N2O Release from agro-biofuel production negates global warming reduction by replacing fossil fuels

The relationship, on a global basis, between the amount of N fixed by chemical, biological or atmospheric processes entering the terrestrial biosphere, and the total emission of nitrous oxide (N2O), has been re-examined, using known global atmospheric removal rates and concentration growth of N2O as a proxy for overall emissions. For both the pre-industrial period and in recent times, after taking into account the large-scale changes in synthetic N fertiliser production, we find an overall conversion factor of 3–5 % from newly fixed N to N2O–N. We assume the same factor to be valid for biofuel production systems. It is covered only in part by the default conversion factor for ‘direct’ emissions from agricultural crop lands (1 %) estimated by IPCC (2006), and the default factors for the ‘indirect’ emissions (following volalilization/deposition and leaching/runoff of N: 0.35–0.45 %) cited therein. However, as we show in the paper, when additional emissions included in the IPCC methodology, e.g. those from livestock production, are included, the total may not be inconsistent with that given by our “top-down” method. When the extra N2O emission from biofuel production is calculated in “CO2-equivalent” global warming terms, and compared with the quasi-cooling effect of ‘saving’ emissions of fossil fuel derived CO2, the outcome is that the production of commonly used biofuels, such as biodiesel from rapeseed and bioethanol from corn (maize), depending on N fertilizer uptake efficiency by the plants, can contribute as much or more to global warming by N2O emissions than cooling by fossil fuel savings. Crops with less N demand, such as grasses and woody coppice species, have more favourable climate impacts. This analysis only considers the conversion of biomass to biofuel. It does not take into account the use of fossil fuel on the farms and for fertilizer and pesticide production, but it also neglects the production of useful co-products. Both factors partially compensate each other. This needs to be analyzed in a full life cycle assessment

N2O emissions from agricultural lands: a synthesis of simulation approaches

Nitrous oxide (N2O) is primarily produced by the microbially-mediated nitrification and denitrification processes in soils. It is influenced by a suite of climate (i.e. temperature and rainfall) and soil (physical and chemical) variables, interacting soil and plant nitrogen (N) transformations (either competing or supplying substrates) as well as land management practices. It is not surprising that N2O emissions are highly variable both spatially and temporally. Computer simulation models, which can integrate all of these variables, are required for the complex task of providing quantitative determinations of N2O emissions. Numerous simulation models have been developed to predict N2O production. Each model has its own philosophy in constructing simulation components as well as performance strengths. The models range from those that attempt to comprehensively simulate all soil processes to more empirical approaches requiring minimal input data. These N2O simulation models can be classified into three categories: laboratory, field and regional/global levels. Process-based field-scale N2O simulation models, which simulate whole agroecosystems and can be used to develop N2O mitigation measures, are the most widely used. The current challenge is how to scale up the relatively more robust field-scale model to catchment, regional and national scales. This paper reviews the development history, main construction components, strengths, limitations and applications of N2O emissions models, which have been published in the literature. The three scale levels are considered and the current knowledge gaps and challenges in modelling N2O emissions from soils are discussed

AUTOMATED GAS SAMPLING SYSTEM FOR LABORATORY ANALYSIS OF CH\u3csub\u3e4\u3c/sub\u3e AND N\u3csub\u3e2\u3c/sub\u3eO

Analyzing the flux of CH4 and N2O from soil is labor intensive when conventional hand injection techniques are utilized in gas chromatography. An automated gas sampling system was designed and assembled from a prototype developed at the National Soil Tilth Laboratory in Ames, IA. The sampler was evaluated for accuracy and precision when attached to a Varian 3700 gas chromatograph configured with flame ionization and electron capture detectors. The automated gas sampling system is easy to operate and provides acceptable results (standards ranging from 1.0–5.0 ppmv CH4 and 0.342–2.0 ppmv N2O had coefficients of variation ranging from 1.7–5.6%) while providing an economical approach for analyzing large numbers of gas samples with minimal labor and equipment cost

Patchiness in microbial nitrogen transformations in groundwater in a riparian forest

We measured microbial N transformations in 15 cm diam. by 40 cm intact horizontal sections of aquifer material (mesocosms), taken from a riparian forest in Rhode Island, USA, incubated under ambient conditions. The mesocosms allowed us to measure these transformations on the same scale as hydrologic tracer methods (Br-/NO3/- ratios) that measure net NO3/- removal. Our objective was to reconcile discrepancies between hydrologic tracer and microbial measurements in previous studies where laboratory-based microbial NO3/- consumption measurements were much lower than in situ hydrologic measurements of net NO3/- removal. We hypothesized that small \u27patches\u27 of organic matter in the aquifer matrix, which are easily missed when sampling for microbial measurements, are \u27hotspots\u27 of NO3/- removal and are responsible for these discrepancies. Mesocosms were subjected to three treatments [Br- only, Br- + 15NO3/-, Br- + 15NO3/- + dissolved organic carbon (DOC)]. Solution (NH4/-, NO3/-, dissolved organic N) and gaseous (N2O, 15N2O, and 15N2) inputs and outputs to the mesocosms were measured over a 132-d incubation, followed by destructive sampling for the presence of patches and residual 15N in aquifer matrix and patch material. Total (gross) NO3/- consumption by denitrification and immobilization was greater than net removal of NO3/- measured by Br- /NO3/- ratios. Net NO3/- consumption was only observed in mesocosms that contained \u27patches\u27 of organic matter and was not increased by addition of DOC, suggesting that these patches, which represent \u3c1% of aquifer weight, are critical to groundwater NO3/- removal in riparian forests

Net Global Warming Potential and Greenhouse Gas Intensity in Irrigated Cropping Systems in Northeastern Colorado

The impact of management on global warming potential (GWP), crop production, and greenhouse gas intensity (GHGI) in irrigated agriculture is not well documented. A no-till (NT) cropping systems study initiated in 1999 to evaluate soil organic carbon (SOC) sequestration potential in irrigated agriculture was used in this study to make trace gas flux measurements for 3 yr to facilitate a complete greenhouse gas accounting of GWP and GHGI. Fluxes of CO2, CH4, and N2O were measured using static, vented chambers, one to three times per week, year round, from April 2002 through October 2004 within conventional-till continuous corn (CT-CC) and NT continuous corn (NT-CC) plots and in NT corn–soybean rotation (NT-CB) plots. Nitrogen fertilizer rates ranged from 0 to 224 kgN ha-1. Methane fluxes were small and did not differ between tillage systems. Nitrous oxide fluxes increased linearly with increasing N fertilizer rate each year, but emission rates varied with years. Carbon dioxide efflux was higher in CT compared to NT in 2002 but was not different by tillage in 2003 or 2004. Based on soil respiration and residue C inputs, NT soils were net sinks of GWP when adequate fertilizer was added to maintain crop production. The CT soils were smaller net sinks for GWP than NT soils. The determinant for the net GWP relationship was a balance between soil respiration and N2O emissions. Based on soil C sequestration, only NT soils were net sinks for GWP. Both estimates of GWP and GHGI indicate that when appropriate crop production levels are achieved, net CO2 emissions are reduced. The results suggest that economic viability and environmental conservation can be achieved by minimizing tillage and utilizing appropriate levels of fertilizer

CO2 enhances productivity, alters species composition, and reduces digestibility of Shortgrass Steppe vegetation

Includes bibliographical references (pages 218-219).The impact of increasing atmospheric CO2 concentrations has been studied in a number of field experiments, but little information exists on the response of semiarid rangelands to CO2, or on the consequences for forage quality. This study was initiated to study the CO2 response of the shortgrass steppe, an important semiarid grassland on the western edge of the North American Great Plains, used extensively for livestock grazing. The experiment was conducted for five years on native vegetation at the USDA-ARS Central Plains Experimental Range in northeastern Colorado, USA. Three perennial grasses dominate the study site, Bouteloua gracilis, a C4 grass, and two C3 grasses, Pascopyrum smithii and Stipa comata. The three species comprise 88% of the aboveground phytomass. To evaluate responses to rising atmospheric CO2, we utilized six open-top chambers, three with ambient air and three with air CO2 enriched to 720 mmol/mol, as well as three unchambered controls. We found that elevated CO2 enhanced production of the shortgrass steppe throughout the study, with 41% greater aboveground phytomass harvested annually in elevated compared to ambient plots. The CO2-induced production response was driven by a single species, S. comata, and was due in part to greater seedling recruitment. The result was species movement toward a composition more typical of the mixed-grass prairie. Growth under elevated CO2 reduced the digestibility of all three dominant grass species. Digestibility was also lowest in the only species to exhibit a CO2-induced production enhancement, S. comata. The results suggest that rising atmospheric CO2 may enhance production of lower quality forage and a species composition shift toward a greater C3 component

A credit system to solve agricultural nitrogen pollution

'Commentary' paper. Increasing amounts of nitrogen fertilizer have been used in agriculture during the last decades to boost food production for the increasing global human population. The marked increase in reactive nitrogen use has also contributed to severe nitrogen pollution and multiple impacts on human and ecosystems' health. Nitrogen is an important precursor to air pollution (e.g., fine particulate matter, near-surface ozone), water pollution (algal blooms, nitrate contamination), biodiversity loss (nitrogen deposition and eutrophication), soil acidification (ammonium fertilizer use), and global warming (nitrous oxide). Agricultural nitrogen pollution has decreased in some high-income countries, such as those in the European Union (EU), during the last decades, but the remaining nitrogen pollution still causes serious damage. The societal cost of nitrogen pollution by agriculture in the EU has been estimated to range from €35 to €230 billion per year and this cost appears to be greater than the farm profits from nitrogen fertilizer use, which range from €20 to €80 billion per year. Socioeconomic trade-offs between farmers and society need to be introduced to decrease nitrogen pollution

Effect of elevated carbon dioxide on growth and nitrogen fixation of two soybean cultivars in northern China

The effect of elevated carbon dioxide (CO2) concentration on symbiotic nitrogen fixation in soybean under open-air conditions has not been reported. Two soybean cultivars (Glycine max (L.) Merr. cv. Zhonghuang 13 and cv. Zhonghuang 35) were grown to maturity under ambient (415 ± 16 μmol mol -1) and elevated (550 ± 17 μmol mol -1) [CO2] at the free-air carbon dioxide enrichment experimental facility in northern China. Elevated [CO2] increased above- and below-ground biomass by 16-18% and 11-20%, respectively, but had no significant effect on the tissue C/N ratio at maturity. Elevated [CO2] increased the percentage of N derived from the atmosphere (%Ndfa, estimated by natural abundance) from 59% to 79% for Zhonghuang 13, and the amount of N fixed from 166 to 275 kg N ha -1, but had no significant effect on either parameter for Zhonghuang 35. These results suggest that variation in N2 fixation ability in response to elevated [CO2] should be used as key trait for selecting cultivars for future climate with respect to meeting the higher N demand driven by a carbon-rich atmosphere