (2004) during the experiment's first rotation.

What was learned
Hoosbeek et al. (2006) note that "during the first rotation … total soil C content increased more under ambient CO2 treatment than under FACE," which "may have been caused by a priming effect of the newly incorporated litter [in the FACE treatment that] increased microbial activity with as side effect the increased decomposition of older soil organic matter." However, they report that data from the first two years of the second rotation "show a larger increase of total C% under FACE than under ambient CO2." Based on these observations, they opine that "the priming effect ceased during the second rotation."

What it means
The three researchers infer from their findings that the POPFACE ecosystem "switched from a state where extra labile C and sufficient N-availability (due to the former agricultural use of the soil) caused a priming effect (first rotation), to a state where extra C input is accumulating due to limited N-availability (second rotation)." Consequently, the big question for the future, as they put it, is "whether the currently observed larger increase of total soil C and the increase of labile C under FACE will eventually result in long-term C sequestration in refractory and stable organic matter fractions." Clearly, only time will tell. Hence, it is extremely important that long-term studies such as this one, as well as others described in the materials archived under Long-Term Studies in our Subject Index, be maintained for as long as is humanly possible.

Reference
Hoosbeek, M.R., Lukac, M., Van Dam, D., Godbold, D.L., Velthorst, E.J., Biondi, F.A., Peressotti, A., Cotrufo, M.F., De Angelis, P. and Scarascia-Mugnozza, G. 2004. More new carbon in the mineral soil of a Poplar plantation under Free Air Carbon Enrichment (POPFACE): Cause of increased priming effect? Global Biogeochemical Cycles 18: GB1040.