Another function of fast-growing tree species such as poplar is their potential role as an effective carbon sink. Intensive management of SRP poplar for biomass energy could help to partially offset carbon dioxide emissions due to short-term turnover of fine roots and long-term accumulation and decomposition associated with larger roots and stumps. Rytter

[66] provides calculations that are based not only on above and belowground biomass pro­duction data from field experiments, but also on fine root turnover, litter decomposition, and increased production levels from commercial plantations. Carbon accumulation in woody biomass, above and belowground, was estimated at 76.6-80.1 MgC ha-1 and accumulation of carbon in the soil at 9.0-10.3 MgC ha-1 over the first 20-22 seasons of plantation growth. The average rates of carbon sequestration were 3.5-4.0 MgC ha-1 yr-1 in woody biomass and 0.4-0.5 MgC ha-1 yr-1 in the soil. In each of his calculations, SRP poplar showed a higher carbon sink potential than for willow. Similar studies were carried out in China

[67] where they also found that SRP poplar had higher carbon sequestration capacity than any annual cropping system in their country. They reported that carbon concentrations in poplar organs ranged from 459 to 526 gC kg-1 DM with the highest levels in stemwood and the lowest concentrations in coarse roots.

Jaoude et al. [68] expressed doubts regarding the ability of poplar plantations to have a positive effect on carbon storage, arguing that if intensive management practices and commercial fertilizers were used, increasing emissions could reduce carbon storage in the soils. The processes for increasing carbon dioxide emission from short-rotation plantations were connected with soil respiration and included the following components: root respira­tion, heterotroph respiration (including microbial respiration of plant residues, turnover of soil organic matter, and rhizomicrobial respiration). It was found that coppicing increased carbon dioxide efflux from soil compared to the pre-coppicing period, but when nitrogen fertilizers were applied it caused a rapid and significant reduction of total soil carbon dioxide efflux by changing the metabolic pathways for both for hetero — and autotrophs.

The long-term effects of SRP poplar on soil properties is a matter of discussion in many countries, where some opponents of woody crop plantations have alleged that after 25 years of such management, soil nutrient levels are exhausted and special, long-lasting rehabilita­tion is needed. Recent studies in Germany [69] helped dispel this myth by providing data for sites where short-rotation poplar was grown for four rotations. The most important soil parameter (i. e. soil organic matter) was improved by 6.2 Mg ha-1 during the 12 years of poplar growth. Higher microbial activity was also recorded. There was some depletion in phosphorus and potassium but no negative yield effects and, furthermore, those nutrients can be easily supplemented with good management. With regard to soil physical properties, soil bulk density decreased and pore volume increased during the 12 years of short-rotation poplar growth.

Luo and Polle [70] evaluated effects of elevated atmospheric carbon dioxide concentra­tions on three poplar genotypes grown in SRPs to determine if the energy content would change. They found that changes in carbon dioxide concentration modified biomass com­position more than nitrogen fertilizers. Long-term elevated carbon dioxide concentrations increased the quantity of lignin in the wood. Since lignin has the highest calorific value of all wood components, this suggests that elevated carbon dioxide could actually result in better poplar biomass if it is burnt directly as a fuel. The other important observation was that higher nitrogen rates were necessary for the poplar to utilize the additional carbon dioxide in the atmosphere.

Environmental benefits associated with converting arable land to short-rotation poplar were presented by Updegraff et al. [71]. With regard to potential greenhouse gas mitigation, they noted high differences in calculations of carbon content. Other benefits included a reduction in erosion and agricultural runoff that can lead to surface water protection. They also pointed out that short-rotation poplar plantations cannot be treated as conservation system because of the intensive agricultural practices that are used to sustain the plantations, but the management strategies and environmental benefits are attained by the site and growing conditions. Updegraff et al. [71] also considered the environmental benefits of converting arable land into SRP poplar by constructing three scenarios of 10, 20, or 30% conversion in Minnesota, U. S.A. They assumed two scenarios for utilization of the poplar biomass — wood production or energy generation — and included an assumption that an offset for carbon sequestration would be introduced. Modeling of the three scenarios gave results that had a very high level of uncertainty because of difficulty in quantifying the most crucial environmental benefit (i. e. carbon sequestration). They simply could not obtain an accurate estimate of belowground biomass and carbon dynamics. Therefore, they concluded that the benefits, when treated as offsets in monetary terms, could only be estimated with a very wide range of between $44 and $96 ha-1.