Annual CH4 Emissions Research Articles

Effect on greenhouse gas balance of converting rice paddies to vegetable production

Rice paddies are increasingly being converted to vegetable production due to economic benefits related, in part, to changes in demand during recent decades. Here, we implemented a parallel field experiment to simultaneously measure annual emissions of CH4 and N2O, and soil organic carbon (SOC) stock changes, in rice paddies (RP), rice paddy–converted conventional vegetable fields (CV), and rice paddy–converted greenhouse vegetable fields (GV). Changing from rice to vegetable production reduced CH4 emissions by nearly 100%, and also triggered substantial N2O emissions. Furthermore, annual N2O emissions from GV significantly exceeded those from CV due to lower soil pH and higher soil temperature. Marginal SOC losses occurred after one year of cultivation of RP, CV, and GV, contributing an important share (3.4%, 32.2%, and 10.3%, respectively) of the overall global warming potential (GWP) balance. The decline in CH4 emissions outweighed the increased N2O emissions and SOC losses in CV and GV, leading to a 13%–30% reduction in annual GWP as compared to RP. These results suggest that large-scale expansion of vegetable production at the expense of rice paddies is beneficial for mitigating climate change in terms of the overall GWP.

Land use legacies and nitrogen fertilization affect methane emissions in the early years of rice field development

Methane (CH4) emissions are critical to greenhouse gas (GHG) management in agriculture, especially in areas growing rice (Oryza sativa). However, studies on CH4 emissions and the nitrogen (N) fertilization effect in new rice fields in subtropical regions are still scarce. In this study, we designed a split-plot field experiment in Jiangxi Province, southern China, to examine whether land-use legacies and N fertilization would influence CH4 emissions. Using static chambers and gas chromatography, we measured CH4 fluxes in a newly developed rice paddy and a 10-year-old rice paddy. We also measured climatic factors and soil chemical and physical properties to match the flux measurements. The results showed that annual CH4 emissions in the new rice plots were significantly lower than in the old rice plots regardless of N fertilization. Annual CH4 emissions increased with the land-use years of rice paddies, following the order of 1 year < 2 years < 3 years < 10 years. N fertilization significantly decreased CH4 emissions by 36.9% in the first year after the new rice plots were developed, whereas it had no significant effects on CH4 emissions in the old rice plots or the new rice plots in the second and third years. The results suggest that land-use legacies have significant effects on CH4 emissions and may influence the N fertilization effect on CH4 emissions in rice fields in subtropical regions. The findings suggest that land-use legacies should be considered in managing and estimating GHG emissions in rice-growing regions.

Smallholder farms in eastern African tropical highlands have low soil greenhouse gas fluxes

Abstract. Few field studies examine greenhouse gas (GHG) emissions from African agricultural systems, resulting in high uncertainty for national inventories. This lack of data is particularly noticeable in smallholder farms in sub-Saharan Africa, where low inputs are often correlated with low yields, often resulting in food insecurity as well. We provide the most comprehensive study in Africa to date, examining annual soil CO2, CH4 and N2O emissions from 59 smallholder plots across different vegetation types, field types and land classes in western Kenya. The study area consists of a lowland area (approximately 1200 m a.s.l.) rising approximately 600 m to a highland plateau. Cumulative annual fluxes ranged from 2.8 to 15.0 Mg CO2-C ha−1, −6.0 to 2.4 kg CH4-C ha−1 and −0.1 to 1.8 kg N2O-N ha−1. Management intensity of the plots did not result in differences in annual GHG fluxes measured (P = 0.46, 0.14 and 0.67 for CO2, CH4 and N2O respectively). The similar emissions were likely related to low fertilizer input rates (≤ 20 kg N ha−1). Grazing plots had the highest CO2 fluxes (P = 0.005), treed plots (plantations) were a larger CH4 sink than grazing plots (P = 0.05), while soil N2O emissions were similar across vegetation types (P = 0.59). This study is likely representative for low fertilizer input, smallholder systems across sub-Saharan Africa, providing critical data for estimating regional or continental GHG inventories. Low crop yields, likely due to low fertilization inputs, resulted in high (up to 67 g N2O-N kg−1 aboveground N uptake) yield-scaled emissions. Improvement of crop production through better water and nutrient management might therefore be an important tool in increasing food security in the region while reducing the climate footprint per unit of food produced.

Field measurements and modeling to resolve m2 to km2 CH4 emissions for a complex urban source: An Indiana landfill study

Large spatial and temporal uncertainties for landfill CH4 emissions remain unresolved by short-term field campaigns and historic greenhouse gas (GHG) inventory models. Using four field methods (aircraft-based mass balance, tracer correlation, vertical radial plume mapping, static chambers) and a new field-validated process-based model (California Landfill Methane Inventory Model, CALMIM 5.4), we investigated the total CH4 emissions from a central Indiana landfill as well as the partitioned emissions inclusive of methanotrophic oxidation for the various cover soils at the site. We observed close agreement between whole site emissions derived from the tracer correlation (8 to 13 mol s–1) and the aircraft mass balance approaches (7 and 17 mol s–1) that were statistically indistinguishable from the modeling result (12 ± 2 mol s–1 inclusive of oxidation). Our model calculations indicated that approximately 90% of the annual average CH4 emissions (11 ± 1 mol s–1; 2200 ± 250 g m–2 d–1) derived from the small daily operational area. Characterized by a thin overnight soil cover directly overlying a thick sequence of older methanogenic waste without biogas recovery, this area constitutes only 2% of the 0.7 km2 total waste footprint area. Because this Indiana landfill is an upwind source for Indianapolis, USA, the resolution of m2 to km2 scale emissions at various temporal scales contributes to improved regional inventories relevant for addressing GHG mitigation strategies. Finally, our comparison of measured to reported CH4 emissions under the US EPA National GHG Reporting program suggests the need to revisit the current IPCC (2006) GHG inventory methodology based on CH4 generation modeling. The reasonable prediction of emissions at individual U.S. landfills requires incorporation of both cover-specific landfill climate modeling (e.g., soil temperature/moisture variability over a typical annual cycle driving CH4 transport and oxidation rates) as well as operational issues (e.g., cover thickness/properties, extent of biogas recovery).

Investigating seasonal methane emissions in Northern California using airborne measurements and inverse modeling

Seasonal methane (CH4) emissions in Northern California are evaluated during this study by using airborne measurement data and inverse model simulations. This research applies Alpha Jet Atmospheric eXperiment (AJAX) measurements obtained during January–February 2013, July 2014, and October–November 2014 over the San Francisco Bay Area (SFBA) and northern San Joaquin Valley (SJV) in order to constrain seasonal CH4 emissions in Northern California. The California Greenhouse Gas Emissions Measurement (CALGEM) a priori emission inventory was applied in conjunction with the Weather Research and Forecasting and Stochastic Time-Inverted Lagrangian Transport model and inverse modeling techniques to optimize CH4 emissions. Comparing model-predicted CH4 mixing ratios with airborne measurements, we find substantial underestimates suggesting that CH4 emissions were likely larger than the year 2008 a priori CALGEM emission inventory in Northern California. Using AJAX measurements to optimize a priori emissions resulted in CH4 flux estimates from the SFBA/SJV of 1.77 ± 0.41, 0.83 ± 0.31, and 1.06 ± 0.39 Tg yr−1 when using winter, summer, and fall flight data, respectively. Averaging seasonal a posteriori emission estimates (weighted by posterior uncertainties) results in SFBA/SJV annual CH4 emissions of 1.28 ± 0.38 Tg yr−1. A posteriori uncertainties are reduced more effectively in the SFBA/SJV region compared to state-wide values indicating that the airborne measurements are most sensitive to emissions in this region. A posteriori estimates during this study suggest that dairy livestock was the source with the largest increase relative to the a priori CALGEM emission inventory during all seasons.

Estimating methane emissions in California's urban and rural regions using multitower observations

AbstractWe present an analysis of methane (CH4) emissions using atmospheric observations from 13 sites in California during June 2013 to May 2014. A hierarchical Bayesian inversion method is used to estimate CH4 emissions for spatial regions (0.3° pixels for major regions) by comparing measured CH4 mixing ratios with transport model (Weather Research and Forecasting and Stochastic Time‐Inverted Lagrangian Transport) predictions based on seasonally varying California‐specific CH4 prior emission models. The transport model is assessed using a combination of meteorological and carbon monoxide (CO) measurements coupled with the gridded California Air Resources Board (CARB) CO emission inventory. The hierarchical Bayesian inversion suggests that state annual anthropogenic CH4 emissions are 2.42 ± 0.49 Tg CH4/yr (at 95% confidence), higher (1.2–1.8 times) than the current CARB inventory (1.64 Tg CH4/yr in 2013). It should be noted that undiagnosed sources of errors or uncaptured errors in the model‐measurement mismatch covariance may increase these uncertainty bounds beyond that indicated here. The CH4 emissions from the Central Valley and urban regions (San Francisco Bay and South Coast Air Basins) account for ~58% and 26% of the total posterior emissions, respectively. This study suggests that the livestock sector is likely the major contributor to the state total CH4 emissions, in agreement with CARB's inventory. Attribution to source sectors for subregions of California using additional trace gas species would further improve the quantification of California's CH4 emissions and mitigation efforts toward the California Global Warming Solutions Act of 2006 (Assembly Bill 32).

On the methane paradox: Transport from shallow water zones rather than in situ methanogenesis is the major source of CH4 in the open surface water of lakes

AbstractEstimates of global methane (CH4) emissions from lakes and the contributions of different pathways are currently under debate. In situ methanogenesis linked to algae growth was recently suggested to be the major source of CH4 fluxes from aquatic systems. However, based on our very large data set on CH4 distributions within lakes, we demonstrate here that methane‐enriched water from shallow water zones is the most likely source of the basin‐wide mean CH4 concentrations in the surface water of lakes. Consistently, the mean surface CH4 concentrations are significantly correlated with the ratio between the surface area of the shallow water zone and the entire lake, fA,s/t, but not with the total surface area. The categorization of CH4 fluxes according to fA,s/t may therefore improve global estimates of CH4 emissions from lakes. Furthermore, CH4 concentrations increase substantially with water temperature, indicating that seasonally resolved data are required to accurately estimate annual CH4 emissions.

Drainage, no-tillage and crop rotation decreases annual cumulative emissions of methane and nitrous oxide from a rice field in Southwest China

Permanently flooded rice fields, a special kind of all year-round flooded rice fields in China, where the crop system is summer rice (Oryza sativa ‘Q You 6′) with winter fallow, contribute to both CH4 and N2O emissions. To investigate their CH4 and N2O emissions over a whole year (November 2009 to October 2010) and responses to long-term tillage-cropping systems, four treatments after the conversion of such rice fields were examined: conventional tillage with a single summer rice and floodwater winter fallow (CTRF) or drained winter rapeseed (Brassica napus ‘W You 25′) (CTRR), no-tillage narrow- or wide-ridge with a rice and rapeseed rotation (NTNRR or NTWRR). Results showed that CTRF emitted the highest CH4 owing to permanently flooding water layer and higher soil organic carbon concentrations. Compared to CTRF, CH4 emissions under other three tillage-cropping systems were decreased not only in the winter season but also in the rice-growing season. In contrast, N2O emissions over a whole one-year rice-rapeseed rotation cycle were almost equivalent to each other under these four tillage-cropping systems. Also compared to CTRR, the two no-tillage-cropping systems tended to enhance CH4 while decrease N2O emissions, though with insignificant effects. The annual cumulative emissions of CH4 and N2O were highest under CTRF (1.07±0.20 kg CO2-eqha−1kg−1 yield) and significantly decreased under CTRR, NTNRR and NTWRR (0.59±0.10, 0.67±0.05 and 0.58±0.09 kg CO2-eqha−1kg−1 yield, respectively), indicating that the summer rice-winter rapeseed rotation system, irrespective of tillage management, rather than the summer rice-winter fallow system, had achieved the objective of higher yields with less greenhouse gas emissions. These results demonstrate that the no-tillage wide-ridge with a rice and rapeseed rotation (NTWRR) is the most efficient management in terms of decreasing CH4 and N2O emissions in Southwest China.

Drainage and tillage practices in the winter fallow season mitigate CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O emissions from a double-rice field in China

Abstract. Traditional land management (no tillage, no drainage, NTND) during the winter fallow season results in substantial CH4 and N2O emissions from double-rice fields in China. A field experiment was conducted to investigate the effects of drainage and tillage during the winter fallow season on CH4 and N2O emissions and to develop mitigation options. The experiment had four treatments: NTND, NTD (drainage but no tillage), TND (tillage but no drainage), and TD (both drainage and tillage). The study was conducted from 2010 to 2014 in a Chinese double-rice field. During winter, total precipitation and mean daily temperature significantly affected CH4 emission. Compared to NTND, drainage and tillage decreased annual CH4 emissions in early- and late-rice seasons by 54 and 33 kg CH4 ha−1 yr−1, respectively. Drainage and tillage increased N2O emissions in the winter fallow season but reduced it in early- and late-rice seasons, resulting in no annual change in N2O emission. Global warming potentials of CH4 and N2O emissions were decreased by 1.49 and 0.92 t CO2 eq. ha−1 yr−1, respectively, and were reduced more by combining drainage with tillage, providing a mitigation potential of 1.96 t CO2 eq. ha−1 yr−1. A low total C content and high C / N ratio in rice residues showed that tillage in the winter fallow season reduced CH4 and N2O emissions in both early- and late-rice seasons. Drainage and tillage significantly decreased the abundance of methanogens in paddy soil, and this may explain the decrease of CH4 emissions. Greenhouse gas intensity was significantly decreased by drainage and tillage separately, and the reduction was greater by combining drainage with tillage, resulting in a reduction of 0.17 t CO2 eq. t−1. The results indicate that drainage combined with tillage during the winter fallow season is an effective strategy for mitigating greenhouse gas releases from double-rice fields.

Importance of vegetation classes in modeling CH4 emissions from boreal and subarctic wetlands in Finland

Boreal/arctic wetlands are dominated by diverse plant species, which vary in their contribution to CH4 production, oxidation and transport processes. Earlier studies have often lumped the processes all together, which may induce large uncertainties into the results. We present a novel model, which includes three vegetation classes and can be used to simulate CH4 emissions from boreal and arctic treeless wetlands. The model is based on an earlier biogeophysical model, CH4MODwetland. We grouped the vegetation as graminoids, shrubs and Sphagnum and recalibrated the vegetation parameters according to their different CH4 production, oxidation and transport capacities. Then, we used eddy-covariance-based CH4 flux observations from a boreal (Siikaneva) and a subarctic fen (Lompolojänkkä) in Finland to validate the model. The results showed that the recalibrated model could generally simulate the seasonal patterns of the Finnish wetlands with different plant communities. The comparison between the simulated and measured daily CH4 fluxes resulted in a correlation coefficient (R2) of 0.82 with a slope of 1.0 and an intercept of −0.1mgm−2h−1 for the Siikaneva site (n=2249, p<0.001) and an R2 of 0.82 with a slope of 1.0 and an intercept of 0.0mgm−2h−1 for the Lompolojänkkä site (n=1826, p<0.001). Compared with the original model, the recalibrated model in this study significantly improved the model efficiency (EF), from −5.5 to 0.8 at the Siikaneva site and from −0.4 to 0.8 at the Lompolojänkkä site. The simulated annual CH4 emissions ranged from 7 to 24gm−2yr−1, which was consistent with the observations (7–22gm−2yr−1). However, there are some discrepancies between the simulated and observed daily CH4 fluxes for the Siikaneva site (RMSE=50.0%) and the Lompolojänkkä site (RMSE=47.9%). Model sensitivity analysis showed that increasing the proportion of the graminoids would significantly increase the CH4 emission levels. Our study demonstrated that the parameterization of the different vegetation processes was important in estimating long-term wetland CH4 emissions.

Estimates of Wildfire Emissions in Boreal Forests of China

Wildfire emissions in the boreal forests yield an important contribution to the chemical budget of the troposphere. To assess the contribution of wildfire to the emissions of atmospheric trace species in the Great Xing’an Mountains (GXM), which is also the most severe fire-prone boreal forest region in China, we estimated various wildfire activities by combining explicit spatio-temporal remote sensing data with fire-induced emission models. We observed 9998 fire scars with 46,096 km2 in the GXM between the years 1986 and 2010. The years 1987 and 2003 contributed 33.2% and 22.9%, respectively, in burned area during the 25 years. Fire activity is the strongest in May. Most large fires occurred in the north region of the GXM between 50° N and 54° N latitude due to much drier weather and higher fire danger in the northern region than in the southern region of the study domain. Evergreen and deciduous needleleaf forest and deciduous broadleaf forest are the main sources of emissions, accounting for 84%, 81%, 84%, 87%, 89%, 86%, 85% and 74% of the total annual CO2, CH4, CO, PM10, PM2.5, SO2, BC and NOx emissions, respectively. Wildfire emissions from shrub, grassland and cropland only account for a small fraction of the total emissions level (approximately 4%–11%). Comparisons of our results with other published estimates of wildfire emissions show reasonable agreement.

Thinning Intensity Affects Soil-Atmosphere Fluxes of Greenhouse Gases and Soil Nitrogen Mineralization in a Lowland Poplar Plantation

Thinning is one of the intensive forest management techniques commonly applied to increase the merchantable timber volume. However, how thinning affects soil–atmospheric fluxes of greenhouse gases (GHGs) is poorly understood. A field experiment with four treatments (CK: unthinned; MB: medium intensity thinning from below; HB: high intensity thinning from below; and HI: high intensity thinning by removing every alternative row of trees) was conducted to assess the impact of thinning regimes on soil–atmospheric fluxes of GHGs (CO2, CH4, and N2O) and soil nitrogen mineralization in a poplar plantation established on a lowland. Thinning significantly increased soil water content and water table in the high thinning treatments (HB and HI) and tended to increase soil temperature (p &lt; 0.10). The result of the one-year study showed that estimated annual emissions of CO2 and CH4 were higher in HB and HI than in other treatments, while the highest emission of N2O was in the CK. The thinning treatments increased the annual emission of CO2 by 23%–64% and that of CH4 by 190%–1200%, but decreased that of N2O by 41%–62%. Thinning increased annual N mineralization by 50.3% in HI and 30.1%in HB. Changes in soil temperature and water table drove CO2, CH4, and N2O emissions, while soil water content was the most important factor driving CH4 emission. We conclude that the moderate thinning (MB) regime is the best thinning option to minimize the impact on GHG emissions for lowland poplar plantations with similar conditions to those tested in this study.

Economic and environmental benefits of landfill gas utilisation in Oman.

Municipal solid waste disposed in landfill sites decomposes under anaerobic conditions and produces so-called landfill-gas, which contains 30%-40% of carbon dioxide (CO2) and 50%-60% of methane (CH4). Methane has the potential of causing global warming 25 times more than CO2 Therefore, migration of landfill-gas from landfills to the surrounding environment can potentially affect human life and environment. Thus, this research aims to determine municipal solid waste generation in Oman over the years 1971-2030, to quantify annual CH4 emissions inventory that resulted from this waste over the same period of time, and to determine the economic and environmental benefits of capturing the CH4 gas for energy production. It is found that cumulative municipal solid waste landfilled in Oman reaches 3089 Giga gram (Gg) in the year 2030, of which approximately 85 Gg of CH4 emissions are produced in the year 2030. The study also found that capturing CH4 emissions between the years 2016 and 2030 could attract revenues of up to US$333 million and US$291 million from the carbon reduction and electricity generation, simultaneously. It is concluded that CH4 emissions from solid waste in Oman increases enormously with time, and capture of this gas for energy production could provide a sustainable waste management solution in Oman.

Measured versus modeled methane emissions from separated liquid dairy manure show large model underestimates

Liquid manure in storage is a significant and poorly quantified source of methane (CH4). A micrometeorological technique was used to measure CH4 emissions from stored liquid manure on a dairy farm with approximately 146 milking cows and a screw-press solid-liquid separator. Higher than expected CH4 emissions were observed from the liquid manure fraction in the concrete storage tank likely due the high biodegradability of the liquid manure fraction when a long retention time coincided with high temperature. Almost all of the annual CH4 emissions occurred when both manure volume and temperature were high during the five-month period from July to November. In Year 1, 85% of annual emissions occurred from July to September, with 53% occurring in August. In Year 2, 93% of annual emissions occurred from July to November, with 30% occurring in October and November (combined). Removing manure in early fall reduced CH4 emissions substantially: we observed 21% lower annual emissions when manure was removed in early fall (33Mg CH4 in Year 1: September removal) compared to removal in late fall (42Mg CH4 in Year 2: end of November). Comparisons between measured and modeled CH4 emissions showed that both the IPCC methane conversion factor (0.17) for cool climates (10°C or less), and the USEPA model, underestimated annual emissions by up to 60%. We calculated CH4 conversion factors ranging from 0.58 to 0.89 using the measured VS loading rate both with and without VS in the sludge that remained after emptying. Our results suggest that farmers can substantially reduce CH4 emissions by removing manure in late summer and early fall.

Alternate wetting and drying in high yielding direct-seeded rice systems accomplishes multiple environmental and agronomic objectives

Rice (Oryza sativa L.) cultivation is critically important for global food security, yet it also represents a significant fraction of agricultural greenhouse gas (GHG) emissions and water resource use. Alternate wetting and drying (AWD) of rice fields has been shown to reduce both methane (CH4) emissions and water use, but its effect on grain yield is variable. In this three-year study we measured CH4 and nitrous oxide (N2O) emissions, rice grain total arsenic (As) concentrations, yield response to N rate, and grain yield from two AWD treatments (drill-seeded and water-seeded) and a conventionally managed water-seeded treatment (control). Grain yields (average=10Mgha−1) were similar or higher in the AWD treatments compared to the control and required similar or lower N rates to achieve these yields. Furthermore, AWD reduced growing season CH4 emissions by 60–87% while maintaining low annual N2O emissions (average=0.38kgN2O–Nha−1);N2O emissions accounted for <15% of the annual global warming potential (GWP) in all treatments. Fallow season emissions did not vary by treatment and accounted for 22–53% of annual CH4 emissions and approximately one third of annual GWP on average. The AWD treatments reduced annual GWP by 57–74% and growing season yield-scaled GWP by 59–88%. Milled grain total As, which averaged 0.114mgkg−1 in the control, was reduced by 59–65% in the AWD treatments. These results show that AWD has the potential to mitigate GHG emissions associated with rice cultivation and reduce rice grain total As concentrations without sacrificing grain yield or requiring higher N inputs; however future research needs to focus on adapting AWD to field scales if adoption of this technology is to be realized.

Methane emission from aquatic ecosystems of Mexico City

Mexico City is a large city, populated by 8.8 million inhabitants. This population density, combined with poor wastewater management, results in aquatic ecosystems receiving a large volume of wastewater which may promote methane (CH4) emission. We measured water quality and CH4 emission from 11 aquatic ecosystems in Mexico City during 1 year, including reservoirs, rivers, lakes, canals and chinampas (system of floating garden on shallow lakes). The total CH4 emission from aquatic ecosystems was estimated as 3679 Mg CH4 year−1, which represents 3.5 % of the annual CH4 emission of Mexico City. The main contributors are chinampas (33 %), followed by lakes (27 %), reservoirs (19 %), rivers (12 %) and canals (9 %). Water quality indicators were positively correlated with CH4 emission, therefore a decrease in untreated wastewater discharge may result in a significant reduction of the greenhouse gas footprint of Mexico City, after a transitional period during which the organic content of the sediment would be degraded.

Effects of slow/controlled release urea on annual CH4 and N2O emissions in paddy field.

Present study examined the influence of different types of slow/controlled release urea on rice yield and annual greenhouse gas emissions in a paddy field, and assessed the greenhouse gas intensity (GHGI, equivalent to global warming potential GWP/rice yield). The results indicated that the optimized fertilization (OPT) treatment recorded the similar yield with reduced nitrogen fertilizer (21.4%) supply compared with the farmers' fertilizer practice (FFP) treatment, and decreased the annual emissions of CH4 (12.6%) and N2O (12.5%) during the rice season, and N2O emission (33.3%) during the fallow period. Application of controlled release urea (CRU) reduced CH4 emission by 28.9% during the rice-growing season with respect to OPT treatment, and showed negligible CH4 emission during the fallow season. However, nitrification inhibitor (DMPP) treatment was found to reduce the CH4 emissions by 41.6% and 76.9%, and N2O emissions by 85.7% and 6.5%, during the rice growing season and fallow season, respectively, compared with OPT treatment. In the fallow season, the N2O emissions accounted for 76.8%-94.9% of annual N2O emissions, which was clearly a key point for evaluation of greenhouse gas emissions in paddy. The average values of GHGI in OPT, CRU and DMPP treatments were 0.50, 0.41 and 0.33 kg·kg-1, respectively. Considering the benefits of higher rice yield and lower annual greenhouse gas emissions, combined application of urea and nitrification inhibitor could be the best combination in paddy fields.

Greenhouse gas emission factors associated with rewetting of organic soils

Drained organic soils are a significant source of greenhouse gas (GHG) emissions to the atmosphere. Rewetting these soils may reduce GHG emissions and could also create suitable conditions for return of the carbon (C) sink function characteristic of undrained organic soils. In this article we expand on the work relating to rewetted organic soils that was carried out for the 2014 Intergovernmental Panel on Climate Change (IPCC) Wetlands Supplement. We describe the methods and scientific approach used to derive the Tier 1 emission factors (the rate of emission per unit of activity) for the full suite of GHG and waterborne C fluxes associated with rewetting of organic soils. We recorded a total of 352 GHG and waterborne annual flux data points from an extensive literature search and these were disaggregated by flux type (i.e. CO2, CH4, N2O and DOC), climate zone and nutrient status. Our results showed fundamental differences between the GHG dynamics of drained and rewetted organic soils and, based on the 100 year global warming potential of each gas, indicated that rewetting of drained organic soils leads to: net annual removals of CO2 in the majority of organic soil classes; an increase in annual CH4 emissions; a decrease in N2O and DOC losses; and a lowering of net GHG emissions. Data published since the Wetlands Supplement (n = 58) generally support our derivations. Significant data gaps exist, particularly with regard to tropical organic soils, DOC and N2O. We propose that the uncertainty associated with our derivations could be significantly reduced by the development of country specific emission factors that could in turn be disaggregated by factors such as vegetation composition, water table level, time since rewetting and previous land use history.

Large contribution of boreal upland forest soils to a catchment‐scale CH4 balance in a wet year

AbstractUpland forest soils affect the atmospheric methane (CH4) balance, not only through the soil sink but also due to episodic high emissions in wet conditions. We measured methane fluxes and found that during a wet fall the forest soil turned from a CH4 sink into a large source for several months, while the CH4 emissions from a nearby wetland did not increase. When upscaled to the whole catchment area the contribution of forests amounted to 60% of the annual CH4 emission from the wetlands, while in a normal year the forest soil consumes 10% of the wetland emission. The period of high upland soil emission was also captured by the nearby atmospheric concentration measurement station. Since the land cover within the catchment is representative of larger regions, our findings imply that upland forests in the boreal zone constitute an important part in the global CH4 cycle not previously accounted for.

Effects of different straw returning modes on greenhouse gas emissions and crop yields in a rice–wheat rotation system

Significant efforts have been made to assess the common straw returning modes on crop yields and greenhouse gas (GHG) emissions. However, the effects of a novel straw returning mode, namely ditch-buried straw returning on GHG emissions are still unknown. We conducted a 2-year field experiment, including four wheat straw returning modes (no straw returning (CK), wheat straw returning with rotary tillage (WR), wheat straw returning with plowing (WP), and ditch-buried wheat straw returning (WD)) to evaluate crop yields and GHG emissions under rice–wheat rotation system. Methane (CH4) and nitrous oxide (N2O) fluxes were measured using the static chamber method from the 2013 rice season to 2015 wheat season. The results indicated that wheat straw returning treatments before rice transplantation significantly increased seasonal CH4 emissions during both the rice seasons and wheat seasons, compared to CK. Annual CH4 emission was lower under WD than that under WR and WP. Straw returning significantly increased N2O emission during the first rice season and second wheat season, compared to CK. Annual N2O emission under WD was significantly lower than that under WP, but significantly higher than that under WR and CK. Straw returning increased both rice and wheat yields compared with CK, and WD had significantly higher annual grain yields of both rice and wheat, with an increase of 7.1%. Across the two rotation cycles, annual yield-scaled GWP of CH4 and N2O emissions under WD was 10.8% lower than that of WR. These results indicated that compared with straw returning via rotary tillage or plowing, ditch-buried wheat straw in rice seasons may reduce GHG emissions while sustaining or even increasing crop yields in the rice–wheat rotation system.