Net Global Warming Potential Research Articles

Consequential life cycle assessment of biogas, biofuel and biomass energy options within an arable crop rotation

AbstractFeed in tariffs (FiTs) and renewable heat incentives (RHIs) are driving a rapid expansion in anaerobic digestion (AD) coupled with combined heat and power (CHP) plants in the UK. Farm models were combined with consequential life cycle assessment (CLCA) to assess the net environmental balance of representative biogas, biofuel and biomass scenarios on a large arable farm, capturing crop rotation and digestate nutrient cycling effects. All bioenergy options led to avoided fossil resource depletion. Global warming potential (GWP) balances ranged from −1732 kg CO2e Mg−1 dry matter (DM) for pig slurry AD feedstock after accounting for avoided slurry storage to +2251 kg CO2e Mg−1 DM for oilseed rape biodiesel feedstock after attributing indirect land use change (iLUC) to displaced food production. Maize monoculture for AD led to net GWP increases via iLUC, but optimized integration of maize into an arable rotation resulted in negligible food crop displacement and iLUC. However, even under best‐case assumptions such as full use of heat output from AD‐CHP, crop–biogas achieved low GWP reductions per hectare compared with Miscanthus heating pellets under default estimates of iLUC. Ecosystem services (ES) assessment highlighted soil and water quality risks for maize cultivation. All bioenergy crop options led to net increases in eutrophication after displaced food production was accounted for. The environmental balance of AD is sensitive to design and management factors such as digestate storage and application techniques, which are not well regulated in the UK. Currently, FiT payments are not dependent on compliance with sustainability criteria. We conclude that CLCA and ES effects should be integrated into sustainability criteria for FiTs and RHIs, to direct public money towards resource‐efficient renewable energy options that achieve genuine climate protection without degrading soil, air or water quality.

The impact of alternative cropping systems on global warming potential, grain yield and groundwater use

The large consumption of groundwater for irrigating winter wheat has resulted in a continuous decline in the groundwater table on the North China Plain in recent decades. Alternative cropping systems have been proposed to substitute for the conventional winter wheat–summer maize rotation system for the sustainable use of groundwater in the future. However, the impact of these cropping systems on net global warming potential (net GWP), and greenhouse gas emissions on the basis of per unit of yield (greenhouse gas intensity, GHGI) is poorly documented. Measurements of greenhouse gases were conducted over a four-year period to gain insight into net GWP and GHGI on a crop rotation scale based on an ongoing long-term field experiment on the North China Plain. The cropping systems investigated include one conventional winter wheat–summer maize system (Chem. W/M) as the control and four alternative cropping systems, namely an optimized winter wheat–summer maize system (Opt. W/M), two winter wheat–summer maize (or soybean)–spring maize system with three crops in two years (W/M–M, W/S–M), and a single spring maize per year (M). Compared with the Chem. W/M control, the grain yields in Opt. W/M increased significantly by 19% while the net GWP, GHGI and fertilizer N decreased by 29%, 40% and 40%, respectively, but still consumed as much groundwater (264mmyr−1) as Chem. W/M. In the two-year rotation cycle fertilizer N, groundwater use, net GWP and GHGI in W/M–M, W/S–M and M declined by 56–70%, 43–63%, 50–58% and 30–50%, respectively, compared to Chem. W/M. Moreover, these cropping systems consumed only 108–159mmyr−1 groundwater for irrigation, a value close to the theoretical value of 150mmyr−1 to avoid a continuing decline in the groundwater table in this region. However, W/S–M treatment had grain yield reductions of −23% and M treatment had −30%, and only W/M–M maintained grain yields relative to Chem. W/M. We therefore recommend the W/M–M management package as a preferred option to maintain grain yields together with low GWP and GHGI while mitigating the decline in the groundwater table in areas with a high water deficit.

The effects of household management practices on the global warming potential of urban lawns

Nitrous oxide (N2O) emissions are an important component of the greenhouse gas (GHG) budget for urban turfgrasses. A biogeochemical model DNDC successfully captured the magnitudes and patterns of N2O emissions observed at an urban turfgrass system at the Richland Creek Watershed in Nashville, TN. The model was then used to study the long-term (i.e. 75 years) impacts of lawn management practice (LMP) on soil organic carbon sequestration rate (dSOC), soil N2O emissions, and net Global Warming Potentials (net GWPs). The model simulated N2O emissions and net GWP from the three management intensity levels over 75 years ranged from 0.75 to 3.57 kg Nha−1yr−1 and 697 to 2443 kg CO2-eq ha−1yr−1, respectively, which suggested that turfgrasses act as a net carbon emitter. Reduction of fertilization is most effective to mitigate the global warming potentials of turfgrasses. Compared to the baseline scenario, halving fertilization rate and clipping recycle as an alternative to synthetic fertilizer can reduce net GWPs by 17% and 12%, respectively. In addition, reducing irrigation and mowing are also effective in lowering net GWPs. The minimum-maintenance LMP without irrigation and fertilization can reduce annual N2O emissions and net GWPs by approximately 53% and 70%, respectively, with the price of gradual depletion of soil organic carbon, when compared to the intensive-maintenance LMP. A lawn age-dependent best management practice is recommended: a high dose fertilizer input at the initial stage of lawn establishment to enhance SOC sequestration, followed by decreasing fertilization rate when the lawn ages to minimize N2O emissions. A minimum-maintained LMP with clipping recycling, and minimum irrigation and mowing, is recommended to mitigate global warming effects from urban turfgrass systems. Among all practices, clipping recycle may be a relatively malleable behavior and, therefore, a good target for interventions seeking to reduce the environmental impacts of lawn management through public education. Our results suggest that a long-term or a chronosequence study of turfgrasses with varying ages is warranted to capture the complete dynamics of contribution of turfgrasses to global warming.

Comparison of Net Global Warming Potential and Greenhouse Gas Intensity Affected by Management Practices in Two Dryland Cropping Sites

Little is known about the effect of management practices on net global warming potential (GWP) and greenhouse gas intensity (GHGI) that account for all sources and sinks of greenhouse gas (GHG) emissions in dryland cropping systems. The objective of this study was to compare the effect of a combination of tillage, cropping system, and N fertilization on GWP and GHGI under dryland cropping systems with various soil and climatic conditions from 2008 to 2011 in western North Dakota and eastern Montana, USA. Treatments in western North Dakota with sandy loam soil and 373 mm annual precipitation were conventional till malt barley (Hordeum vulgarie L.) with 67 kg N ha-1 (CTB/N1), conventional till malt barley with 0 kg N ha-1 (CTB/N0), no-till malt barley-pea (Pisum sativum L.) with 67 kg N ha-1 (NTB-P/N1), no-till malt barley with 67 kg N ha-1 (NTB/N1), and no-till malt barley with 0 kg N ha-1 (NTB/N0). In eastern Montana with loam soil and 350 mm annual precipitation, treatments were conventional till malt barley-fallow with 80 kg N ha-1 (CTB-F/N1), conventional till malt barley-fallow with 0 kg N ha-1 (CTB-F/N0), no-till malt barley-pea with 80 kg N ha-1 (NTB-P/N1), no-till malt barley with 80 kg N ha-1 (NTB/N1), and no-till malt barley with 0 kg N ha-1 (NTB/N0). Carbon dioxide sink as soil C sequestration rate at the 0 - 10 cm depth was greater in NTB-P/N1 and NTB/N1 than the other treatments at both sites and greater in eastern Montana than western North Dakota. Carbon dioxide sources were greater with N fertilization than without and greater with conventional till than no-till. Soil total annual N2O and CH4 fluxes varied among treatments, years, and locations. Net GWP and GHGI were lower in NTB-P/N1 than the other treatments in western North Dakota and lower in NTB-P/N1 and NTB/N1 than the other treatments in eastern Montana. Net GWP across similar treatments was lower in eastern Montana than western North Dakota, but GHGI was similar. Annualized crop yield was greater in the treatments with N fertilization than without. Because of greater grain yield but lower GWP and GHGI, no-till malt barley-pea rotation with adequate N fertilization can be used as a robust management practice to mitigate net GHG emissions while sustaining dryland crop yields, regardless of soil and climatic conditions. Loam soil reduced GWP and crop yields compared with sandy loam soil.

Nitrous oxide and methane emissions from a Chinese wheat–rice cropping system under different tillage practices during the wheat-growing season

The annual wheat (Triticum aestivum L.)–rice (Oryza sativa L.) cropping system is the most important cereal production system in the Yangtze River Valley of China, in which various tillage systems are currently implemented during the wheat-growing season. The emissions of nitrous oxide (N2O) and methane (CH4) from the different tillage systems in this system remain unclear. We conducted a 3-year field experiment in a wheat–rice cropping system in a silt clay loam soil to investigate the effects of the type of tillage employed during the wheat-growing season (no-tillage (NT), reduced tillage (RT) or conventional tillage (CT)) on the emissions of N2O and CH4 using the static chamber method over three annual rotation cycles from the 2008 wheat season to the 2011 rice season. The results revealed that the adoption of an NT system during the wheat-growing season significantly increased CH4 emissions during both the wheat-growing season and the following rice-growing season. Over the three annual rotation cycles studied, the annual N2O emissions from the NT (2.24kgN2O–Nha−1) and CT (2.01kgN2O–Nha−1) treatments were similar to each other and significantly higher than those from the RT treatment (1.73kgN2O–Nha−1); the annual CH4 emissions were significantly higher from the NT (100.1kg CH4–Cha−1) than the CT (83.7kg CH4–Cha−1) and RT (73.9kg CH4–Cha−1) systems. The overall results regarding the net global warming potential associated with annual N2O and CH4 emissions indicate that the conversion of conventional tillage to no-tillage systems during the wheat-growing season would intensify the radiative forcing in wheat–rice cropping systems in China.

Effect of no-tillage with weed cover mulching versus conventional tillage on global warming potential and nitrate leaching

Abandoned agricultural land could potentially accumulate soil organic carbon (SOC) when it is no longer used for cultivation and is allowed to revert to natural vegetation. In Japan, no tillage with weed mulching will be adopted in marginal farmland as a new organic farming system because this system minimizes the disturbance of the soil ecosystem and reduces the cost for crop production. The present study aimed to compare the effects of two organic farming systems, namely no-tillage with weed cover mulching and conventional tillage (CT), and two organic fertilizer application modes, namely no fertilizer (N−) and organic fertilizer (N+; 50kgNha−1 during 2010 and 2011 and 80kgNha−1 during 2012) on greenhouse gas (GHG) fluxes, soil carbon sequestration, net global warming potential (GWP), and nitrate leaching. Pumpkin (Cucurbita spp.) was cultivated as the main crop in 2010 and 2011, whereas mixed cropping of okra (Abelmoschus esculentus L.), bell pepper (Capsicum annuum L.), and eggplant (Solanum melongena L.) was implemented in 2012. Tillage management increased CH4 uptake immediately after the tillage; however, the effects did not continue in the long term. On the contrary, NTW increased CH4 uptake, and the soil carbon content at the soil surface linearly increased every year after conversion to NTW indicating that improving soil physics by continuing NTW contributed to enhanced CH4 uptake. N2O emissions in NTW were higher only immediately after a weed mowing; however, NTW did not increase the annual N2O emission. In addition, the difference between initial and final SOC (ΔSOC) was greater in NTW than in CT, which significantly decreased net GWP in NTW in comparison with CT. Nitrate leaching was 48.6% and 47.3% lower in NTW than in CT at soil depths of 30–60 and 60–90cm, respectively. These results show that no-tillage with weed cover mulching contributed to conserve the regional and global environment by reducing nitrate leaching and net GWP from the agro-ecosystem by increasing the annual CH4 uptake and soil carbon sequestration. This system will be adopted for abandoned agricultural land because it reduces net GWP shortly after conversion to this management.

Resurgent beaver ponds in the northeastern United States: implications for greenhouse gas emissions.

Beaver ponds, a wetland type of increasing density in the northeastern United States, vary spatially and temporally, creating high uncertainty in their impact to greenhouse gas (GHG) emissions. We used floating static gas chambers to assess diffusive fluxes of methane (CH), carbon dioxide (CO), and nitrous oxide (NO) from the air-water interface of three beaver ponds (0.05-8 ha) in Rhode Island from fall 2012 to summer 2013. Gas flux was based on linear changes of gas concentrations in chambers over 1 h. Our results show that these beaver ponds generated considerable CH and CO emissions. Methane flux (18-556 mg m d) showed no significant seasonal differences, but the shallowest pond generated significantly higher CH flux than the other ponds. Carbon dioxide flux (0.5-22.0 g m d) was not significantly different between sites, but it was significantly higher in the fall, possibly due to the degradation of fresh leaves. Nitrous oxide flux was low (0-2.4 mg m d). Overall, CH and CO comprised most of the global warming potential, 61 and 38%, respectively. The shallowness of the beaver ponds may have limited the time needed for CH oxidation to CO before CH escaped to the atmosphere. Beaver dams also increase the aerial extent of hydric soils, which may transform riparian areas from upland GHG sinks to wetland GHG sources thereby changing the net global warming potential. Further studies tracking the pattern and conditions of beaver pond creation and abandonment will be essential to understanding their role as GHG sources.

Combined effects of nitrogen fertilization and biochar on the net global warming potential, greenhouse gas intensity and net ecosystem economic budget in intensive vegetable agriculture in southeastern China

Field experiments were conducted to determine the effects of nitrogen (N) fertilization and biochar addition on the net global warming potential (net GWP), greenhouse gas intensity (GHGI) and net ecosystem economic budget (NEEB). These experiments were conducted in an intensive vegetable field with 4 consecutive vegetable crops in 2012 and 2013 in southeastern China. The experiment was conducted with a 32 factorial design in triplicate at N fertilizer rates of 0, 1475, 1967 kg N ha−1 and biochar rates of 0, 20, and 40 t ha−1. Although CH4 emissions were not obviously affected by N fertilization, N2O emissions increased by 27.2–116.2% and the net GWP increased by 30.6–307.2%. Consequently, the GHGI increased significantly, but vegetable yield and the NEEB did not improve. Furthermore, biochar amendments did not significantly influence CH4 emissions, but significantly decreased the N2O emissions by 1.7–25.4%, the net GWP by 89.6–700.5%, and the GHGI by 89.5–644.8%. In addition, vegetable yields significantly increased by 2.1–74.1%, which improved the NEEB. Thus, N fertilization did not increase vegetable yields or the NEEB. However, N fertilization did increase the net GWP and GHGI. In contrast, biochar additions resulted in lower N2O emissions and net GWP and GHGI, but increased vegetable yield and the NEEB in the intensive vegetable production system. Therefore, appropriate biochar amendment should be studied to combat changing climate and to improve the economic profits of vegetable production.

Effects of long-term straw incorporation on the net global warming potential and the net economic benefit in a rice–wheat cropping system in China

Straw incorporation has multiple effects on greenhouse gas emissions and soil productivity. However, few studies have comprehensively evaluated the effects of long-term straw incorporation. An ongoing long-term straw incorporation experiment in a rice–wheat cropping system in China was established in 1990 and was used in the present study to evaluate the net global warming potential (NGWP) and the net economic benefit (NEB) of the straw return. The following four field treatments were included: a control (CK); N, P and K fertilization (NPK); fertilization plus a moderate rate of straw application (NPKS1); and fertilization plus a high rate of straw application (NPKS2). We calculated the increase in the soil organic carbon (SOC) and the straw-induced emissions of CH4 and N2O, which were expressed as the global warming potential (GWP) in units of CO2-equivalent (CO2-eq) at the 100-year scale. The straw-induced NEB was defined as the difference between the economic income, which was calculated by multiplying the increase in straw-induced crop grain yield by the grain price, and the economic loss was computed by multiplying the increase in straw-induced CO2-eq emissions by the carbon price. The results showed that long-term straw incorporation significantly increased the CH4 emissions and the topsoil SOC density. The GWP of the straw-induced CH4 emissions was 3.21–3.92 times that of the straw-induced SOC sequestration rate, suggesting that long-term direct straw incorporation in the rice–wheat systems worsens rather than mitigates the climate change. Additionally, continuous straw incorporation slightly enhanced the rice and wheat grain yields, contributing to the production of the NEB. We determined that under the current carbon price, ranging from 2.55 to 31.71 EUR per ton CO2-eq, the direct straw incorporation will produce a positive NEB, ranging from 156 to 658 RMBha−1year−1, if the grain yield prices do not fluctuate, which does not provide a significant incentive for farmers to change from their traditional direct straw incorporation pattern. Considering the other benefits that the straw application produced, such as improving soil fertility and the water retention capacity, we recommend that the government should establish an incentive for ecological compensation to encourage farmers to implement proper straw incorporation, such as composting straw under aerobic conditions before application.

Methane and nitrous oxide emissions from irrigated lowland rice paddies after wheat straw application and midseason aeration

Straw application and midseason drainage play role in controlling methane (CH4) and nitrous oxide (N2O) emissions from rice paddy fields, but little information is available on their integrative effect on CH4 and N2O emissions. A two-year field experiment was conducted to study the combined effect of timing and duration of midseason aeration and wheat straw incorporation on mitigation of global warming potential (GWP) of CH4 and N2O emissions from irrigated lowland rice paddy fields. Results showed that incorporation of wheat straw increased CH4 by a factor of 5-9 under various water regimes, but simultaneously decreased N2O emission by 19-42 % during the rice growing season. Without straw incorporation, prolonged aeration significantly reduced the net 100-year GWP of CH4 and N2O emissions by 6 %, but also decreased rice production when compared with normal aeration. With straw incorporation, the lowest GWP was found by early aeration, which reduced GWP by 7 and 20 % in 2007 and 2008, respectively. Estimation of net GWPs of CH4 and N2O emissions indicated that early midsea- son drainage with straw incorporation offered the potential to mitigate CH4 and N2O emissions from irrigated lowland rice paddies in China.

Differences in CH4 and N2O emissions between rice nurseries in Chinese major rice cropping areas

Studies on greenhouse gas (GHG) emissions from paddy field have primarily focused on the post-transplanting period, however, recent researches raise new concerns about GHGs emission from rice nursery. In this study, CH4 and N2O fluxes were determined from different nurseries under major rice cropping systems in China. The tested nurseries included flooded nursery (FN), moist nursery (MN) and dry nursery (DN). Methane emissions from FN were significantly higher than those from MN and DN under all the rice cropping systems. When comparing with FN, MN decreased total CH4 emissions by 74.2%, 72.1% and 49.6% under the rice–upland rotation cropping system (RUR), and the double rice cropping system for the early rice (EDR) and the late rice (LDR), respectively. DN decreased CH4 emissions by 99.2%, 92.0%, 99.0% and 78.6% compared to FN under the single rice cropping system (SR), RUR, EDR and LDR, respectively. When comparing with FN, MN and DN increased N2O emissions by 58.1–134.1% and 28.2–332.7%, respectively. Ultimately, compared with FN across the cropping systems, MN and DN decreased net global warming potentials (GWPs) of CH4 and N2O by 33–68% and 43–86%, respectively. The mitigating effect of MN and DN on total GWPs varied greatly across the systems, ranging from 30.8% in the LDR to 86.5% in the SR. Chinese actual emission from rice nurseries was reduced to 956.66 × 103 t CO2 eq from the theoretical estimate of 2242.59 × 103 t CO2 eq if under the flooded nursery scenario in 2012. Taking into account the large rice nursery area (2032.52 × 103 ha) in China, the results of this study clearly indicate the importance to estimate and mitigate GHGs emission from flooded rice nursery. Being effective to reduce GHG emissions and increase rice yield, dry nursery technique is a promising candidate for climate smart rice cropping.

Greenhouse Gas Emissions from Nontilled, Permanent Raised, and Conventionally Tilled Beds in the Central Highlands of Mexico

Organic matter content increases in soil with no-tilled permanent raised beds (PBs) compared with soil with conventionally tilled beds (CBs), and this might affect greenhouse gas (GHG) emissions. Greenhouse gas (CO2, N2O, and CH4) emissions were measured from PBs, from which crop residue was either removed or retained and from CBs where crop residue was retained. The CO2 emission was not affected by tillage, but CH4 and N2O emissions were lower in PBs when residue was retained than in CBs. Removing crop residue from PBs reduced CO2 emissions compared with when it was retained, but it had no effect on N2O and CH4 emissions. The global warming potential (GWP) of GHG emissions was higher in CBs (801 kg CO2/ha/year) than in PBs (517 kg CO2/ha/year) with crop-residue retention, but more C was sequestered in the 0–60 cm soil layer in PBs (83.4 Mg C/ha) than in CBs (79.2 Mg C/ha). Crop-residue removal in PBs had little effect on the GWP of GHG compared with PBs with crop residue retained, but less C was sequestered in the latter (63.1 Mg C/ha). Net GWP (considering soil C sequestration, GHG emissions, fuel used, glyphosate application, fertilizer and seed production) was lower in CBs with crop-residue retention (1062 kg CO2/ha/year) than in PBs with crop-residue removal (6,120 kg CO2/ha/year), but it was larger than in PBs with crop-residue retention (−681 kg CO2/ha/year). We found that reduced tillage when beds were made permanent and crop-residue retention greatly reduced net GWP compared with when beds were tilled and remade each year.We found that retention of crop residue in PBs increased the emission of CO2 compared with where it was removed, but tillage did not affect fluxes of CO2. Emission of CH4 and N2O was larger from CBs than from PBs, but crop-residue management in PBs had no significant effect on fluxes of CH4 and N2O. Concentrations of mineral N were larger in CBs than in PBs, whereas the removal of crop residue from PBs increased mineral N concentration. Soil temperature was higher in CBs than in PBs and in PBs with crop residue retained compared with where it was removed. Soil water was better preserved in PBs than in CBs and in PBs where residue was retained than where it was removed. The higher water content in the PB compared with the CB will favor plant growth during dry spells. However, retaining crop residues in PBs will require sufficient application of inorganic N, as mineral N in soil is lower in PBs than in CBs or PBs with crop residue removed. Limited N availability in PBs with crop residue retained might reduce yields as poor farmers in the central highlands of Mexico apply little or no N fertilizer. Reduced tillage on PBs and crop-residue retention strongly reduced the net GWP of the system compared with the case when beds were remade each year. PBs with residue retention reduced net GWP by 50% compared with CBs with residue retention, but the removal of residues from the PBs more than doubled it.

Direct emission of methane and nitrous oxide from switchgrass and corn stover: implications for large‐scale biomass storage

AbstractLittle is known about the contributions of biomass feedstock storage to the net greenhouse gas emissions from cellulosic biofuels. Direct emissions of methane and nitrous oxide during decomposition in storage may contribute substantially to the global warming potential of biofuels. In this study, laboratory‐scale bales of switchgrass and corn stover were stored under a range of moisture (13.0–32.9%) and temperature (5–35 °C) conditions and monitored for O2 consumption and CO2, CH4, and N2O production over 8 weeks. Gas concentrations and emissions rates were highly variable within and between experimental groups. Stover bales produced higher CO2 concentrations (P = 0.0002) and lower O2 (P < 0.0001) during storage than switchgrass bales. Methane concentrations (1.8–2100 ppm) were inversely correlated with bale moisture (P < 0.05), with emissions rates ranging from 4.4–914.9 μg kg−1 DM day−1. Nitrous oxide concentrations ranged from 0 to 31 ppm, and emissions from switchgrass bales inversely correlated with temperature and moisture (P < 0.0001). Net global warming potential from each treatment (0–2.4 gCO2e kg−1 DM) suggests that direct emission of methane and nitrous oxide from aerobically stored feedstocks have a small effect on net global warming potential of cellulosic biofuels.

Cattle feed or bioenergy? Consequential life cycle assessment of biogas feedstock options on dairy farms

AbstractOn‐farm anaerobic digestion (AD) of wastes and crops can potentially avoid greenhouse gas (GHG) emissions, but incurs extensive environmental effects via carbon and nitrogen cycles and substitution of multiple processes within and outside farm system boundaries. Farm models were combined with consequential life cycle assessment (CLCA) to assess plausible biogas and miscanthus heating pellet scenarios on dairy farms. On the large dairy farm, the introduction of slurry‐only AD led to reductions in global warming potential (GWP) and resource depletion burdens of 14% and 67%, respectively, but eutrophication and acidification burden increases of 9% and 10%, respectively, assuming open tank digestate storage. Marginal GWP burdens per Mg dry matter (DM) feedstock codigested with slurry ranged from –637 kg CO2e for food waste to +509 kg CO2e for maize. Codigestion of grass and maize led to increased imports of concentrate feed to the farm, negating the GWP benefits of grid electricity substitution. Attributing grass‐to‐arable land use change (LUC) to marginal wheat feed production led to net GWP burdens exceeding 900 kg CO2e Mg−1 maize DM codigested. Converting the medium‐sized dairy farm to a beef‐plus‐AD farm led to a minor reduction in GWP when grass‐to‐arable LUC was excluded, but a 38% GWP increase when such LUC was attributed to marginal maize and wheat feed required for intensive compensatory milk production. If marginal animal feed is derived from soybeans cultivated on recently converted cropland in South America, the net GWP burden increases to 4099 kg CO2e Mg−1 maize DM codigested – equivalent to 55 Mg CO2e yr−1 per hectare used for AD‐maize cultivation. We conclude that AD of slurry and food waste on dairy farms is an effective GHG mitigation option, but that the quantity of codigested crops should be strictly limited to avoid potentially large international carbon leakage via animal feed displacement.

From waste‐to‐worth: energy, emissions, and nutrient implications of manure processing pathways

AbstractFour manure processing pathways are evaluated to provide a system‐level understanding of their impacts on different sustainability indicators. In particular we look at how solid‐liquid separation (SLS), anaerobic digestion (AD), and AD + SLS affect depletion of fossil fuels (DFF), nutrient balances, global warming potential (GWP), and ammonia emissions when compared to the base‐case (BC) pathway of direct land application. Lifecycle sustainability assessment techniques are applied to develop inventory data and model a Wisconsin dairy farm. For the BC, net GWP is 101.2 kg CO2‐eq, DFFS is 106.1 MJ, ammonia emissions are 2.62 kg, and nitrogen availability is 2.45 kg per ton of excreted manure. Net GWP is reduced in all pathways compared to BC by 19% for SLS, 48% for AD, and 47% for AD + SLS. DFF is reduced by 43% for AD and 40% for AD + SLS, but increased by 13% for SLS. Ammonia emissions are increased in all pathways by 2% for SLS, 40% for AD, and 44% for AD + SLS. Nitrogen availability remains the same in SLS but decreases in AD and AD + SLS due to higher ammonia volatilization, which could be reduced by injecting manure. Ratios of fossil energy (FERAD) and energy return on investment (EROIAD) of 3.7 and 0.98–1.8 are determined for AD pathways, compared to FER of 0.29 and EROI of 0.27 for grid electricity. When allocating results to specific outputs, variability can be reduced by applying system subdivision and allocation. Sensitivity analyses highlight the importance of reducing emissions during manure storage and the influence of changes in fertilizer and sand bedding. © 2014 Society of Chemical Industry and John Wiley & Sons, Ltd

Net Global Warming Potential and Greenhouse Gas Intensity Influenced by Irrigation, Tillage, Crop Rotation, and Nitrogen Fertilization

Little information exists about how global warming potential (GWP) is affected by management practices in agroecosystems. We evaluated the effects of irrigation, tillage, crop rotation, and N fertilization on net GWP and greenhouse gas intensity (GHGI or GWP per unit crop yield) calculated by soil respiration (GWP and GHGI) and organic C (SOC) (GWP and GHGI) methods after accounting for CO emissions from all sources (irrigation, farm operations, N fertilization, and greenhouse gas [GHG] fluxes) and sinks (crop residue and SOC) in a Lihen sandy loam from 2008 to 2011 in western North Dakota. Treatments were two irrigation practices (irrigated vs. nonirrigated) and five cropping systems (conventional-till malt barley [ L.] with N fertilizer [CTBN], conventional-till malt barley with no N fertilizer [CTBO], no-till malt barley-pea [ L.] with N fertilizer [NTB-P], no-till malt barley with N fertilizer, and no-till malt barley with no N fertilizer [NTBO]). While CO equivalents were greater with irrigation, tillage, and N fertilization than without, NO and CH fluxes were 2 to 218 kg CO eq. ha greater in nonirrigated NTBN and irrigated CTBN than in other treatments. Previous year's crop residue and C sequestration rate were 202 to 9316 kg CO eq. ha greater in irrigated NTB-P than in other treatments. Compared with other treatments, GWP and GWP were 160 to 9052 kg CO eq. ha lower in irrigated and nonirrigated NTB-P. Similarly, GHGI and GHGI were lower in nonirrigated NTB-P than in other treatments. Regardless of irrigation practices, NTB-P may lower net GHG emissions more than other treatments in the northern Great Plains.

Environmental life cycle assessment of bio-fuel production via fast pyrolysis of corn stover and hydroprocessing

A comparative life cycle assessment (LCA) of biofuel production from corn stover fast pyrolysis and subsequent hydrotreating and hydrocracking was conducted based on the Greenhouse gases Regulated Emissions and Energy use in Transportation (GREET) model. Three different cases were investigated as a result of different hydrogen treatment in bio-oil upgrading process. Environmental impacts such as non-renewable energy demand and Global Warming Potential (GWP) were primarily evaluated for 1MJ biofuel produced. All the results showed obvious environmental benefits compared with conventional gasoline and diesel based on the year of 2010. The results of Case 1 in which hydrogen was provided by external natural gas reforming displayed a reduction of 67.5% in net non-renewable energy demand and a reduction of 69.1% in net GWP in contrast to conventional gasoline and diesel. Net non-renewable energy demand was lowered by 76.6% and net GWP was reduced by 73.0% in Case 2 where 35% of bio-oil aqueous phase was consumed by steam reforming for hydrogen production. Case 3 gained an obvious advantage on both environmental aspects over the others, in which surplus hydrogen from steam reforming of whole aqueous phase of bio-oil was obtained as a coproduct. The results displayed a significant net non-renewable energy reduction of 147.5% and a net GWP reduction of 119.4%. However, the outcomes indicated decreasing biofuel yields from Case 1 to Case 3 compared with increasing environmental benefits. The biofuel yields of three cases were 31.2%, 23.7% and 11.5% respectively in this work.

Effects of biochar and other amendments on the physical properties and greenhouse gas emissions of an artificially degraded soil

Short and long-term impacts of biochar on soil properties under field conditions are poorly understood. In addition, there is a lack of field reports of the impacts of biochar on soil physical properties, gaseous emissions and C stability, particularly in comparison with other amendments. Thus, three amendments – biochar produced from oak at 650°C, humic acid (HA) and water treatment residual – (WTR) were added to a scalped silty-loam soil @ 0.5% (w/w) in triplicated plots under soybean. Over the 4-month active growing season, all amendments significantly increased soil pH, but the effect of biochar was the greatest. Biochar significantly increased soil-C by 7%, increased sub-nanopore surface area by 15% and reduced soil bulk density by 13% compared to control. However, only WTR amendment significantly increased soil nanopore surface area by 23% relative to the control. While total cumulative CH4 and CO2 emissions were not significantly affected by any amendment, cumulative N2O emission was significantly decreased in the biochar-amended soil (by 92%) compared to control over the growing period. Considering both the total gas emissions and the C removed from the atmosphere as crop growth and C added to the soil, WTR and HA resulted in net soil C losses and biochar as a soil C gain. However, all amendments reduced the global warming potential (GWP) of the soil and biochar addition even produced a net negative GWP effect. The short observation period, low application rate and high intra-treatment variation resulted in fewer significant effects of the amendments on the physicochemical properties of the soils than one might expect indicating further possible experimentation altering these variables. However, there was clear evidence of amendment–soil interaction processes affecting both soil properties and gaseous emissions, particularly for biochar, that might lead to greater changes with additional field emplacement time.

To what extent can zero tillage lead to a reduction in greenhouse gas emissions from temperate soils?

Soil tillage practices have a profound influence on the physical properties of soil and the greenhouse gas (GHG) balance. However there have been very few integrated studies on the emission of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) and soil biophysical and chemical characteristics under different soil management systems. We recorded a significantly higher net global warming potential under conventional tillage systems (26–31% higher than zero tillage systems). Crucially the 3-D soil pore network, imaged using X-ray Computed Tomography, modified by tillage played a significant role in the flux of CO2 and CH4. In contrast, N2O flux was determined mainly by microbial biomass carbon and soil moisture content. Our work indicates that zero tillage could play a significant role in minimising emissions of GHGs from soils and contribute to efforts to mitigate against climate change.

Soil greenhouse gas fluxes from different tree species on Taihang Mountain, North China

Abstract. The objectives of this study were to investigate seasonal variation of greenhouse gas fluxes from soils on sites dominated by plantation (Robinia pseudoacacia, Punica granatum, and Ziziphus jujube) and natural regenerated forests (Vitex negundo var. heterophylla, Leptodermis oblonga, and Bothriochloa ischcemum), and to identify how tree species, litter exclusion, and soil properties (soil temperature, soil moisture, soil organic carbon, total N, soil bulk density, and soil pH) explained the temporal and spatial variation in soil greenhouse gas fluxes. Fluxes of greenhouse gases were measured using static chamber and gas chromatography techniques. Six static chambers were randomly installed in each tree species. Three chambers were randomly designated to measure the impacts of surface litter exclusion, and the remaining three were used as a control. Field measurements were conducted biweekly from May 2010 to April 2012. Soil CO2 emissions from all tree species were significantly affected by soil temperature, soil moisture, and their interaction. Driven by the seasonality of temperature and precipitation, soil CO2 emissions demonstrated a clear seasonal pattern, with fluxes significantly higher during the rainy season than during the dry season. Soil CH4 and N2O fluxes were not significantly correlated with soil temperature, soil moisture, or their interaction, and no significant seasonal differences were detected. Soil organic carbon and total N were significantly positively correlated with CO2 and N2O fluxes. Soil bulk density was significantly negatively correlated with CO2 and N2O fluxes. Soil pH was not correlated with CO2 and N2O emissions. Soil CH4 fluxes did not display pronounced dependency on soil organic carbon, total N, soil bulk density, and soil pH. Removal of surface litter significantly decreased in CO2 emissions and CH4 uptakes. Soils in six tree species acted as sinks for atmospheric CH4. With the exception of Ziziphus jujube, soils in all tree species acted as sinks for atmospheric N2O. Tree species had a significant effect on CO2 and N2O releases but not on CH4 uptake. The lower net global warming potential in natural regenerated vegetation suggested that natural regenerated vegetation were more desirable plant species in reducing global warming.