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Agricultural soils play a dual role in regulating the Earth's climate by releasing or sequestering carbon dioxide (CO2) in soil organic carbon (SOC) and emitting non‐CO2 greenhouse gases (GHGs) such as nitrous oxide (N2O) and methane (CH4). To understand how agricultural soils can play a role in climate solutions requires a comprehensive assessment of net soil GHG balance (i.e., sum of SOC‐sequestered CO2 and non‐CO2 GHG emissions) and the underlying controls. Herein, we used a model‐data integration approach to understand and quantify how natural and anthropogenic factors have affected the magnitude and spatiotemporal variations of the net soil GHG balance in U.S. croplands during 1960–2018. Specifically, we used the dynamic land ecosystem model for regional simulations and used field observations of SOC sequestration rates and N2O and CH4 emissions to calibrate, validate, and corroborate model simulations. Results show that U.S. agricultural soils sequestered 13.2±1.16$$ 13.2\pm 1.16 $$ Tg CO2‐C year−1 in SOC (at a depth of 3.5 m) during 1960–2018 and emitted 0.39±0.02$$ 0.39\pm 0.02 $$ Tg N2O‐N year−1 and 0.21±0.01$$ 0.21\pm 0.01 $$ Tg CH4‐C year−1, respectively. Based on the GWP100 metric (global warming potential on a 100‐year time horizon), the estimated national net GHG emission rate from agricultural soils was 122.3±11.46$$ 122.3\pm 11.46 $$ Tg CO2‐eq year−1, with the largest contribution from N2O emissions. The sequestered SOC offset ~28% of the climate‐warming effects resulting from non‐CO2 GHG emissions, and this offsetting effect increased over time. Increased nitrogen fertilizer use was the dominant factor contributing to the increase in net GHG emissions during 1960–2018, explaining ~47% of total changes. In contrast, reduced cropland area, the adoption of agricultural conservation practices (e.g., reduced tillage), and rising atmospheric CO2 levels attenuated net GHG emissions from U.S. croplands. Improving management practices to mitigate N2O emissions represents the biggest opportunity for achieving net‐zero emissions in U.S. croplands. Our study highlights the importance of concurrently quantifying SOC‐sequestered CO2 and non‐CO2 GHG emissions for developing effective agricultural climate change mitigation measures. [ABSTRACT FROM AUTHOR]

Human activities have caused considerable perturbations of the nitrogen (N) cycle, leading to a ~20% increase in the concentration of atmospheric nitrous oxide (N2O) since the preindustrial era. While substantial efforts have been made to quantify global and regional N2O emissions from cropland, there is large uncertainty regarding how climate change and variability have altered net N2O fluxes at annual and decadal time scales. Herein, we applied a process‐based dynamic land ecosystem model (DLEM) to estimate global N2O emissions from cropland driven by synthetic N fertilizer application and multiple environmental factors (i.e., elevated CO2, atmospheric N deposition, and climate change). We estimate that global cropland N2O emissions increased by 180% (from 1.1 ± 0.2 to 3.3 ± 0.1 Tg N year−1; mean ±1 standard deviation) during 1961–2014. Synthetic N fertilizer applications accounted for ~70% of total emissions during 2000–2014. At the regional scale, Europe and North America were two leading regions for N2O emissions in the 1960s. However, East Asia became the largest emitter after the 1990s. Compared with estimates based on linear and nonlinear emission factors, our results were 150% and 186% larger, respectively, at the global scale during 2000–2014. Our higher estimates of N2O emissions could be attributable to the legacy effect from previous N addition to cropland as well as the interactive effect of N addition and climate change. To reduce future cropland N2O emissions, effective mitigation strategies should be implemented in regions that have received high levels of N fertilizer and regions that would be more vulnerable to future climate change. Key Points: Synthetic N fertilizer contributed to N2O emissions of 2.0 ± 0.1 Tg N year−1 from global cropland between 2000 and 2014Uncertainty remains in fertilizer‐induced N2O emissions due to temporal and spatial differences in current N fertilizer data setsChanges in environmental factors increased cropland N2O emissions; however, the magnitude of environmental effects remain highly uncertain [ABSTRACT FROM AUTHOR]

Excessive ammonia (NH3) emitted from nitrogen (N) fertilizer applications in global croplands plays an important role in atmospheric aerosol production, resulting in visibility reduction and regional haze. However, large uncertainty exists in the estimates of NH3 emissions from global and regional croplands, which utilize different data and methods. In this study, we have coupled a process‐based Dynamic Land Ecosystem Model (DLEM) with the bidirectional NH3 exchange module in the Community Multiscale Air‐Quality (CMAQ) model (DLEM‐Bi‐NH3) to quantify NH3 emissions at the global and regional scale, and crop‐specific NH3 emissions globally at a spatial resolution of 0.5° × 0.5° during 1961–2010. Results indicate that global NH3 emissions from N fertilizer use have increased from 1.9 ± 0.03 to 16.7 ± 0.5 Tg N/year between 1961 and 2010. The annual increase of NH3 emissions shows large spatial variations across the global land surface. Southern Asia, including China and India, has accounted for more than 50% of total global NH3 emissions since the 1980s, followed by North America and Europe. Rice cultivation has been the largest contributor to total global NH3 emissions since the 1990s, followed by corn and wheat. In addition, results show that empirical methods without considering environmental factors (constant emission factor in the IPCC Tier 1 guideline) could underestimate NH3 emissions in context of climate change, with the highest difference (i.e., 6.9 Tg N/year) occurring in 2010. This study provides a robust estimate on global and regional NH3 emissions over the past 50 years, which offers a reference for assessing air quality consequences of future nitrogen enrichment as well as nitrogen use efficiency improvement. Based on the process‐based DLEM‐Bi‐NH3 module, global NH3 emissions from N fertilizer use have increased by 14.8 Tg N/year during the period 1961–2010. At the regional scale, southern Asia, including China and India, has accounted for more than 50% of total global NH3 emissions since the 1980s. Rice cultivation has been the largest contributor to total global NH3 emissions since the 1990s, followed by corn and wheat. In addition, results show that empirical methods without considering environmental factors (constant emission factor in the IPCC Tier 1 guideline) could underestimate NH3 emissions in the context of climate change, with the highest difference (i.e., 6.9 Tg N/year) occurring in 2010. [ABSTRACT FROM AUTHOR]

Soil pore structure has a strong influence on water retention, and is itself influenced by plant and microbial dynamics such as root proliferation and microbial exudation. Although increased nitrogen (N) availability and elevated atmospheric CO2 concentrations (eCO2) often have interacting effects on root and microbial dynamics, it is unclear whether these biotic effects can translate into altered soil pore structure and water retention. This study was based on a long-term experiment (7 yr at the time of sampling) in which a C4 pasture grass ( Paspalum notatum) was grown on a sandy loam soil while provided factorial additions of N and CO2. Through an analysis of soil aggregate fractal properties supported by 3D microtomographic imagery, we found that N fertilization induced an increase in intra-aggregate porosity and a simultaneous shift toward greater accumulation of pore space in larger aggregates. These effects were enhanced by eCO2 and yielded an increase in water retention at pressure potentials near the wilting point of plants. However, eCO2 alone induced changes in the opposite direction, with larger aggregates containing less pore space than under control conditions, and water retention decreasing accordingly. Results on biotic factors further suggested that organic matter gains or losses induced the observed structural changes. Based on our results, we postulate that the pore structure of many mineral soils could undergo N-dependent changes as atmospheric CO2 concentrations rise, having global-scale implications for water balance, carbon storage, and related rhizosphere functions. [ABSTRACT FROM AUTHOR]

Accurately estimating biogenic methane (CH4) and nitrous oxide (N20) fluxes in terrestrial ecosystems is critical for resolving global budgets of these greenhouse gases (GHGs) and continuing to mitigate climate warming. Here, we assess contemporary biogenic CH4 and N20 budgets and probable climate-change-related impacts on CH4 and N20 emissions in terrestrial North America. Multi-approach estimations show that, during 1990-2010, biogenic CH4 emissions ranged from 0.159 to 0.502 petagrams of carbon dioxide (C02) equivalents per year (Pg CO2eq yr-1 where 1 Pg = 1 x 1015 g) and N20 emissions ranged from 0.802 to 1.016 Pg C02eq yr-1, which offset 47-166% of ter-restrial C02 sequestration (0.915-2.040 Pg C02eq yr-1 as indicated elsewhere in this Special Issue). According to two future climate scenarios, CH4 and N20 emissions are projected to continue increasing by 137-151% and 157-227%, respectively, by the end of this century, as compared with levels during 2000-2010. Strategies to miti-gate climate change must account for non-C02 GHG emissions, given their substantial warming potentials. INSET: In a nutshell:. [ABSTRACT FROM AUTHOR]

Long-term cillage and manure application can alter a soil's ability to sequester nutrients and mineralize C and N. A laboratory incubation study (C and N mineralization) evaluated the long-term impact of poultry litter (PL) application (>10 yr) and tillage practice (>25 yr). Soil chemical properties (pH, total C, total N, and Mehlich-l extractable P, K, Ca, and Mg) were also assessed. Soil was collected (0-5-, 5-10-, and 10-20-cm depths) from continuous soybean [Glycine max (L.) Merr.] and corn (Zea mays L.) systems managed under conventional tillage (CT) or no-till (NT) with either PL or inorganic fertilizer (IF) applications. The study was located in northeast Alabama on a Hartsells fine sandy loam (a fine-loamy, siliceous, subactive, thermic Typic Hapludult). Poultry litter and NT increased soil nutrients (N, P, K, Ca, and Mg), primarily at the 0 to 5-cm depth. No-till concentrated nutrients near the soil surface as opposed to the more even distribution seen under CT. The NT-PL treatment had higher soil C for corn and soybean (2.25 and 1.83 gkg-1 C, respectively), followed by NT-IF (1.73 and 1.11 gkg-1 C, respectively). Carbon and N mineralization was higher at the 0 to 5-cm depth for NT and CT compared with lower depths. Long-term PL application increased C and N mineralization more than IF. As depth increased, more C and N mineralization occurred under CT due to plow layer mixing. Results indicated that long-term tillage with PL application can increase soil C and N mineralization, nutrient retention, and organic matter. [ABSTRACT FROM AUTHOR]

The article discusses research investigating the growth of tropical spiderwort (Commelina benghalensis L.) under elevated atmospheric carbon dioxide (CO2) conditions. The researchers note that despite considerable efforts to study exotic plant pests, few researchers consider how invasive plants might react to the increasing CO2 concentration in the atmosphere. Topics include a brief overview of the economic impact on agriculture in the U.S. from the invasive plant C. benghalensis L. and results from a study measuring growth responses of C. benghalensis L. using plants grown in containers with soilless potting medium under ambient and elevated levels of CO2 in open-field chambers.

Carbon is an essential component of life and, in its organic form, plays a pivotal role in the soil's fertility, productivity, and water retention. It is an integral part of the atmospheric-terrestrial C exchange cycle mediated via photosynthesis; furthermore, it emerged recently as a new trading commodity, i.e., "carbon credits." When carefully manipulated, C sequestration by the soil could balance and mitigate anthropogenic CO2 emissions into the atmosphere that are believed to contribute to global warming. The pressing need for assessing the soil's C stocks at local, regional, and global scales, now in the forefront of much research, is considerably hindered by the problems besetting dry-combustion chemical analyses, even with state-of-the-art procedures. To overcome these issues, we developed a new method based on gamma-ray spectroscopy induced by inelastic neutron scattering (INS). The INS method is an in situ, nondestructive, multielemental technique that can be used in stationary or continuous-scanning modes of operation. The results from data acquired from an investigated soil mass of a few hundred kilograms to an approximate depth of 30 cm are reported immediately. Our initial experiments have demonstrated the feasibility of our proposed approach; we obtained a linear response with C concentration and a detection limit between 0.5 and 1% C by weight. [ABSTRACT FROM AUTHOR]

Increasing atmospheric CO2 concentration has led to concerns about potential effects on production agriculture as well as agriculture's role in sequestering C. In the fall of 1997, a study was initiated to compare the response of two crop management systems (conventional and conservation) to elevated CO2. The study used a split-plot design replicated three times with two management systems as main plots and two CO2 levels (ambient=375 μL L−1 and elevated CO2=683 μL L−1) as split-plots using open-top chambers on a Decatur silt loam (clayey, kaolinitic, thermic Rhodic Paleudults). The conventional system was a grain sorghum (Sorghum bicolor(L.) Moench.) and soybean (Glycine max(L.) Merr.) rotation with winter fallow and spring tillage practices. In the conservation system, sorghum and soybean were rotated and three cover crops were used (crimson clover (Trifolium incarnatumL.), sunn hemp (Crotalaria junceaL.), and wheat (Triticum aestivumL.)) under no-tillage practices. The effect of management on soil C and biomass responses over two cropping cycles (4 years) were evaluated. In the conservation system, cover crop residue (clover, sunn hemp, and wheat) was increased by elevated CO2, but CO2 effects on weed residue were variable in the conventional system. Elevated CO2 had a greater effect on increasing soybean residue as compared with sorghum, and grain yield increases were greater for soybean followed by wheat and sorghum. Differences in sorghum and soybean residue production within the different management systems were small and variable. Cumulative residue inputs were increased by elevated CO2 and conservation management. Greater inputs resulted in a substantial increase in soil C concentration at the 0–5 cm depth increment in the conservation system under CO2-enriched conditions. Smaller shifts in soil C were noted at greater depths (5–10 and 15–30 cm) because of management or CO2 level. Results suggest that with conservation management in an elevated CO2 environment, greater residue amounts could increase soil C storage as well as increase ground cover. [ABSTRACT FROM AUTHOR]

A critical global climate change issue is how increasing concentrations of atmospheric CO2 and ground-level O3 will affect agricultural productivity. This includes effects on decomposition of residues left in the field and availability of mineral nutrients to subsequent crops. To address questions about decomposition processes, a 2-year experiment was conducted to determine the chemistry and decomposition rate of aboveground residues of soybean (Glycine max(L.) Merr.) grown under reciprocal combinations of low and high concentrations of CO2 and O3 in open-top field chambers. The CO2 treatments were ambient (370 μmol mol−1) and elevated (714 μmol mol−1) levels (daytime 12 h averages). Ozone treatments were charcoal-filtered air (21 nmol mol−1) and nonfiltered air plus 1.5 times ambient O3 (74 nmol mol−1) 12 h day−1. Elevated CO2 increased aboveground postharvest residue production by 28–56% while elevated O3 suppressed it by 15–46%. In combination, inhibitory effects of added O3 on biomass production were largely negated by elevated CO2. Plant residue chemistry was generally unaffected by elevated CO2, except for an increase in leaf residue lignin concentration. Leaf residues from the elevated O3 treatments had lower concentrations of nonstructural carbohydrates, but higher N, fiber, and lignin levels. Chemical composition of petiole, stem, and pod husk residues was only marginally affected by the elevated gas treatments. Treatment effects on plant biomass production, however, influenced the content of chemical constituents on an areal basis. Elevated CO2 increased the mass per square meter of nonstructural carbohydrates, phenolics, N, cellulose, and lignin by 24–46%. Elevated O3 decreased the mass per square meter of these constituents by 30–48%, while elevated CO2 largely ameliorated the added O3 effect. Carbon mineralization rates of component residues from the elevated gas treatments were not significantly different from the control. However, N immobilization increased in soils containing petiole and stem residues from the elevated CO2, O3, and combined gas treatments. Mass loss of decomposing leaf residue from the added O3 and combined gas treatments was 48% less than the control treatment after 20 weeks, while differences in decomposition of petiole, stem, and husk residues among treatments were minor. Decreased decomposition of leaf residues was correlated with lower starch and higher lignin levels. However, leaf residues only comprised about 20% of the total residue biomass assayed so treatment effects on mass loss of total aboveground residues were relatively small. The primary influence of elevated atmospheric CO2 and O3 concentrations on decomposition processes is apt to arise from effects on residue mass input, which is increased by elevated CO2 and suppressed by O3. [ABSTRACT FROM AUTHOR]

A critical global climate change issue is how increasing concentrations of atmospheric CO2 and ground-level O3 will affect agricultural productivity. This includes effects on decomposition of residues left in the field and availability of mineral nutrients to subsequent crops. To address questions about decomposition processes, a 2-year experiment was conducted to determine the chemistry and decomposition rate of aboveground residues of soybean (Glycine max(L.) Merr.) grown under reciprocal combinations of low and high concentrations of CO2 and O3 in open-top field chambers. The CO2 treatments were ambient (370 μmol mol−1) and elevated (714 μmol mol−1) levels (daytime 12 h averages). Ozone treatments were charcoal-filtered air (21 nmol mol−1) and nonfiltered air plus 1.5 times ambient O3 (74 nmol mol−1) 12 h day−1. Elevated CO2 increased aboveground postharvest residue production by 28–56% while elevated O3 suppressed it by 15–46%. In combination, inhibitory effects of added O3 on biomass production were largely negated by elevated CO2. Plant residue chemistry was generally unaffected by elevated CO2, except for an increase in leaf residue lignin concentration. Leaf residues from the elevated O3 treatments had lower concentrations of nonstructural carbohydrates, but higher N, fiber, and lignin levels. Chemical composition of petiole, stem, and pod husk residues was only marginally affected by the elevated gas treatments. Treatment effects on plant biomass production, however, influenced the content of chemical constituents on an areal basis. Elevated CO2 increased the mass per square meter of nonstructural carbohydrates, phenolics, N, cellulose, and lignin by 24–46%. Elevated O3 decreased the mass per square meter of these constituents by 30–48%, while elevated CO2 largely ameliorated the added O3 effect. Carbon mineralization rates of component residues from the elevated gas treatments were not significantly different from the control. However, N immobilization increased in soils containing petiole and stem residues from the elevated CO2, O3, and combined gas treatments. Mass loss of decomposing leaf residue from the added O3 and combined gas treatments was 48% less than the control treatment after 20 weeks, while differences in decomposition of petiole, stem, and husk residues among treatments were minor. Decreased decomposition of leaf residues was correlated with lower starch and higher lignin levels. However, leaf residues only comprised about 20% of the total residue biomass assayed so treatment effects on mass loss of total aboveground residues were relatively small. The primary influence of elevated atmospheric CO2 and O3 concentrations on decomposition processes is apt to arise from effects on residue mass input, which is increased by elevated CO2 and suppressed by O3. [ABSTRACT FROM AUTHOR]

Abstract Differences in plant morphology, physiology, life form, and symbiotic relationships can generate differences in species responses to CO2 -enrichment, which can alter competitive interactions, thus affecting community structure and function. Here, we present data from a two-year study, examining the species and community responses to elevated [CO2 ] of a model regenerating longleaf pine community. The model community was constructed from an assemblage of early successional forest species representing major functional guilds within a typical longleaf pine–wiregrass community: (1) a C3 evergreen conifer (Pinus palustris ); (2) a C4 bunch grass (Aristida stricta ); (3) a C3 broadleaf tree (Quercus margaretta ); (4) a C3 perennial herbaceous legume (Crotalaria rotundifolia ); and (5) a C3 herbaceous perennial (Asclepias tuberosa ). After 2 years, CO2 -enriched plots had 109% greater above-ground biomass than ambient plots, mainly due to a 117% increase in pine biomass. Community structure was altered by CO2 enrichment; Crotalaria and Asclepias had higher mortality and less biomass in high-CO2 plots, suggesting that not all species will perform well as global [CO2 ] rises. Our data suggest that longleaf pine communities as a whole will perform well in a future higher CO2 world, but some species may fall prey to altered competitive interactions for light and soil moisture. [ABSTRACT FROM AUTHOR]

Examines the effects of atmospheric carbon dioxide on the root dynamics of Pinus taeda. Root mortality; Relative root turnover; Ecosystem-level root productivity.

Summary • Developmental and inducible changes in metal-based (nickel (Ni)) and organic defences (phenolics) are compared in the Ni-hyperaccumulating shrub, Psychotria douarrei . • Young and old leaves of P. douarrei shrubs, subjected to different degrees of simulated herbivory, were analyzed for metals, tannins, macronutrients and total carbon, and compared with a co-occuring nonhyperaccumulator shrub, Ficus webbiana . • Leaf age affected both nickel Ni-based and organic defences in P. douarrei ; foliar metal concentrations were higher in mature leaves, whereas organic defences were higher in young leaves. Neither metal-based nor organic defences were increased by simulated herbivore damage, implying noninducibility, although some organic defence compounds were significantly reduced. P. douarrei had a greater percentage of total phenolics, condensed tannins and protein precipitation ability than F. webbianai . Since total carbon content did not differ between species, Psychotria invests more of its leaf carbon budget in organic defences than does Ficus . • Data suggest that P. douarrei foliage is well protected by Ni, but tannins have multiple functions. The high concentrations of tannins in Psychotria leaves might function as a detoxification mechanism for elevated cytoplasmic metal concentrations, in addition to providing defensive benefits. [ABSTRACT FROM AUTHOR]

SummaryConsequences of increasing atmospheric CO2 concentration on plant structure, an important determinant of physiological and competitive success, have not received sufficient attention in the literature. Understanding how increasing carbon input will influence plant developmental processes, and resultant form, will help bridge the gap between physiological response and ecosystem level phenomena. Growth in elevated CO2 alters plant structure through its effects on both primary and secondary meristems of shoots and roots. Although not well established, a review of the literature suggests that cell division, cell expansion, and cell patterning may be affected, driven mainly by increased substrate (sucrose) availability and perhaps also by differential expression of genes involved in cell cycling (e.g. cyclins) or cell expansion (e.g. xyloglucan endotransglycosylase). Few studies, however, have attempted to elucidate the mechanistic basis for increased growth at the cellular level. Regardless of specific mechanisms involved, plant leaf size and anatomy are often altered by growth in elevated CO2, but the magnitude of these changes, which often decreases as leaves mature, hinges upon plant genetic plasticity, nutrient availability, temperature, and phenology. Increased leaf growth results more often from increased cell expansion rather than increased division. Leaves of crop species exhibit greater increases in leaf thickness than do leaves of wild species. Increased mesophyll and vascular tissue cross-sectional areas, important determinates of photosynthetic rates and assimilate transport capacity, are often reported. Few studies, however, have quantified characteristics more reflective of leaf function such as spatial relationships among chlorenchyma cells (size, orientation, and surface area), intercellular spaces, and conductive tissue. Greater leaf size and/or more leaves per plant are often noted; plants grown in elevated CO2 exhibited increased leaf area per plant in 66% of studies, compared to 28% of observations reporting no change, and 6% reported a decrease in whole plant leaf area. This resulted in an average net increase in leaf area per plant of 24%. Crop species showed the greatest average increase in whole plant leaf area (+ 37%) compared to tree species (+ 14%) and wild, nonwoody species (+ 15%). Conversely, tree species and wild, nontrees showed the greatest reduction in specific leaf area (– 14% and – 20%) compared to crop plants (– 6%). Alterations in developmental processes at the shoot apex and within the vascular cambium contributed to increased plant height, altered branching characteristics, and increased stem diameters. The ratio of internode length to node number often increased, but the length and sometimes the number of branches per node was greater, suggesting reduced apical dominance. Data concerning effects of elevated CO2 on stem/branch anatomy, vital for understanding potential shifts in functional relationships of leaves with stems, roots with stems, and leaves with roots, are too few to make generalizations. Growth in elevated CO2 typically leads to increased root length, diameter, and altered branching patterns. Altered branching characteristics in both shoots and roots may impact competitive relationships above and below the ground. Understanding how increased carbon assimilation affects growth processes (cell division, cell expansion, and cell patterning) will facilitate a better understanding of how plant form will change as atmospheric CO2 increases. Knowing how basic growth processes respond to increased carbon inputs may also provide a mechanistic basis for the differential phenotypic plasticity exhibited by different plant species/functional types to elevated CO2. [ABSTRACT FROM AUTHOR]

Presents information on research conducted on the effect of atmospheric carbon dioxide, types of soil and water, on the development of the leaf surface wax, forming on the Pinus palustris (Pinaceae) plant. Materials and methods employed in the research; Reference to the work of Keeling et al., (1989), Sundquist (1993), Amthor (1995) and others; Analysis of the results; Statistical information supporting the results.

Grain sorghum [Sorghum bicolor (L.) Moench, a C4 crop] and soybean [Glycine max (L.) Merr. cv. Stonewall, a C3 crop] plants were grown in ambient (c. 360 μl l-1) and twice-ambient (c. 720 μl l-1) CO2 levels in open-top chambers in soil without root constriction. Plant dry mass, energy content, composition and construction cost (i. e. amount of carbohydrate required to synthesize a unit plant dry mass) were assessed at the end of the growing season. Elevated CO2 (a) increased phytomass accumulation (kg per plant) in both species, (b) had little affect on energy concentration (MJ kg-1 plant) but caused large increases in the amount of plant energy per ground area (MJ m-2 ground), and (c) did not alter specific growth cost (kg carbohydrate kg-1 plant growth) but greatly increased growth cost per ground area (kg carbohydrate m-2 ground) because growth was enhanced. For soybean, twice-ambient CO2 resulted in a 50% increase in the amount of nitrogen and energy in grain (seed plus pod) per ground area. This response to elevated CO2 has important implications for agricultural productivity during the next century because the rate of human population growth is exceeding the rate of increase of land used for agriculture so that future food demands can only be met by greater production per ground area. [ABSTRACT FROM AUTHOR]

Inelastic neutron scattering measures the average carbon weight percent in the 8-cm soil layer for any carbon depth profile shape. Accounting for system background returns an accuracy comparable to the dry combustion method. The herein described inelastic neutron scattering (INS) method of measuring soil carbon was based on a new procedure for extracting the net carbon signal (NCS) from the measured γ spectra and determination of the average carbon weight percent in the upper ~8-cm soil layer (AvgCw%8). The NCS extraction utilized the net-INS spectrum, which was the difference between the INS and thermal neutron capture (TNC) spectra and the net-INS system background spectrum. The proportionality between NCS and AvgCw%8 for any shape of soil carbon depth distribution was demonstrated theoretically. The theoretical model for NCS calculations accounted for carbon depth distribution and neutron and γ ray propagation laws in our analysis; previous model results were verified by comparison with a Monte Carlo simulation using Geant4. The experimental results confirmed the identified proportionality. The mobile INS system was calibrated using pits filled with synthetic soil; this calibration was used for AvgCw%8 determinations in INS field measurements. The AvgCw%8 was also determined by the dry combustion method. Benchmarking the soil carbon determination by INS demonstrated results that coincided with dry combustion technique (DCT) results (within experimental error limits). Given the agreement between these methods, the described INS measurement system can be recommended as a reliable alternative means for measuring soil carbon. [ABSTRACT FROM AUTHOR]

Increasing global atmospheric CO2 concentration has led to concerns regarding its potential effects on the terrestrial environment. Attempts to balance the atmospheric carbon (C) budget have met with a large shortfall in C accounting (approx. 1.4 x 1015g C y-1) and this has led to the hypothesis that C is being stored in the soil of terrestrial ecosystems. This study examined the effects Of CO2 enrichment on soil C storage in C3 soybean (Glycine max L.) Merr. and C4 grain sorghum (Sorghum bicolor L.) Moench. agroecosystems established on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). The study was a split-plot design replicated three times with two crop species (soybean and grain sorghum) as the main plots and two CO2 concentration (ambient and twice ambient) as subplots-using open top field chambers. Carbon isotopic techniques using delta13C were used to track the input of new C into the soil system. At the end of two years, shifts in delta13C content of soil organic matter carbon were observed to a depth of 30 cm. Calculated new C in soil organic matter with grain sorghum was greater for elevated CO2 vs. ambient CO2 (162 and 29 g m-2, respectively), butwith soybean the new C in soil organic matter was less for elevated CO2 vs. ambient CO2 (120 and 291 g m-2, respectively). A significant increase in mineral associated organic C was observed in 1993 which may result in increased soil C storage over the long-term, however, little change in total soil organic C was observed under either plant species. These data indicate that elevated atmospheric CO2 resulted in changes in soil C dynamics in agro-ecosystems that are crop species dependent. [ABSTRACT FROM AUTHOR]