Net Source Of Greenhouse Gases Research Articles

The greenhouse gas budget for China's terrestrial ecosystems.

The first greenhouse gas (GHG) budget accounting over China shows that China's land ecosystems is close to GHG neutral, in contrast to the net GHG source of global land ecosystems.

Predicting The Great Lakes Wetlands' Resilience to Climate Change in Response to Phragmites australis subsp. australis Removal

Phragmites australis subsp. australis (hereon Phargmites), has become a dominating threat to the Great Lakes wetlands at a time when climate change pressures are also compromising the integrity of the native habitat. Understanding the role this invader plays in climate change resiliency is crucial in order to improve management decisions to protect and restore wetlands. This literature review was conducted to predict if the Great Lakes wetland’s resiliency to climate change effects could improve in absence of Phragmites. Peer-reviewed articles were analyzed to determine the overall impact of Phragmites on the Great Lakes wetlands by drawing connections between study findings and how this invader may influence the wetland’s ability to mitigate climate change. Evaluation of Phragmites invasion impacts included effects on floral and faunal species diversity, richness and composition, methane and carbon emissions, nutrient availability, and water levels and quality. This examination of studies showed floral, avian, and turtle species diversity to be negatively related to Phragmites invasion, with no clear impacts on frogs and macroinvertebrates. Studies also showed that Phragmites-dominated wetlands increased carbon sequestration, but also increased methane emissions which has greater radiative forcing power, resulting in a net source of greenhouse gases. Soil nutrient availability was found to be negatively impacted by this invader, and effects on water levels and quality were inconclusive due to insufficient available evidence. Overall, this information provides indication that the Great Lakes wetlands could benefit from Phragmites removal, likely enhancing the wetland’s ability to resist climate change effects.

Basin‐scale estimates of greenhouse gas emissions from the Mara River, Kenya: Importance of discharge, stream size, and land use/land cover

AbstractGreenhouse gas fluxes (CO2, CH4, and N2O) from African streams and rivers are under‐represented in global datasets, resulting in uncertainties in their contributions to regional and global budgets. We conducted year‐long sampling of 59 sites in a nested‐catchment design in the Mara River, Kenya in which fluxes were quantified and their underlying controls assessed. We estimated annual basin‐scale greenhouse gas emissions from measured in‐stream gas concentrations, modeled gas transfer velocities, and determined the sensitivity of up‐scaling to discharge. Based on the total annual CO2‐equivalent emissions calculated from global warming potentials (GWP), the Mara basin was a net greenhouse gas source (294 ± 35 Gg CO2 eq yr−1). Lower‐order streams (1–3) contributed 81% of the total fluxes, and higher stream orders (4–8) contributed 19%. Cropland‐draining streams also exhibited higher fluxes compared to forested streams. Seasonality in stream discharge affected stream widths (and stream area) and gas exchange rates, strongly influencing the basin‐wide annual flux, which was 10 times higher during the high and medium discharge periods than the low discharge period. The basin‐wide estimate was underestimated by up to 36% if discharge was ignored, and up to 37% for lower stream orders. Future research should therefore include seasonality in stream surface areas in upscaling procedures to better constrain basin‐wide fluxes. Given that agricultural activities are a major factor increasing riverine greenhouse gas fluxes in the study region, increased conversion of forests and agricultural intensification has the possibility of increasing the contribution of the African continent to global greenhouse gas sources.

Climatic variation drives loss and restructuring of carbon and nitrogen in boreal forest wildfire

Abstract. The boreal forest landscape covers approximately 10 % of the earth's land area and accounts for almost 30 % of the global annual terrestrial sink of carbon (C). Increased emissions due to climate-change-amplified fire frequency, size, and intensity threaten to remove elements such as C and nitrogen (N) from forest soil and vegetation at rates faster than they accumulate. This may result in large areas within the region becoming a net source of greenhouse gases, creating a positive feedback loop with a changing climate. Meter-scale estimates of area-normalized fire emissions are limited in Eurasian boreal forests, and knowledge of their relation to climate and ecosystem properties is sparse. This study sampled 50 separate Swedish wildfires, which occurred during an extreme fire season in 2018, providing quantitative estimates of C and N loss due to fire along a climate gradient. Mean annual precipitation had strong positive effects on total fuel, which was the strongest driver for increasing C and N losses. Mean annual temperature (MAT) influenced both pre- and postfire organic layer soil bulk density and C : N ratio, which had mixed effects on C and N losses. Significant fire-induced loss of C estimated in the 50 plots was comparable to estimates in similar Eurasian forests but approximately a quarter of those found in typically more intense North American boreal wildfires. N loss was insignificant, though a large amount of fire-affected fuel was converted to a low C : N surface layer of char in proportion to increased MAT. These results reveal large quantitative differences in C and N losses between global regions and their linkage to the broad range of climate conditions within Fennoscandia. A need exists to better incorporate these factors into models to improve estimates of global emissions of C and N due to fire in future climate scenarios. Additionally, this study demonstrated a linkage between climate and the extent of charring of soil fuel and discusses its potential for altering C and N dynamics in postfire recovery.

Seasonal patterns of greenhouse gas emissions from a forest‐to‐bog restored site in northern Scotland: Influence of microtopography and vegetation on carbon dioxide and methane dynamics

AbstractNorthern peatlands play an important role in the regulation of the atmospheric greenhouse gas (GHG) balance, functioning as a net carbon sink with low rates of organic decomposition. However, perturbations such as drainage increase peat oxidation, which may lead to enhanced gaseous release of carbon. For this reason, the number of restoration projects that aim to rewet blanket bogs has increased in the last few years, but there is still a lack of understanding of the impact of restoration on emissions of greenhouse gases, such as methane, particularly in sites restored from forestry. In this paper, we investigate the seasonal greenhouse gas dynamics in a forest‐to‐bog restoration site in Scotland. We analyse the effects of restoration on both carbon dioxide and methane fluxes, and investigate which site factors (microtopography, vegetation type, soil moisture and temperature) drive the processes of gaseous exchange between the bog surface and the atmosphere. Our results show that the original surface is near greenhouse gase equilibrium at −0.28 gCO2eq m2·day−1 and that microtopographic features act as a net greenhouse gas sink (ridges = −0.94 gCO2eq m2·day−1 and furrows = −0.86 gCO2eq m2·day−1), whereas the bog pool is a net source of greenhouse gases (0.98 gCO2eq m2·day−1). We found different vegetation species play a key role in greenhouse gas flux dynamics, especially in forestry‐derived microtopographical features, and their presence and influence on greenhouse gas dynamics should be accounted for to provide a more comprehensive understanding of emissions associated with restoration management practices.Highlights GHG (CO2 and CH4) dynamics in a boreal peatland restored from forestry are mainly affected by microtopography and vegetation. Forestry‐derived microforms (ridges and furrows) are better GHG sinks than pools (GHG emitters) and original surfaces (near GHG equilibrium). The presence of Trichophorum cespitosum leads to higher CH4 emissions. Restoration practices like terraforming may create a short‐term pulse net GHG emission due to an increase of both CH4 and CO2 fluxes.

Mapeo digital de carbono orgánico en suelos de Ecuador

Loayza, V., Sevilla, V., Olivera, C., Guevara, M., Olmedo, G., Vargas, R., Oyonarte, C., Jimenez, W. 2020. Digital mapping of organic carbon in Ecuador soils. Ecosistemas 29(1):1852. https://doi.org/10.7818/ECOS.1852 Soils are the largest organic carbon terrestrial reservoir (COS), compared to the oceans, terrestrial vegetation, and the atmosphere. Soil organic carbon (COS) is dynamic and, due to anthropogenic action, it can become a sink or a net source of greenhouse gases (GHG). For this reason, Ecuador joined efforts and contributed to the construction of the World Map of Organic Soil Carbon (GSOCmap), with which it was possible to estimate, by means of digital soil mapping, the COS content within the 30 cm depth level. national as well as its spatial variability by identifying the environmental factors involved in its storage and those covariates that influence its uncertainty regarding its content. To achieve this, a geostatistical model (Regression – Kriging) was used, which combined 12 924 data from profiles of Heritage soils at 1: 25 000 scale along with 140 edaphic and environmental covariates. The model obtained with a spatial resolution of 1 km, estimated the COS reserve between 10 to 297.8 t ha-1, (mean of 55.69 t ha-1), and suggests a COS reservoir in the surface 30 cm of 1.37 Pg. However, in order to strengthen the confidence of the reported value, it was essential to calculate the uncertainty through this study through ordinary kriging between the differences in the COS values ​​estimated by the regression equation and the real values, through cross-validation (5%), and later by external validation, from which an uncertainty value of COS stored in the soils of Ecuador of 1.63 ± 0.38 Pg of COS was obtained. It was obtained from this model that 40% of the spatial variation of COS presented a root of the root mean square error (RMSE) of 0.52 t ha-1 and a correlation (R2) of 0.41. The covariates with the greatest weight and which directly influence the carbon stock are temperature, soil type, altitude, and the topographic humidity index. The highest concentration of COS is present in the Andes region (> 120 t ha-1), while in the Coast region and Insular region the values ​​fluctuate between medium to low and in the Amazon region the values ​​are low (<40 t ha-1). The analysis allowed to know the areas that present greater uncertainty and less precision, with which it will be possible to strengthen monitoring in areas that currently have information gaps in future studies.

Environmental controls on methane fluxes in a cool temperate bog

Peatlands are important ecosystems for understanding global methane (CH4) and greenhouse gas budgets. We evaluated the CH4 emissions from a cool temperate bog in northern Japan and their responses to environmental drivers based on four years of quasi-continuous measurements with the eddy covariance method. The bog acted as a 13 to 19 g C m−2 yr−1 CH4 source. High annual CH4 emissions outweighed the annual carbon dioxide (CO2) sink strength (-223 to -79 g C m−2 yr−1), except in one year, resulting in a net greenhouse gas source at the annual timescale, although greater uncertainties in the annual CO2 budget (14%) than the CH4 budget (5%) caused uncertainties in the greenhouse gas budget. The seasonal variations in the CH4 fluxes were well explained by soil temperatures in the deep layers and the leaf area index of vascular plants. Pressure-driven CH4 emissions were measured at synoptic timescales, indicating high contributions of ebullition emissions. The episodic CH4 emissions during two typhoon events accounted for 5.1% of the annual fluxes in 2018. Nighttime CH4 emissions were greater than daytime emissions during the early growing season, possibly due to the hydrodynamic transport associated with soil temperature instability at night. The annual CH4 emissions were high when the dissolved oxygen concentration at 10 cm was low and the annual CO2 sink was great, suggesting the importance of anaerobic conditions and substrate availability, which facilitate CH4 production and restrict CH4 oxidation. The climate conditions in the study years were within the normal range; thus, further long-term measurements covering a wide range of climatic conditions will be required.

Impacts of nitrogen management and organic matter application on nitrous oxide emissions and soil organic carbon from spring maize fields in the North China Plain

Soil carbon dioxide (CO2) and nitrous oxide (N2O) emissions and carbon (C) sequestration are important processes for determining net greenhouse gas (GHG) emissions from maize fields. Farming management practices (FMPs) substantially affect crop productivity, soil organic carbon (SOC), and N2O emissions. However, relative knowledge is not sufficient, and further research is required for sustainable agriculture development. A multi-year experiment was performed on a spring maize field in the North China Plain (NCP) to quantify changes in SOC stocks, N2O emissions, and maize yields. Seven treatments were involved in the experiment, including no N fertilization (CK), farmers’ conventional N fertilization (FP), sole application of manure N (M), a combination of M and FP (MFP), FP combined with returned maize straw (SFP), fertilization with reduced synthetic N amount (RN), and fertilization with both reduced synthetic and manure N (RMN). The field measurements demonstrated that urea applications and returned maize straw increased N2O emission. The seasonal total N2O emissions under FP were significantly (P < 0.05) higher than those under CK by 176% to 222% and lower than those under SFP by 20% to 61% throughout the three maize growing seasons. The application of organic manure or maize straw enhanced SOC storage for the studied cropping system. The SOC stocks were relatively high in the plots that received applications of organic manure or maize straw, with the SOC stock of the M plot being significantly higher (P < 0.05) than that of the CK, FP, SFP, RN, and RMN plots in 2016 after eight years of manure amendments. The average net GHG emission from 2012 to 2014 was 203 kg CO2 equivalent ha−1 year−1 (i.e., a net GHG source) under FP. The other treatments converted the maize fields from net GHG sources to GHG sinks. By considering both the maize yields and net GHG emissions, RMN would be an optimal management option, although MFP, SFP, and RN also mitigated the GHG emissions and maintained the yields. Large variations were observed in the contributions of the SOC changes and N2O emissions to net GHG emissions among the treatments. The large variations demonstrate the need to consider both SOC changes and N2O emissions when evaluating the impacts of alternative FMPs on net GHG emissions.

Seasonal and diel variation in greenhouse gas emissions from an urban pond and its major drivers

AbstractSmall water systems are important hotspots of greenhouse gas (GHG) emission, but estimates are poorly constrained as data are scarce. Small ponds are often constructed in urban areas, where they receive large amounts of nutrients and therefore tend to be highly productive. Here, we investigated GHG emissions, seasonal and diel variation, and net ecosystem production (NEP) from an urban pond. In monthly 24‐h field campaigns during 11 months, diffusive water–atmosphere methane (CH4) and carbon dioxide (CO2) fluxes and CH4 ebullition and oxidation were quantified. With oxygen (O2) measurements, NEP was assessed. The pond was a net GHG source the entire year, with an emission of 3.4 kg CO2 eq m−2 yr−1. The dominant GHG emission pathway was CH4 ebullition (bubble flux, 50%), followed by diffusive emissions of CO2 (38%) and CH4 (12%). Sediment CH4 release was primarily driven by temperature and especially ebullition increased exponentially above a temperature threshold of 15°C. The pond's atmospheric CO2 exchange was not related to NEP or temperature but likely to a high allochthonous carbon (C) input via runoff and anaerobic mineralization of C. We expect urban ponds to show a large increase in GHG emission with increasing temperature, which should be considered carefully when constructing ponds in urban areas. Emissions may partly be counteracted by pond management focusing on a reduction of nutrient and organic matter input.

Chinese cropping systems are a net source of greenhouse gases despite soil carbon sequestration.

Soil carbon sequestration is being considered as a potential pathway to mitigate climate change. Cropland soils could provide a sink for carbon that can be modified by farming practices; however, they can also act as a source of greenhouse gases (GHG), including not only nitrous oxide (N2 O) and methane (CH4 ), but also the upstream carbon dioxide (CO2 ) emissions associated with agronomic management. These latter emissions are also sometimes termed "hidden" or "embedded" CO2 . In this paper, we estimated the net GHG balance for Chinese cropping systems by considering the balance of soil carbon sequestration, N2 O and CH4 emissions, and the upstream CO2 emissions of agronomic management from a life cycle perspective during 2000-2017. Results showed that although soil organic carbon (SOC) increased by 23.2±8.6TgC per year, the soil N2 O and CH4 emissions plus upstream CO2 emissions arising from agronomic management added 269.5±21.1TgC-eq per year to the atmosphere. These findings demonstrate that Chinese cropping systems are a net source of GHG emissions and that total GHG emissions are about 12 times larger than carbon uptake by soil sequestration. There were large variations between different cropping systems in the net GHG balance ranging from 328 to 7,567kgC-eqha-1 year-1 , but all systems act as a net GHG source to the atmosphere. The main sources of total GHG emissions are nitrogen fertilization (emissions during production and application), power use for irrigation, and soil N2 O and CH4 emissions. Optimizing agronomic management practices, especially fertilization, irrigation, plastic mulching, and crop residues to reduce total GHG emissions from the whole chain is urgently required in order to develop a low-carbon future for Chinese crop production.

A radiative forcing analysis of tropical peatlands before and after their conversion to agricultural plantations.

The tropical peat swamp forests of South-East Asia are being rapidly converted to agricultural plantations of oil palm and Acacia creating a significant global "hot-spot" for CO2 emissions. However, the effect of this major perturbation has yet to be quantified in terms of global warming potential (GWP) and the Earth's radiative budget. We used a GWP analysis and an impulse-response model of radiative forcing to quantify the climate forcing of this shift from a long-term carbon sink to a net source of greenhouse gases (CO2 and CH4 ). In the GWP analysis, five tropical peatlands were sinks in terms of their CO2 equivalent fluxes while they remained undisturbed. However, their drainage and conversion to oil palm and Acacia plantations produced a dramatic shift to very strong net CO2 -equivalent sources. The induced losses of peat carbon are ~20× greater than the natural CO2 sequestration rates. In contrast, a radiative forcing model indicates that the magnitude of this shift from a net cooling to warming effect is ultimately related to the size of an individual peatland's carbon pool. The continuous accumulation of carbon in pristine tropical peatlands produced a progressively negative radiative forcing (i.e., cooling) that ranged from -2.1 to -6.7nW/m2 per hectare peatland by 2010 CE, referenced to zero at the time of peat initiation. Peatland conversion to plantations leads to an immediate shift from negative to positive trend in radiative forcing (i.e., warming). If drainage persists, peak warming ranges from +3.3 to +8.7nW/m2 per hectare of drained peatland. More importantly, this net warming impact on the Earth's radiation budget will persist for centuries to millennia after all the peat has been oxidized to CO2 . This previously unreported and undesirable impact on the Earth's radiative balance provides a scientific rationale for conserving tropical peatlands in their pristine state.

Mitigation potential of soil carbon management overestimated by neglecting N2O emissions

International initiatives such as the ‘4 per 1000’ are promoting enhanced carbon (C) sequestration in agricultural soils as a way to mitigate greenhouse gas emissions 1 . However, changes in soil organic C turnover feed back into the nitrogen (N) cycle 2 , meaning that variation in soil nitrous oxide (N2O) emissions may offset or enhance C sequestration actions 3 . Here we use a biogeochemistry model on approximately 8,000 soil sampling locations in the European Union 4 to quantify the net CO2 equivalent (CO2e) fluxes associated with representative C-mitigating agricultural practices. Practices based on integrated crop residue retention and lower soil disturbance are found to not increase N2O emissions as long as C accumulation continues (until around 2040), thereafter leading to a moderate C sequestration offset mostly below 47% by 2100. The introduction of N-fixing cover crops allowed higher C accumulation over the initial 20 years, but this gain was progressively offset by higher N2O emissions over time. By 2060, around half of the sites became a net source of greenhouse gases. We conclude that significant CO2 mitigation can be achieved in the initial 20–30 years of any C management scheme, but after that N inputs should be controlled through appropriate management. Agricultural soils can be targeted for carbon (C) sequestration. Research considering C and nitrogen (N) dynamics confirms that significant CO2 mitigation can be achieved, but after 20–30 years N inputs also need controlling to prevent the C sequestration being offset by N2O emissions.

Stover retention rather than no-till decreases the global warming potential of rainfed continuous maize cropland

During the past two decades, conservation management practices to sequester soil carbon have been recommended to mitigate greenhouse gas (GHG) emissions. However, the long-term effects of no-till, stover retention, and their interaction on soil organic carbon (SOC) stocks and GHG emissions from rainfed croplands remain uncertain. In this study, tillage practice and stover management effects were investigated in a long-term rainfed continuous maize cropping system. Measurements of soil nitrous oxide (N2O) and methane (CH4) fluxes and SOC change were conducted in four treatments: conventional tillage with stover removal (CT), conventional tillage with stover retention (CS), no tillage with stover removal (NT) and no tillage with stover retention (NS). Annual N2O emissions with stover retention (CS and NS, 0.52–0.74 kg N ha−1 yr−1) were significantly higher (P < 0.0001) than those with stover removal (CT and NT, 0.40–0.55 kg N ha−1 yr−1), but N2O emissions were not affected by tillage practice. Net CH4 consumption occurred in all treatments, but no significant effect of tillage practice or stover management was found. Surface (0–20 cm) SOC stocks decreased with both stover removal and no tillage practice, while deep SOC (20–100 cm) was not affected by tillage practice or stover management over ten years. Stover retention led to a net GHG sink with annual global warming potential (GWP) values of −2.52 ± 0.05 and −1.03 ± 0.02 Mg CO2-eq. ha−1 yr−1 for CS and NS, respectively, but stover removal practices were a net GHG source with annual GWP values of 0.83 ± 0.04 and 1.40 ± 0.04 Mg CO2-eq. ha−1 yr−1 for CT and NT, respectively. Our results highlight the importance of C input from crop residues for increasing SOC stocks and mitigating GHG emissions. Therefore, conventional tillage with crop residue return is the most promising management system for simultaneously achieving maximum yield and minimum GWP.

Greenhouse gas emissions from the energy crop oilseed rape (Brassica napus); the role of photosynthetically active radiation in diurnal N2O flux variation

AbstractOilseed rape (OSR, Brassica napus L.) is an important feedstock for biodiesel; hence, carbon dioxide (CO2), methane (CH4) and particularly fertilizer‐derived nitrous oxide (N2O) emissions during cultivation must be quantified to assess putative greenhouse gas (GHG) savings, thus creating an urgent and increasing need for such data. Substrates of nitrification [ammonium (NH4)] and denitrification [nitrate (NO3)], the predominant N2O production pathways, were supplied separately and in combination to OSR in a UK field trial aiming to: (i) produce an accurate GHG budget of fertilizer application; (ii) characterize short‐ to medium‐term variation in GHG fluxes; (iii) establish the processes driving N2O emission. Three treatments were applied twice, 1 week apart: ammonium nitrate fertilizer (NH4NO3, 69 kg‐N ha−1) mimicking the farm management, ammonium chloride (NH4Cl, 34.4 kg‐N ha−1) and sodium nitrate (NaNO3, 34.6 kg‐N ha−1). We deployed SkyLine2D for the very first time, a novel automated chamber system to measure CO2, CH4 and N2O fluxes at unprecedented high temporal and spatial resolution from OSR. During 3 weeks following the fertilizer application, CH4 fluxes were negligible, but all treatments were a net sink for CO2 (ca. 100 g CO2 m−2). Cumulative N2O emissions (ca. 120 g CO2‐eq m−2) from NH4NO3 were significantly greater (P &lt; 0.04) than from NaNO3 (ca. 80 g CO2‐eq m−2), but did not differ from NH4Cl (ca. 100 g CO2‐eq m−2) and reduced the carbon sink of photosynthesis so that OSR was a net GHG source in the fertilizer treatment. Diurnal variation in N2O emissions, peaking in the afternoon, was more strongly associated with photosynthetically active radiation (PAR) than temperature. This suggests that the supply of carbon (C) from photosynthate may have been the key driver of the observed diurnal pattern in N2O emission and thus should be considered in future process‐based models of GHG emissions.

Greenhouse-gas budgets for irrigated dairy pasture and a winter-forage kale crop

Managed grasslands can be net sources or sinks for three major greenhouse gases (CO2, CH4 and N2O). We measured the exchange of these gases for three years over an irrigated, intensively-managed dairy pasture in New Zealand that was grazed ten times per year. We also measured the greenhouse gas (GHG) exchanges over a neighbouring dryland pasture for two years, including its conversion to a winter-forage kale crop in the second year and the grazing of that crop. From the gas exchanges, measurements of the grazed biomass, and estimates of other imports and exports we obtained annual net carbon (C) budgets and GHG budgets for these ecosystems. The irrigated pasture system (excluding cows) removed CO2 from the atmosphere, 423 (±23)gC m−2yr−1 on average, and when considering the other C inputs and outputs it was also a net C sink in each year, gaining 81 (±27)gCm−2yr−1 on average. The net CO2 uptake of the dryland in the conversion year was about half that of the irrigated pasture, and its net C budget was neutral. The irrigated pasture, without grazing cows, emitted CH4 throughout all seasons. These emissions were about 15 times greater than emissions expected just from cow dung; we cannot discern what fractions originated from the soil and the pasture plants, respectively. At the dryland/kale site, CH4 emissions of the same magnitude occurred. The emissions of N2O from the irrigated pasture were 0.68 (±0.026)gNm−2yr−1 on average (± standard error), and about half that from the kale crop. These results agree reasonably well with expected emissions based on the N inputs from fertiliser and excreta, using emission factors from New Zealand’s national GHG inventory; however, it is unclear what fraction of the observed emissions can be considered as non-anthropogenic background fluxes. For the irrigated pasture, the global-warming potential of the N2O emissions (expressed as CO2-equivalent mass) was approximately equal to the net C uptake. Hence, the pasture was offsetting its own N2O emissions. However, CH4 emissions directly from cows (calculated from the cows’ feed intake) were two to three times greater than the N2O emissions, and about six times greater than the pasture’s CH4 emissions. Therefore, the dairy system including pasture and cows was a net GHG source.

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.

EMISIÓN DE METANO Y ÓXIDO NITROSO DE LOS SEDIMENTOS DE MANGLAR DE LA CIÉNAGA GRANDE DE SANTA MARTA, CARIBE COLOMBIANO

In Colombia there is little information on the role of mangroves in relation to greenhouse gases (GHG), their release rates under different environmental conditions, or their role in the global carbon cycle. For these reasons, in this study we evaluated the fluxes of CH4 and N2O, in four sectors of the Ciénaga Grande de Santa Marta (CGSM) with different degrees of conservation of mangrove forests, to determine their role as a source or sink of GHG. The fluxes were measured by the method of the static chambers and showed variations between 34.7-1179.7 and nd-31569.2 μg.m-2.h-1 for N2O and CH4, respectively, showing that mangrove sediments of CGSM are a net source of GHG, and furthermore are of the same magnitude as levels recorded world-wide in mangroves subjected to sewage input. Statistical analyses showed differences between sectors but not between climatic periods. N2O emissions were highest in the Agua Negras station (AN, 847.3 ± 265.7 μg.m-2.h-1), a locality in the process of natural regeneration with a direct influence from the Magdalena River and in Caño Dragado (CD, 438.7 ± 235.3 μg.m-2.h-1); while emissions were lower in the recovery sites Caño Grande (CG) and Rinconada (RIN), (104.7 ± 49.4 and 152.1 ± 36.0 μg.m-2.h-1, respectively). The highest CH4 emission was recorded in recovery sectors: CG and AN (9573.4 ± 8623.8 and 4328.2 ± 7569.5 μg.m-2.h-1, respectively). In terms of CO2-equivalent, N2O emissions account for over 50% of the total, and this has been documented for agricultural systems and constitutes evidence of deterioration of CD. A correlation analysis with environmental factors showed that N2O emissions vary inversely with salinity and positively with nitrites, suggesting production mainly via nitrification. Finally, a coarse estimation of GHG emissions per hectare indicated that, depending on the state of conservation or deterioration of the mangrove, emissions can vary from 10.2 to 27.1 tCO2-eq.ha-1.a-1.

The fluvial flux of particulate organic matter from the UK: the emission factor of soil erosion

AbstractSoil erosion has been identified as a potential global carbon sink since eroded organic matter is replaced at source and eroded material is readily buried. However, this argument has relied on poor estimates of the total fate of in‐transit particulates and could erroneously imply soil erosion could be encouraged to generate carbon stores. These previous estimates have not considered that organic matter can also be released to the atmosphere as a range of greenhouse gases, not only carbon dioxide (CO2), but also the more powerful greenhouse gases methane (CH4) and nitrous oxide (N2O). As soil carbon lost by erosion is only replaced by uptake of CO2, this could represent a considerable imbalance in greenhouse gas warming potential, even if it is not significant in terms of overall carbon flux. This work therefore considers the flux of particulate organic matter through UK rivers with respect to both carbon fluxes and greenhouse gas emissions. The results show that, although emissions to the atmosphere are dominated by CO2, there are also considerable fluxes of CH4 and N2O. The results suggest that soil erosion is a net source of greenhouse gases with median emission factors of 5.5, 4.4 and 0.3 tonnes CO2eq/yr for one tonne of fluvial carbon, gross carbon erosion and gross soil erosion, respectively. This study concludes that gross soil erosion would therefore only be a net sink of both carbon and greenhouse gases if all the following criteria are met: the gross soil erosion rate were very low (&lt;91 tonnes/km2/yr); the eroded carbon were completely replaced by new soil organic matter; and if less than half of the gross erosion made it into the stream network. By establishing the emission factor for soil erosion, it becomes possible to properly account for the benefits of good soil management in minimizing losses of greenhouse gases to the atmosphere as a by‐product of soil erosion. Copyright © 2015 John Wiley &amp; Sons, Ltd.

The greenhouse gas balance of European grasslands.

The greenhouse gas (GHG) balance of European grasslands (EU-28 plus Norway and Switzerland), including CO2 , CH4 and N2 O, is estimated using the new process-based biogeochemical model ORCHIDEE-GM over the period 1961-2010. The model includes the following: (1) a mechanistic representation of the spatial distribution of management practice; (2) management intensity, going from intensively to extensively managed; (3) gridded simulation of the carbon balance at ecosystem and farm scale; and (4) gridded simulation of N2 O and CH4 emissions by fertilized grassland soils and livestock. The external drivers of the model are changing animal numbers, nitrogen fertilization and deposition, land-use change, and variable CO2 and climate. The carbon balance of European grassland (NBP) is estimated to be a net sink of 15±7gCm(-2) year(-1) during 1961-2010, equivalent to a 50-year continental cumulative soil carbon sequestration of 1.0±0.4PgC. At the farm scale, which includes both ecosystem CO2 fluxes and CO2 emissions from the digestion of harvested forage, the net C balance is roughly halved, down to a small sink, or nearly neutral flux of 8gCm(-2) year(-1) . Adding CH4 and N2 O emissions to net ecosystem exchange to define the ecosystem-scale GHG balance, we found that grasslands remain a net GHG sink of 19±10g C-CO2 equiv. m(-2) year(-1) , because the CO2 sink offsets N2 O and grazing animal CH4 emissions. However, when considering the farm scale, the GHG balance (NGB) becomes a net GHG source of -50gC-CO2 equiv. m(-2) year(-1) . ORCHIDEE-GM simulated an increase in European grassland NBP during the last five decades. This enhanced NBP reflects the combination of a positive trend of net primary production due to CO2 , climate and nitrogen fertilization and the diminishing requirement for grass forage due to the Europe-wide reduction in livestock numbers.

Modeling impacts of changes in temperature and water table on C gas fluxes in an Alaskan peatland

AbstractNorthern peatlands have accumulated a large amount of organic carbon (C) in their thick peat profile. Climate change and associated variations in soil environments are expected to have significant impacts on the C balance of these ecosystems, but the magnitude is still highly uncertain. Verifying and understanding the influences of changes in environmental factors on C gas fluxes in biogeochemical models are essential for forecasting feedbacks between C gas fluxes and climate change. In this study, we applied a biogeochemical model, DeNitrification‐DeComposition (DNDC), to assess impacts of air temperature (TA) and water table (WT) on C gas fluxes in an Alaskan peatland. DNDC was validated against field measurements of net ecosystem exchange of CO2 (NEE) and CH4 fluxes under manipulated surface soil temperature and WT conditions in a moderate rich fen. The validation demonstrates that DNDC was able to capture the observed impacts of the manipulations in soil environments on C gas fluxes. To investigate responses of C gas fluxes to changes in TA and soil water condition, we conducted a series of simulations with varying TA and WT. The results demonstrate that (1) uptake rates of CO2 at the site were reduced by either too colder or warmer temperatures and generally increased with increasing soil moisture; (2) CH4 emissions showed an increasing trend as TA increased or WT rose toward the peat surface; and (3) the site could shift from a net greenhouse gas (GHG) sink into a net GHG source under some warm and/or dry conditions. A sensitivity analysis evaluated the relative importance of TA and WT to C gas fluxes. The results indicate that both TA and WT played important roles in regulating NEE and CH4 emissions and that within the investigated ranges of the variations in TA and WT, changes in WT showed a greater impact than changes in TA on NEE, CH4 fluxes, and net C gas fluxes at the study fen.