• Energy Research

Abstract Rates of soil carbon (C) accumulation under 7 recently established tree plantations in Tennessee and South Carolina (USA) were estimated by comparing soil C stocks under the plantations to adjacent reference (nonplantation) sites. Estimated rates of C accumulation in surface (0– 40 cm ) mineral soil were 40– 170 g C m −2 yr −1 during the first decade following plantation establishment. Most soil C at each site was found in mineral-associated organic matter (i.e., soil C associated with the silt–clay fraction). Soils with high sand content and low initial C stocks exhibited the greatest gains in particulate organic matter C (POM-C). Labile soil C stocks (consisting of forest floor and mineral soil POM-C) became an increasingly important component of soil C storage as loblolly pine stands aged. Rates of mineral soil C accumulation were highly variable in the first decade of plantation growth, depending on location, but the findings support a hypothesis that farm to tree plantation conversions can result in high initial rates of soil C accumulation in the southeastern United States.

Abstract The potential for soil carbon (C) sequestration under short-rotation woody crops, like hybrid poplar ( Populus spp.), is a significant uncertainty in our understanding of how managed tree plantations might be used to partially offset increasing atmospheric CO 2 concentrations. Through development of a multi-compartment model, we reviewed information from studies on hybrid poplar and analyzed the potential impact of changes in plant traits and nitrogen (N) fertilization on soil C storage. For a hypothetical setting in the southeastern U.S.A., and starting from soils that are relatively depleted in organic matter (2.5 kg C m −2 ), the model predicted an increase in mineral soil C stocks (1.7 kg C m −2 ) over four 7-year rotations of hybrid poplar. However, at the end of the fourth rotation, both cumulative soil C gains and annual rates of soil C accrual (23–93 g C m −2 yr −1 ) varied widely depending on fertilization rate, biomass yield, and rates of dead root decomposition (three factors that were examined in a factorial model-based experiment). Our analysis indicated that processes linked to genetically modifiable poplar traits (aboveground biomass production, belowground C allocation, root decomposition) are potential controls on soil C sequestration. Key measures of model performance were sensitive to how aboveground biomass production responded to N fertilization. Site specific properties that were independent of plant traits were also important to predicted soil C accumulation and point to possible genotype x site interactions that may explain contradictory data from both empirical and theoretical studies of C sequestration under hybrid poplar plantations.

Projected economic benefits of renewable energy derived from a native prairie grass, switchgrass, include nonmarket values that can reduce net fuel costs to near zero. At a farm gate price of $44.00/dry Mg, an agricultural sector model predicts higher profits for switchgrass than conventional crops on 16.9 million hectares (ha). Benefits would include an annual increase of $6 billion in net farm returns, a $1.86 billion reduction in government subsidies, and displacement of 44-159 Tg/year (1 Tg = 1012 g) of greenhouse gas emissions. Incorporating these values into the pricing structure for switchgrass bioenergy could accelerate commercialization and provide net benefits to the U.S. economy.

A multi-compartment model was developed to summarize existing data and predict soil carbon sequestration beneath switchgrass (Panicum virgatum) in the southeastern USA. Soil carbon sequestration is an important part of sustainable switchgrass production for bioenergy because soil organic matter promotes water retention, nutrient supply, and soil properties that minimize erosion. A literature review was undertaken for the purpose of model parameterization. A sensitivity analysis of the model indicated that predictions of soil carbon sequestration were affected most by changes in aboveground biomass production, the ratio of belowground-to-aboveground biomass production, and mean annual temperature. Simulations indicated that the annual rate of soil carbon sequestration approached steady state after a decade of switchgrass growth while predicted mineral soil carbon stocks were still increasing. A model-based experiment was performed to predict rates of soil carbon sequestration at different levels of nitrogen fertilization and initial soil carbon stocks (to a 30-cm depth). At a mean annual temperature of 13°C, the predicted rate of soil carbon sequestration varied from −28 to 114 g C m−2 year−1 (after 30 years) and was greater than zero in 11 of 12 simulations that varied initial surface soil carbon stocks from 1 to 5 kg C m−2 and nitrogen fertilization from 0 to 18 g N m−2 year−1. The modeling indicated that more research is needed on the process of biomass allocation and on nitrogen loss from mature plantations, respectively, to improve our understanding of carbon and nitrogen dynamics in switchgrass agriculture.

Soil carbon contents were measured on a short-rotation woody crop study located on the US Department of Energy's Savannah River Site outside Aiken, SC. This study included fertilization and irrigation treatments on five tree genotypes (sweetgum, loblolly pine, sycamore and two eastern cottonwood clones). Prior to study installation, the previous pine stand was harvested and the remaining slash and stumps were pulverized and incorporated 30 cm into the soil. One year after harvest soil carbon levels were consistent with pre-harvest levels but dropped in the third year below pre-harvest levels. Tillage increased soil carbon contents, after three years, as compared with adjacent plots that were not part of the study but where harvested, but not tilled, at the same time. When the soil response to the individual treatments for each genotype was examined, one cottonwood clone (ST66), when irrigated and fertilized, had higher total soil carbon and mineral associated carbon in the upper 30 cm compared with the other tree genotypes. This suggests that root development in ST66 may have been stimulated by the irrigation plus fertilization treatment.

Radiocesium is one of the more prevalent radionuclides in the environment as a result of weapons production-related atomic projects in the USA and the former Soviet Union. Radiocesium discharges during the 1950s account for a large fraction of the historical releases from US weapons production facilities. Releases of radiocesium to terrestrial and aquatic ecosystems during the early years of nuclear weapons production provided the opportunity to conduct multidisciplinary studies on the transport mechanisms of this potentially hazardous radionuclide. The major US Department of Energy facilities (Oak Ridge Reservation in Tennessee, Hanford Site near Richland, Washington, and Savannah River Site near Aiken, South Carolina, USA) are located in regions of the country that have different geographical characteristics. The facility siting provided diverse backgrounds for the development of an understanding of environmental factors contributing to the fate and transport of radiocesium. In this paper, we summarize the significant environmental releases of radiocesium in the early years of weapons production and then discuss the historically significant transport mechanisms for 137Cs at the three facilities that were part of the US nuclear weapons complex.

Through cation exchange capacity assay, nitrogen adsorption-desorption surface area measurements, scanning electron microscopic imaging, infrared spectra and elemental analyses, we characterized biochar materials produced from cornstover under two different pyrolysis conditions, fast pyrolysis at 450 °C and gasification at 700 °C. Our experimental results showed that the cation exchange capacity (CEC) of the fast-pyrolytic char is about twice as high as that of the gasification char as well as that of a standard soil sample. The CEC values correlate well with the increase in the ratios of the oxygen atoms to the carbon atoms (O:C ratios) in the biochar materials. The higher O:C ratio was consistent with the presence of more hydroxyl, carboxylate, and carbonyl groups in the fast pyrolysis char. These results show how control of biomass pyrolysis conditions can improve biochar properties for soil amendment and carbon sequestration. Since the CEC of the fast-pyrolytic cornstover char can be about double that of a standard soil sample, this type of biochar products would be suitable for improvement of soil properties such as CEC, and at the same time, can serve as a carbon sequestration agent.

The dynamics of rapid changes in carbon (C) partitioning within forest ecosystems are not well understood, which limits improvement of mechanistic models of C cycling. Our objective was to inform model processes by describing relationships between C partitioning and accessible environmental or physiological measurements, with a special emphasis on short-term C flux through a forest ecosystem. We exposed eight 7-year-old loblolly pine (Pinus taeda L.) trees to air enriched with (13)CO(2) and then implemented adjacent light shade (LS) and heavy shade (HS) treatments in order to manipulate C uptake and flux. The impacts of shading on photosynthesis, plant water potential, sap flow, basal area growth, root growth and soil CO(2) efflux rate (CER) were assessed for each tree over a 3-week period. The progression of the (13)C label was concurrently tracked from the atmosphere through foliage, phloem, roots and surface soil CO(2) efflux. The HS treatment significantly reduced C uptake, sap flow, stem growth and fine root standing crop, and resulted in greater residual soil water content to 1 m depth. Soil CER was strongly correlated with sap flow on the previous day, but not the current day, with no apparent treatment effect on the relationship. Although there were apparent reductions in new C flux belowground, the HS treatment did not noticeably reduce the magnitude of belowground autotrophic and heterotrophic respiration based on surface soil CER, which was overwhelmingly driven by soil temperature and moisture. The (13)C label was immediately detected in foliage on label day (half-life = 0.5 day), progressed through phloem by Day 2 (half-life = 4.7 days), roots by Days 2-4, and subsequently was evident as respiratory release from soil which peaked between Days 3 and 6. The δ(13)C of soil CO(2) efflux was strongly correlated with phloem δ(13)C on the previous day, or 2 days earlier. While the (13)C label was readily tracked through the ecosystem, the fate of root C through respiratory, mycorrhizal or exudative release pathways was not assessed. These data detail the timing and relative magnitude of C flux through various components of a young pine stand in relation to environmental conditions.

AbstractNitrogen (N) cycling can be an important constraint on forest ecosystem response to elevated atmospheric CO2. Our objective was to trace the movement of 15N, injected into tree sap, to labile and stable forms of soil organic matter derived partly from the turnover of tree roots under elevated (545 ppm) and ambient (394 ppm) atmospheric CO2 concentrations at the Oak Ridge National Laboratory (ORNL) FACE (Free‐Air Carbon Dioxide Enrichment) Experiment. Twenty‐four sweetgum trees, divided equally between CO2 treatments, were injected with 3.2 g 15N‐ammonium sulfate (99 atom %), and soil samples were collected beneath the trees over a period of 89 weeks. For 16 cm deep soil samples collected beneath the study trees, there was 28% more fine root (less than or equal to 2 mm diameter) biomass under elevated CO2 (P = 0.001), but no significant treatment effect on the amounts of necromass, coarse root biomass, or on the N concentrations in tree roots and necromass. Nitrogen‐15 moved quickly into roots from the stem injection site and the 15N content of roots, necromass, and labile organic matter (i.e. particulate organic matter, POM) increased over time. At 89 weeks post‐injection, approximately 76% of the necromass 15N originated from fine root turnover. Nitrogen‐15 in POM had a relatively long turnover time (47 weeks) compared with 15N in roots (16 to 22 weeks). Over the 1.7 year period of the study, 15N moved from roots into slower cycling POM and the disparity in turnover times between root N and N in POM could impose progressive limitations on soil N availability with stand maturation irrespective of atmospheric CO2, especially if the release of N through the decomposition of POM is essential to sustain forest net primary production. Published in 2009 by John Wiley & Sons, Ltd.