Paper Reviewed
Jonard, M., Nicolas, M., Coomes, D.A., Caignet, I., Saenger, A. and Ponette, Q. 2017. Forest soils in France are sequestering substantial amounts of carbon. Science of the Total Environment 574: 616-628.

Coupled carbon-climate models predict a positive feedback to global warming will occur if accelerated soil carbon decomposition offsets vegetation growth in a warming climate. Given that soil carbon dynamics in forest ecosystems remain uncertain, it is important to investigate whether and how soil carbon stocks in forest ecosystems have changed in recent decades as temperatures have warmed.

Focusing on the forests of France, Jonard et al. (2017) analyzed data collected from two soil surveys to quantify changes in soil organic carbon (SOC) stocks and estimate the carbon sequestration rates in French forest soils. During the years 1993-1995 and 2007-2012, scientists inventoried forest SOC stocks from the forest floor to a soil depth of 40 cm at 102 plots distributed across France. Identical sampling design and analytical methods used in both surveys allowed the researchers to estimate the change in SOC stock for the forest floor and three soil depths (0-10 cm, 10-20 cm and 20-40 cm) between the two periods.

Results of the analysis revealed a significant enhancement of SOC stocks over time in the forest floor (+0.10 MgC ha^-1 yr^-1; p < 0.01) and 0-10 cm soil layer (+0.25 MgC ha^-1 yr^-1; p < 0.001), which increased in spite of "no temporal trend in litter production." No statistically significant change occurred in the bottom two soil depth layers. Consequently, the total SOC sequestration rate of the French forests amounted to 0.35 MgC ha^-1 yr^-1 (p < 0.001). Extrapolating this finding to the whole of France, Jonard et al. calculate a country-wide SOC sequestration estimate of 5.5 TgC yr^-1.

In light of the above, it would appear that global warming is not diminishing the SOC stocks of French forests and is therefore not acting as a positive feedback to warming. Rather, the enhanced soil carbon uptake falls right in line with the aboveground biomass increases observed by satellites, which have detected a significant greening of the terrestrial biosphere over the past few decades due, in large measure, to the aerial fertilization effect of rising atmospheric CO2 concentrations that is increasing plant production all across the globe (see the many reviews posted under the heading Biospheric Productivity in our Subject Index).