Sinks Of Greenhouse Gases Research Articles

The impact of water management practices on subtropical pasture methane emissions and ecosystem service payments.

Pastures are an extensive land cover type; however, patterns in pasture greenhouse gas (GHG) exchange vary widely depending on climate and land management. Understanding this variation is important, as pastures may be a net GHG source or sink depending on these factors. We quantified carbon dioxide (CO2 ) and methane (CH4 ) fluxes from subtropical pastures in south Florida for three wet-dry seasonal cycles using eddy covariance, and estimated two annual budgets of CO2 , CH4 , and GHG equivalent emissions. We also estimated the impact of water retention practices on pasture GHG emissions and assessed the impact of these emissions on stakeholder payments for water retention services in a carbon market framework. The pastures were net CO2 sinks sequestering up to 163±54gCO2 -C·m-2 ·yr-1 (mean ± 95% CI), but were also strong CH4 sources emitting up to 23.5±2.1gCH4 -C·m-2 ·yr-1 . Accounting for the increased global warming potential of CH4 , the pastures were strong net GHG sources emitting up to 584±78g CO2 -C eq.·m-2 ·yr-1 , and all CO2 uptake was offset by wet season CH4 emissions from the flooded landscape. Our analysis suggests that CH4 emissions due to increased flooding from water management practices is a small component of the pasture GHG budget, and water retention likely contributes 2-11% of net pasture GHG emissions. These emissions could reduce water retention payments by up to ~12% if stakeholders were required to pay for current GHG emissions in a carbon market. It would require at least 93.7kg CH4 -C emissions per acre-foot water storage (1 acre-foot = 1233.48 m3 ) for carbon market costs to exceed water retention payments, and this scenario is highly unlikely as we estimate current practices are responsible for 11.3±7.2kg CH4 -C emissions per acre-foot of water storage. Our results demonstrate that water retention practices aimed at reducing nutrient loading to the Everglades are likely only responsible for a minor increase in pasture GHG emissions and would have a small economic consequence in a carbon market.

Demonstration of spatial greenhouse gas mapping using laser absorption spectrometers on local scales

A system for measuring the two-dimensional (2-D) spatial distribution of atmospheric CO2 over complex industrial sites and urban areas on the order of 1 to 30 km2 every few minutes with a spatial resolution as high as tens of meters has been developed and demonstrated over the past 3 years. The greenhouse gas (GHG) laser imaging tomography experiment (GreenLITE™) provides improved measurement capabilities for applications ranging from automated 24/7 monitoring of ground carbon storage/sequestration (GCS) sites to long-duration real-time analyses of GHG sources and sinks in urban environments. GreenLITE combines a set of sensors based on an intensity modulated continuous wave approach with 2-D sparse tomographic reconstruction mechanisms to compute a 2-D map of CO2 concentrations over the area of interest. GreenLITE systems have recently been deployed at a number of test facilities, including a 4000-h demonstration at a GCS site in Illinois and an urban deployment in Paris, France, from November 2015 to the present. This paper describes the GreenLITE concept and the associated measurement capabilities and provides proof of concept results and analyses of observations from both short-term tests as well as longer-term industrial and urban deployments.

Forestry Sector in India is Net Source of Green House Gases (GHGS)

More than 275 million people are deriving their sustenance and livelihood needs from forests and causing forest degradation due to unsustainable harvest of forest produce. India is committed to achieve sustainable development of forests. The inadequate finance, capacity and research are key issues for sustainable development of forests in India. The consumption of wood and wood-based products has been considered for the estimation of emissions from forestry sector. The carbon sequestration has been estimated on the basis of productivity of forests and tree outside forests. The forestry sector in India will be a net source of Green House Gases (GHGs) till 2051 if policies and programs of forestry sector are implemented at the current pace and commitment. The forestry sector will also be net source of GHG till 2051 if the policies and programs suggested above are implemented moderately but the intensity of emissions will be low. The forestry sector may become net sink of GHG’s only if policies and programs are implemented aggressively with full commitment. The aggressive implementation of policies and programs will not only improve the quality of forests but also improve the quality of the life of the forest dependent communities by providing them a sustained livelihood which in turn benefit environment. The political commitment is very low for the forestry sector in India. If this continue in future, it would not be possible to achieve the scenario based on aggressive scenario. The Intended Nationally Determined Contributions (INDCs) for forestry sector announced by the Government of India on 2nd October, 2015 are extremely difficult to achieve.

Extensive grazing in contrast to mowing is climate-friendly based on the farm-scale greenhouse gas balance

Livestock is both threatened by and contributing to climate change. The contribution of livestock to climate change and greenhouse gas (GHG) emission greatly vary under different management regimes. A number of mitigation options comprise livestock management, although there are a lot of uncertainties as to which management regime to use for a given pedoclimatic and farming system. Therefore, we 1) tested if an extensive cattle livestock farm is a net sink or a net source for GHG (carbon–dioxide, CO2; methane, CH4; nitrous oxide N2O) in Central–Eastern Europe, 2) compared the annual GHG balances between the grazed and mowed treatments of the farm 3) and investigated the role of climate variability in shaping these balances. Net ecosystem exchange of CO2 (NEE) was measured with eddy covariance technique in both the grazed and mowed treatments. Estimations of lateral C fluxes were based on management data. Other GHG fluxes (CH4, N2O) were determined by chamber gas flux measurements technique (in case of soil) and IPCC guidelines (in case of manure decomposition and animal fermentation). Net greenhouse gas balance (NGHG) for the grazed treatment was 228±283g CO2 equivalent m−2year−1 (net sink) and −475±144g CO2 equiv. m−2year−1 (net source) for the mowed treatment. Net source activity at the mowed treatment was due to its higher herbage use intensity compared to the grazed treatment. At the farm scale the system was estimated to be a net sink for NGHG in a year with wet (135g CO2 equiv. m−2year−1), while a net source in years with dry soil moisture conditions (−267±214g CO2 equiv. m−2year−1). We conclude that under a temperate continental climate extended extensive grazing could serve as a potential mitigation of GHG in contrast to mowing. Our study highlights the fact that livestock farming could create a net sink for GHG under proper management regimes.

Assessment of carbon neutrality and sustainability in educational campuses (CaNSEC): A general framework

This study presents the cross-institutional Carbon Neutrality and Sustainability in Educational Campuses (CaNSEC) framework. This framework estimates carbon footprints along with overall sustainability of the institution. The carbon footprint analysis comprises of five indicators based on the sources and sinks (offsets) of greenhouse gas (GHG) emissions. Overall sustainability assessment comprises of four components – environment, society, economics and academics. Twenty four indicators have been identified and categorized under each of these four components based on experts’ opinion and literature review. The framework has been developed by involving experts from India and abroad using questionnaire based survey. Around 60 complete responses (with ∼12% response rate) were received which include ∼65% foreign and ∼35% Indian experts working in the area of sustainability, sustainable development in higher education and sustainable and green campuses. Each indicator has been weighted by experts, and subsequently verified through two brainstorming workshops with experts working in different areas of sustainable development. The study considered two types of scoring methods for indicators; level based scoring and continuous scoring. The CaNSEC framework will enable educational institutions to adopt a qualitatively and quantitatively measured and monitored framework that can effectively assess the state of sustainability on their respective campuses. The framework has been applied for the case study of TERI University. The per capita net CO2e emissions at the campus was found to be ∼0.72 tonnes per annum, which is relatively lower compared to other campuses globally. TERI University has scored ∼7800 out of 10000 points on campus sustainability score. The proposed framework also aims to create a platform for comparing educational institutions based on the performances of their initiatives and to encourage mutual learning through these comparisons.

The research of δ13CO2 by use of wavelet de-noising at 2.008 μm based on tunable diode laser absorption spectroscopy

Development of optical isotope techniques has provided scientists with a set of powerful tools for investigating the sources and sink of atmospheric CO2. Here we describe a continuous, high precision, compact and portable carbon dioxide isotope ratio laser multi-pass cell spectrometer with a tunable distribute feedback laser at 2.008 μm based on tunable diode laser absorption spectroscopy and, the spectrometer has good temperature and pressure stability. In order to deduce the noise, drift effect and background changes associated with low level signals, a superior signal processing technique of wavelet denoising, which possesses multi-level analytical resolutions both in time and frequency-domains, is introduced. After evaluating the method, evaluation ability and applicabilities of several common wavelet functions are analyzed and tested, the wavelet function of Haar is selected as an optimal wavelet basis function. Based on the analysis of the optimal decomposition level of Haa wavelet function, the VISU function is selected as an optimal wavelet threshold function. The denoising effect and measurement precision are evaluated by use of the VISU threshold function in the measurement process of carbon dioxide stable isotope ratio. The measurement results of carbon dioxide stable isotope ratio before and after suppressing the noises are compared in the same experiment conditions and, the inconsistent reasons of the measured results are theoretically analyzed. This technique allows the measurement of the δ-value for carbon dioxide isotopic ratios with a precision of -12.5‰ and and the measuremnt results show that the wavelet denoising measuring results have higher measurement accuracy, and the measurement precise of carbon dioxide isotope ratio is 7.3 times the original measurement results. The application of the wavelet denoising to the carbon dioxide isotope ratio measurement for the first time proves that the capability of the new near-infrared direct absorption technique to measure isotope ratio can permit high-frequency, near-continuous isotope measurement and obtain the high precision and accurate real-time stable isotope data directly in the field. This technique provides an important tool for studying the resource and sink of green house gases in the future.

CSIRO Non-carbon Dioxide Greenhouse Gas Research. Part 1: 1975–90

There are a number atmospheric gases, in addition to carbon dioxide (CO2), that affect the absorption and emission of infrared radiation throughout the atmosphere, the so-called ‘non-CO2 greenhouse gases', and they have a significant impact on climate. In addition, some of these non-CO2 greenhouse gases contain chlorine and/or bromine, and contribute to halogen-catalysed stratospheric ozone depletion. In the mid 1970s, CSIRO at Aspendale became the first southern hemisphere laboratory to initiate research into the atmospheric abundance, trends, sources and sinks of non-CO2 greenhouse gases, and today (2017) is currently observing and modelling the past and present biogeochemical cycling of over eighty of these species, arguably the most comprehensive program of its type globally. The resultant CSIRO data are used to derive global and regional emissions of non-CO2 greenhouse gases and their impact on climate and stratospheric ozone via resultant changes to the planetary radiative budget and the abundance of ‘equivalent chlorine' (weighted sum of chlorine and bromine) in the stratosphere. These data and their impacts are reported nationally to relevant Commonwealth and State Departments—environment, energy, industry, agriculture—and to relevant Australian industries—refrigeration, air-conditioning, aluminium production. They are reported internationally to United Nations agencies responsible for implementing the Vienna Convention for the Protection of the Ozone Layer (1985) and the Framework Convention on Climate Change (1992), who periodically assess the science of climate change and ozone depletion. As the world strives to reduce its greenhouse gas emissions through national, policy-driven, initiatives framed to meet agreed obligations under these international agreements, atmospheric measurement programs, such as those operated by the CSIRO and the Bureau of Meteorology in Australia, are critical in independently verifying the success or otherwise of such endeavours. This paper describes the initial fifteen years (1975–90) of activities in CSIRO that set up the framework for the current, globally significant, CSIRO non-CO2 greenhouse gas research program.

A Life-Cycle Assessment of Forest Resources of the Pacific Northwest, USA*

Abstract Life-cycle inventory (LCI) and life-cycle assessment (LCA) were used to provide quantitative assessments of the environmental impacts of forest management activities that are required to produce feedstock for wood products such as lumber, engineered panels, and pulp. Primary and secondary data were gathered for the Pacific Northwest Douglas-fir region of the United States to produce an attributional LCA that includes planting, growing, and harvesting trees that are destined for use in wood manufacturing. Using the Tool for the Reduction and Assessment of Chemical and other environmental Impacts (TRACI) method, under average management conditions, forest operations can expect to generate from 10 to 18 kg CO2 equivalent (CO2 eq) per cubic meter (m3) of logs ready to leave the landing for the manufacturing facility, depending on the amount of forest residues that are piled and burned. This same cubic meter of log plus bark will have sequestered 960 kg CO2 eq during its growth cycle, for a net greenhouse gas sink of 942 to 950 kg CO2 eq per m3. Forest management impacts are from 1 to 13 percent of the total impacts from the cradle to gate for global warming potential and the potential to increase smog, eutrophication, and acidification. Upstream impacts associated with the production of herbicides are reflected in the ozone potential impact category. These LCA results can be used as upstream processes for wood manufacturers interested in developing Environmental Product Declarations for products that use these resources as inputs.

Effects of urea-fertilization rates on the environmental performance of giant reed lignocellulosic feedstock produced for biorefinery purpose

A life cycle assessment (LCA) was carried out to investigate the environmental constrains of Arundo donax L. (common name giant reed) feedstock cultivation, for biorefinery purpose, treated with different amount of nitrogen fertilizations: 50 and 100 kg ha−1 of N as urea (low input- LI and high input- HI, respectively). The analysis was addressed for a cultivation of giant reed (GR) under Mediterranean conditions, considering primary data for the first eight years of cultivation and further modeled for the whole life cycle (15 years). The goals were, to ascertain and compare the environmental profile as well as to identify the hotspots for both investigated N- management techniques.Results highlighted a potential higher environmental performance for the LI giant reed cultivation, due to the reduced up-stream and direct field emissions linked to urea application. LI impacts resulted to be 24% lower than HI impacts, as averaged values for all impact categories. Crop yield represented a driving parameter and the lower biomass productivity under N low input management partially counteracted the entailed benefits of the lower fertilization rates. Through the inclusion of soil carbon storage, after seven years of harvesting, the whole crop phase revealed a net greenhouse gases sink and also differences between LI (−0.21 kg CO2 eq) and HI (−0.09 kg CO2 eq) appeared amplified. This was due to the enhanced C accumulation rate in LI trials (about 7.7 ton C ha−1 at the end of the seventh harvest year of GR cultivation), likely promoted by reduced soil organic matter mineralization at lower nitrogen availability. Nevertheless, in order to provide a full and comprehensive picture of the environmental performance of GR feedstock under different N-fertilization, further studies are needed encompassing the whole production chain: from the cultivation, through selected efficient conversion processes towards the co-production of added value products in the framework of the BioPoliS project.

Warming of subarctic tundra increases emissions of all three important greenhouse gases - carbon dioxide, methane, and nitrous oxide.

Rapidly rising temperatures in the Arctic might cause a greater release of greenhouse gases (GHGs) to the atmosphere. To study the effect of warming on GHG dynamics, we deployed open-top chambers in a subarctic tundra site in Northeast European Russia. We determined carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2 O) fluxes as well as the concentration of those gases, inorganic nitrogen (N) and dissolved organic carbon (DOC) along the soil profile. Studied tundra surfaces ranged from mineral to organic soils and from vegetated to unvegetated areas. As a result of air warming, the seasonal GHG budget of the vegetated tundra surfaces shifted from a GHG sink of -300 to-198g CO2 -eq m-2 to a source of 105 to 144g CO2 -eq m-2 . At bare peat surfaces, we observed increased release of all three GHGs. While the positive warming response was dominated by CO2 , we provide here the first insitu evidence of increasing N2 O emissions from tundra soils with warming. Warming promoted N2 O release not only from bare peat, previously identified as a strong N2 O source, but also from the abundant, vegetated peat surfaces that do not emit N2 O under present climate. At these surfaces, elevated temperatures had an adverse effect on plant growth, resulting in lower plant N uptake and, consequently, better N availability for soil microbes. Although the warming was limited to the soil surface and did not alter thaw depth, it increased concentrations of DOC, CO2, and CH4 in the soil down to the permafrost table. This can be attributed to downward DOC leaching, fueling microbial activity at depth. Taken together, our results emphasize the tight linkages between plant and soil processes, and different soil layers, which need to be taken into account when predicting the climate change feedback of the Arctic.

Energy use and greenhouse gas emissions from turf management of two Swedish golf courses

Turf management on golf courses entails frequent maintenance activities, such as mowing, irrigation and fertilisation, and relies on purchased inputs for optimal performance and aesthetic quality. Using life cycle assessment (LCA) methodology, this study evaluated energy use and greenhouse gas (GHG) emissions from management of two Swedish golf courses, divided into green, tee, fairway and rough, and identified options for improved management. Energy use and GHG emissions per unit area were highest for greens, followed by tees, fairways and roughs. However, when considering the entire golf course, both energy use and GHG emissions were mainly related to fairway and rough maintenance due to their larger area. Emissions of GHG for the two golf courses were 1.0 and 1.6Mg CO2e ha−1year−1 as an area-weighted average, while the energy use was 14 and 19GJha−1year−1. Mowing was the most energy-consuming activity, contributing 21 and 27% of the primary energy use for the two golf courses. In addition, irrigation and manufacturing of mineral fertiliser and machinery resulted in considerable energy use. Mowing and emissions associated with fertilisation (manufacturing of N fertiliser and soil emissions of N2O occurring after application) contributed most to GHG emissions. Including the estimated mean annual soil C sequestration rate for fairway and rough in the assessment considerably reduced the carbon footprint for fairway and turned the rough into a sink for GHG. Emissions of N2O from decomposition of grass clippings may be a potential hotspot for GHG emissions, but the high spatial and temporal variability of values reported in the literature makes it difficult to estimate these emissions for specific management regimes. Lowering the application rate of N mineral fertiliser, particularly on fairways, should be a high priority for golf courses trying to reduce their carbon footprint. However, measures must be adapted to the prevailing conditions at the specific golf course and the requirements set by golfers.

The role of <i>Phragmites</i> in the CH<sub>4</sub> and CO<sub>2</sub> fluxes in a minerotrophic peatland in southwest Germany

Abstract. Peatlands are interesting as a carbon storage option, but are also natural emitters of the greenhouse gas methane (CH4). Phragmites peatlands are particularly interesting due to the global abundance of this wetland plant (Phragmites australis) and the highly efficient internal gas transport mechanism, which is called humidity-induced convection (HIC). The research aims were to (1) clarify how this plant-mediated gas transport influences the CH4 fluxes, (2) which other environmental variables influence the CO2 and CH4 fluxes, and (3) whether Phragmites peatlands are a net source or sink of greenhouse gases. CO2 and CH4 fluxes were measured with the eddy covariance technique within a Phragmites-dominated fen in southwest Germany. One year of flux data (March 2013–February 2014) shows very clear diurnal and seasonal patterns for both CO2 and CH4. The diurnal pattern of CH4 fluxes was only visible when living, green reed was present. In August the diurnal cycle of CH4 was the most distinct, with 11 times higher midday fluxes (15.7 mg CH4 m−2 h−1) than night fluxes (1.41 mg CH4 m−2 h−1). This diurnal cycle has the highest correlation with global radiation, which suggests a high influence of the plants on the CH4 flux. But if the cause were the HIC, it would be expected that relative humidity would correlate stronger with CH4 flux. Therefore, we conclude that in addition to HIC, at least one additional mechanism must be involved in the creation of the convective flow within the Phragmites plants. Overall, the fen was a sink for carbon and greenhouse gases in the measured year, with a total carbon uptake of 221 g C m−2 yr−1 (26 % of the total assimilated carbon). The net uptake of greenhouse gases was 52 g CO2 eq. m−2 yr−1, which is obtained from an uptake of CO2 of 894 g CO2 eq. m−2 yr−1 and a release of CH4 of 842 g CO2 eq. m−2 yr−1.

Plant manipulations and diel cycle measurements test drivers of carbon dioxide and methane fluxes in a Phragmites australis-invaded coastal marsh

Invasion of coastal marshes by Phragmites australis may alter carbon cycling, including fluxes of the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4). Understanding patterns and drivers of these GHG fluxes in P. australis-invaded coastal marshes is critical to predicting how this widespread biological invasion may impact carbon (C) sequestration in coastal marshes. The objectives of this study were (1) to test effects of P. australis aboveground vegetation removal on GHG fluxes over short timescales (up to 4 months) and (2) to contrast diel patterns of GHG fluxes in P. australis-vegetated and cleared plots. First, effects of mechanical aboveground P. australis biomass removal on GHG fluxes and soil variables were tested over a series of short-term durations (frommin to months). Next, on 3 dates, GHG fluxes were measured every 3h over complete diel cycles. Net daytime CO2 uptake (−60 to −100mmolm−2s−1) was observed where P. australis was left intact. All durations of vegetation removal produced similar CO2 emissions to those measured from intact P. australis plots during evening hours. CH4 fluxes did not differ where P. australis was removed or left intact. Greater daytime CH4 emissions (75–100μmolm−2h−1) were found than at night (20–40μmolm−2h−1) from both cleared and vegetated plots. Results of this study suggest that CO2 fluxes in this system vary primarily due to substantial photosynthetic uptake by P. australis, and that CH4 emissions are likely driven by abiotic factors, such as temperature, that vary on diel cycles. Calculation of net GHG fluxes in this P. australis-invaded coastal marsh indicates that it is a GHG sink during the growing season.

Modeling approaches for agricultural N2O fluxes from large scale areas: A case for sugarcane crops in the state of São Paulo - Brazil

Anthropogenic greenhouse gas (GHG) emissions and climate change are issues that currently receive a great deal of attention, both from society and the scientific community. Combustion of fossil fuels is considered the main reason for the increasing concentrations of GHG in the atmosphere and therefore, the main cause of climate change. In such context, biofuels are usually presented as an energy source that considerably reduces GHG emissions when compared to the use of fossil fuels. However, some issues of biofuel production such as the emissions associated with both change in land use and the use of nitrogen fertilizers are still under scrutiny. The focus of this study is related to N2O soil emissions associated with sugarcane ethanol production in Brazil. Signatory countries of the United Nation's Framework Convention on Climate Change (UNFCCC) are required to prepare and periodically update an inventory of GHG emissions and sinks. The International Panel of Climate Change (IPCC), through the Agriculture, Forest and Other Land Use (AFOLU), establishes guidelines to calculate N2O soil emissions on three different levels, or tiers. The simplest Tier 1 uses few equations and default emissions factors. The most elaborated, Tier 3, uses computation modeling, field measurements and Geographic Information Systems (GIS). This study proposes using the limited data available, procedures for Tier 3 estimates of N2O soil emissions in sugarcane crops in the state of São Paulo - Brazil. It also compares N2O sugarcane emissions calculated by Tier 1 and Tier 3 methods. Results show that in most cases the difference in results from Tier 1 and Tier 3 methods are beyond the error margin of the Brazilian Inventory.

Responses of greenhouse gas fluxes to climate extremes in a semiarid grassland

Climate extremes are expected to increase in frequency and intensity as a consequence of anthropogenic climate change attributed to the rise of atmospheric concentrations of greenhouse gases (GHGs). However, studies on the impacts of climate extremes on terrestrial ecosystems are limited. Here, we experimentally imposed extreme drought and a heat wave (∼60-year recurrence) to investigate their effects on GHGs fluxes of a semiarid grassland in China. We estimated a 16% and 38% percent reduction in net ecosystem CO2 uptake caused by the heat wave and drought respectively, but via different mechanisms. Drought reduced gross ecosystem productively (GEP) and to a lower extent ecosystem respiration (ER). By contrast, the simulated heat wave suppressed only GEP while ER remained stable. The climate extremes also created a legacy effect on GEP and NEE lasting until the end of the growing season, whereas ER recovered immediately. Although CH4 and N2O fluxes were unaffected by the heat wave, drought promoted CH4 uptake and suppressed N2O emission during the treatment period. The effect of drought on GHGs fluxes generally overwhelmed that of the heat wave treatment, and there were no interactive effects of these two types of climate extremes. Our results showed that responses of ecosystem GHGs exchange to climate extremes are strongly regulated by soil moisture status. In conclusion, future amplification of climate extremes could decrease the sink for GHGs, especially CO2, in this semiarid grasslands.

Need for Phytolith-Occluded Carbon Research in India

Mitigation of climate change is a human intervention aimed at reducing the sources or enhancing the sinks of greenhouse gases. Developing technologies to reduce the rate of increase in the atmospheric concentration of carbon dioxide (CO 2 ) from annual emissions of 11 Pg C yr -1 from energy, process industry, land-use conversion and soil cultivation is an important issue of the 21st century.

Component greenhouse gas fluxes and radiative balance from two deltaic marshes in Louisiana: Pairing chamber techniques and eddy covariance

AbstractCoastal marshes take up atmospheric CO2 while emitting CO2, CH4, and N2O. This ability to sequester carbon (C) is much greater for wetlands on a per area basis than from most ecosystems, facilitating scientific, political, and economic interest in their value as greenhouse gas sinks. However, the greenhouse gas balance of Gulf of Mexico wetlands is particularly understudied. We describe the net ecosystem exchange (NEEc) of CO2 and CH4 using eddy covariance (EC) in comparison with fluxes of CO2, CH4, and N2O using chambers from brackish and freshwater marshes in Louisiana, USA. From EC, we found that 182 g C m−2 yr−1 was lost through NEEc from the brackish marsh. Of this, 11 g C m−2 yr−1 resulted from net CH4 emissions and the remaining 171 g C m−2 yr−1 resulted from net CO2 emissions. In contrast, −290 g C m2 yr−1 was taken up through NEEc by the freshwater marsh, with 47 g C m−2 yr−1 emitted as CH4 and −337 g C m−2 yr−1 taken up as CO2. From chambers, we discovered that neither site had large fluxes of N2O. Sustained‐flux greenhouse gas accounting metrics indicated that both marshes had a positive (warming) radiative balance, with the brackish marsh having a substantially greater warming effect than the freshwater marsh. That net respiratory emissions of CO2 and CH4 as estimated through chamber techniques were 2–4 times different from emissions estimated through EC requires additional understanding of the artifacts created by different spatial and temporal sampling footprints between techniques.

Mitigating greenhouse gas emissions from a subtropical Ultisol by using long-term no-tillage in combination with legume cover crops

Greenhouse gas (GHG) emissions can be partially mitigated with conservation agriculture. In this study, we assessed the effects of conventional tillage (CT) and no-tillage (NT), as well as five NT cover crop-based cropping systems, on yield-scaled GHG emissions in two long-term experiments (18 and 19 years) on a subtropical Paleudult. Air samples collected in static closed chambers were used to measure nitrous oxide (N2O) and methane (CH4) fluxes. The annual rate of change in soil organic C from the beginning of the experiments was used as proxy for net CO2 flux. Cumulative annual emissions of the three GHG and the CO2 costs of agricultural inputs and operations were taken in full account when estimating the global warming potential (GWP). Under legume cover crops, NT soil exhibited increased N2O emissions relative to CT soil (531 vs 217kg CO2eqha−1yr−1); however, emissions of this gas from NT soil were fully offset by CO2 retention in soil organic matter (−2063 to −3940kg CO2 ha−1 yr−1). Soil CH4 fluxes were very low with all management systems (−1.5 to 30.5kg CO2eqha−1yr−1). NT soil under legume cover crops behaved as a net sink for GHG (GWP ranged from −971 to −2818kg CO2eqha−1yr−1); by contrast, CT soil and NT soil with a low biomass input were net sources of GHG (GWP ranged from 857 to 2133kg CO2eqha−1yr−1). The legume cover crops increased maize yield and further reduced yield-scaled GHG emissions. This result suggests that conservation management practices involving no-till in combination with legume cover crops provide an effective approach to sustainable low-C footprint food production in subtropical regions.

A Global Meta-Analysis on the Impact of Management Practices on Net Global Warming Potential and Greenhouse Gas Intensity from Cropland Soils.

Management practices, such as tillage, crop rotation, and N fertilization, may affect net global warming potential (GWP) and greenhouse gas intensity (GHGI), but their global impact on cropland soils under different soil and climatic conditions need further evaluation. Available global data from 57 experiments and 225 treatments were evaluated for individual and combined effects of tillage, cropping systems, and N fertilization rates on GWP and GHGI which accounted for CO2 equivalents from N2O and CH4 emissions with or without equivalents from soil C sequestration rate (ΔSOC), farm operations, and N fertilization. The GWP and GHGI were 66 to 71% lower with no-till than conventional till and 168 to 215% lower with perennial than annual cropping systems, but 41 to 46% greater with crop rotation than monocroppping. With no-till vs. conventional till, GWP and GHGI were 2.6- to 7.4-fold lower when partial than full accounting of all sources and sinks of greenhouse gases (GHGs) were considered. With 100 kg N ha-1, GWP and GHGI were 3.2 to 11.4 times greater with partial than full accounting. Both GWP and GHGI increased curvilinearly with increased N fertilization rate. Net GWP and GHGI were 70 to 87% lower in the improved combined management that included no-till, crop rotation/perennial crop, and reduced N rate than the traditional combined management that included conventional till, monocopping/annual crop, and recommended N rate. An alternative soil respiration method, which replaces ΔSOC by soil respiration and crop residue returned to soil in the previous year, similarly reduced GWP and GHGI by 133 to 158% in the improved vs. the traditional combined management. Changes in GWP and GHGI due to improved vs. traditional management varied with the duration of the experiment and inclusion of soil and climatic factors in multiple linear regressions improved their relationships. Improved management practices reduced GWP and GHGI compared with traditional management practices and combined management practices were even more effective than individual management practices in reducing net GHG emissions from cropland soils. Partial accounting overestimated GWP and GHGI values as sinks or sources of net GHGs compared with full accounting when evaluating the effect of management practices.

Nitrogen deposition and greenhouse gas emissions from grasslands: uncertainties and future directions.

Increases in atmospheric nitrogen deposition (Ndep) can strongly affect the greenhouse gas (GHG; CO2, CH4, and N2O) sink capacity of grasslands as well as other terrestrial ecosystems. Robust predictions of the net GHG sink strength of grasslands depend on how experimental N loads compare to projected Ndep rates, and how accurately the relationship between GHG fluxes and Ndep is characterized. A literature review revealed that the vast majority of experimental N loads were higher than levels these ecosystems are predicted to experience in the future. Using a process-based biogeochemical model, we predicted that low levels of Ndep either enhanced or reduced the net GHG sink strength of most grasslands, but as experimental N loads continued to increase, grasslands transitioned to a N saturation-decline stage, where the sensitivity of GHG exchange to further increases in Ndep declined. Most published studies represented treatments well into the N saturation-decline stage. Our model results predict that the responses of GHG fluxes to N are highly nonlinear and that the N saturation thresholds for GHGs varied greatly among grasslands and with fire management. We predict that during the 21st century some grasslands will be in the N limitation stage where others will transition into the N saturation-decline stage. The linear relationship between GHG sink strength and N load assumed by most studies can overestimate or underestimate predictions of the net GHG sink strength of grasslands depending on their N baseline status. The next generation of global change experiments should be designed at multiple N loads consistent with future Ndep rates to improve our empirical understanding and predictive ability.