Could tree species be a key factor on soil carbon balance in temperate forest?

Soils play a key role in regulating atmospheric concentrations of greenhouse gases notably by their action on organic carbon dynamics (storage vs. release). Forests occupy 31% of the continental surface and store around 40% of the continental organic carbon, half of it in soils. Ongoing climate change could alter the balance of this stock, and the effect of temperature on soil carbon fluxes remains an important question. In this study, we use the Q₁₀ parameter (i.e. increase in CO₂ emission for a 10°C rise in temperature) to estimate the temperature sensitivity of soil organic matter in four forest tree species (beech, spruce, douglas fir and silver fir). In addition, soil organic carbon stocks were estimated and compared with Q₁₀ values and forestry data (volume, basal area, density and dead wood).

The mont Beuvray site (Morvan Regional Park, France), a mid-mountain area of around 1,000 ha with a quite homogeneous geology and pedology, was selected. On this site, Beech forests correspond to historical land use, while softwood forests have been gradually introduced over the past 70 years. Thus, 48 soil samples (0-20 cm) were collected (12 per tree species) and the main physicochemical characteristics were determined (bulk density, stone content, pH, organic carbon and total nitrogen contents, water-extractable organic carbon). The Q₁₀ was calculated for a temperature range of 5 to 25°C in the laboratory using a Respicond X (Nordgren Innovations AB, Sweden).

Results show that soil organic carbon and water-extractable organic carbon contents are higher in silver fir and beech stand soils than in Douglas fir stand soils. For soil organic carbon stocks, the average values are slightly higher for beech and silver fir than for Douglas fir and spruce, but there is no statistical difference between the four tree species.  Q₁₀ values range from 2.3 to 3.0, with a statistical higher value for beech (2.8 ± 0.1) than for the other softwood species (2.6 ± 0.1). This last result suggests that, for similar initial soil conditions, CO₂ emissions from soil in beech stands would increase more strongly with temperature than in other species.

In conclusion, several decades after the introduction of softwood species, we did not measure in the top soil (0-20 cm) significant difference in carbon stocks. However, CO₂ emissions and Q₁₀ values are different and related to forest species. Hence, beech stand soils, corresponding to the historical land cover, could see their CO2 flux increase as they are the most sensitive to temperature. Conversely, silver fir stands, with their lower sensitivity to temperature, could be of interest in mitigating emissions. These results need to be confirmed by field data on soil respiration and compared with above-ground forest biomass and stand health.