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Results on productivity and belowground carbon sequestration potential of SRF plantations

Forest soils account for a large part of the stable carbon pool held in terrestrial ecosystems. Future levels of atmospheric CO2 are likely to increase C input into the soils through increased above and below ground production of forest ecosystems. A common response to elevated CO2 is increased allocation of assimilated C below ground. This phenomenon can result in increased belowground C inputs by a shift in C allocation between foliage and roots, increased production and turnover of fine roots, by greater proliferation of mycorrhizal symbionts or by increased root exudation. Assessment of the relative contribution of these processes to the soil C pool is ridden with technical and methodological difficulties. However, it seems that the turnover of ephemeral tissues constitutes the greatest source of C entering the soil C pool.

In EuroFACE experimental facility, elevated CO2 increased both above and belowground biomass production. This effect on root biomass, both coarse and fine, did not diminish even after 6 years of CO2 enrichment and 2 rotation periods. All Populus species have shown similar reaction to elevated CO2, indicating that this increase might be common at least within the genus Populus. From our observations in EuroFACE it appears that the biggest input of C into the soil is from fast turnover belowground biomass, such as fine roots and mycorrhizas attached to them.

Elevated CO2 increased fine root biomass by 85% in P. alba, 86% in P. nigra and 17% P. x euroamericana during second rotation. Moreover, fine root biomass is turning over faster under elevated CO2. This results in a further increased in carbon transferred into the soil through root systems, up to 210% more fine root biomass in P. alba is turned over annually in elevated CO2 compared to ambient control. It is reasonable to assume that mycorrhizal fungi, which have been found colonising fine roots of all three poplar species, will decompose in short time after root death, since fine roots constitute their only source of carbon. This increased input will result in greater sequestration of C only if the additional C enters stable soil C pools.

When assessing whether elevated CO2 increases soil C storage, alongside measuring inputs, equal importance must be given to the estimates of the amount of C leaving the soil. Forest soil respiration has been indicated as a main pathway for C leaving temperate forest ecosystems and therefore plays a major role in determining sequestration potential. Soil CO2 emanating from the soil is a product of both autotrophic and heterotrophic respiration. A likely effect of greater root systems found under elevated CO2 is an increase of autotrophic respiration. Soil CO2 efflux is controlled by diffusion gradient, an increase in CO2 concentration in soil atmosphere resulting from increased respiration will cause increased soil CO2 efflux. The other pathway for C to leave soil leaching of dissolved inorganic carbon to ground water appears to be only of minor significance. Therefore, when considering the effect of elevated CO2 on soil C storage, it is important to consider the response of C inputs and of soil CO2 efflux.

In conclusion, elevated levels of atmospheric CO2 result in increased C allocation below ground. Usually, increased belowground C allocation has been accompanied by greater soil CO2 efflux making predictions of enhanced C sequestration in elevated CO2 difficult. The results from the EuroFACE site suggest that much of the increase in C input is rapidly lost. Whether increased pool of soil C will result in increased C sequestration by temperate forests depends on convolution of various factors such as soil fertility, temperature and moisture. All these factors influence the rate and magnitude of root and microbial respiration and ultimately the fate of extra C allocated to the soil.

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