Mitigation of greenhouse gas emissions from reed canary grass in paludiculture: effect of groundwater level

Background and aims

Combination of rewetting and wetland crop cultivation (paludiculture) is pursued as a wider carbon dioxide (CO₂) mitigation option in drained peatland. However, information on the overall greenhouse gas (GHG) balance for paludiculture is lacking. We investigated the GHG balance of peatlands grown with reed canary grass (RCG) and rewetted to various extents.

Methods

Gas fluxes of CO₂, methane (CH₄) and nitrous oxide (N₂O) were measured with a static chamber technique for 10 months from mesocosms sown with RCG and manipulated to ground water levels (GWL) of 0, ?10, ?20, ?30 and ?40 cm below the soil surface. Gross primary production (GPP) was estimated from the above ground biomass yield.

Results

The mean dry biomass yield across all water table treatments was 6 Mg ha^?1 with no significant differences between the treatments. Raising the GWL to the surface decreased both the net ecosystem exchange (NEE) of CO₂ and N₂O emissions whereas CH₄ emissions increased. Total cumulative GHG emissions (for 10 months) corresponded to 0.08, 0.13, 0.61, 0.68 and 0.98 kg CO₂ equivalents m^?2 from the GWL treatments at 0, ?10, ?20, ?30 and ?40 cm below the soil surface, respectively.

Conclusions

The results showed that a reduction in total GHG emission can be achieved without losing the productivity of newly established RCG when GWL is maintained close to the surface. Further studies should address the practical constrains and long-term productivity of RCG cultivation in rewetted peatlands.     

Measurement and modelling of CO2 flux from a drained fen peatland cultivated with reed canary grass and spring barley

Cultivation of bioenergy crops has been suggested as a promising option for reduction of greenhouse gas (GHG) emissions from arable organic soils (Histosols). Here, we report the annual net ecosystem exchange (NEE) fluxes of CO₂ as measured with a dynamic closed chamber method at a drained fen peatland grown with reed canary grass (RCG) and spring barley (SB) in a plot experiment (n = 3 for each cropping system). The CO₂ flux was partitioned into gross photosynthesis (GP) and ecosystem respiration (R_E). For the data analysis, simple yet useful GP and R_E models were developed which introduce plot‐scale ratio vegetation index as an active vegetation proxy. The GP model captures the effect of temperature and vegetation status, and the R_E model estimates the proportion of foliar biomass dependent respiration (R_fb) in the total R_E. Annual R_E was 1887 ± 7 (mean ± standard error, n = 3) and 1288 ± 19 g CO₂‐C m^?2 in RCG and SB plots, respectively, with R_fb accounting for 32 and 22% respectively. Total estimated annual GP was ?1818 ± 42 and ?1329 ± 66 g CO₂‐C m^?2 in RCG and SB plots leading to a NEE of 69 ± 36 g CO₂‐C m^?2 yr^?1 in RCG plots (i.e., a weak net source) and ?41 ± 47 g CO₂‐C m^?2 yr^?1 in SB plots (i.e., a weak net sink). Standard errors related to spatial variation were small (as shown above), but more significant uncertainties were related to the modelling approach for establishment of annual budgets. In conclusion, the bioenergy cropping system was not more favourable than the food cropping system when looking at the atmospheric CO₂ emissions during cultivation. However, in a broader GHG life‐cycle perspective, the lower fertilizer N input and the higher biomass yield in bioenergy cropping systems could be beneficial.     

High emissions of greenhouse gases from grasslands on peat and other organic soils

Drainage has turned peatlands from a carbon sink into one of the world's largest greenhouse gas (GHG) sources from cultivated soils. We analyzed a unique data set (12 peatlands, 48 sites and 122 annual budgets) of mainly unpublished GHG emissions from grasslands on bog and fen peat as well as other soils rich in soil organic carbon (SOC) in Germany. Emissions and environmental variables were measured with identical methods. Site‐averaged GHG budgets were surprisingly variable (29.2 ± 17.4 t CO₂‐eq. ha^?1 yr^?1) and partially higher than all published data and the IPCC default emission factors for GHG inventories. Generally, CO₂ (27.7 ± 17.3 t CO₂ ha^?1 yr^?1) dominated the GHG budget. Nitrous oxide (2.3 ± 2.4 kg N₂O‐N ha^?1 yr^?1) and methane emissions (30.8 ± 69.8 kg CH₄‐C ha^?1 yr^?1) were lower than expected except for CH₄ emissions from nutrient‐poor acidic sites. At single peatlands, CO₂ emissions clearly increased with deeper mean water table depth (WTD), but there was no general dependency of CO₂ on WTD for the complete data set. Thus, regionalization of CO₂ emissions by WTD only will remain uncertain. WTD dynamics explained some of the differences between peatlands as sites which became very dry during summer showed lower emissions. We introduced the aerated nitrogen stock (N_air) as a variable combining soil nitrogen stocks with WTD. CO₂ increased with N_air across peatlands. Soils with comparatively low SOC concentrations showed as high CO₂ emissions as true peat soils because N_air was similar. N₂O emissions were controlled by the WTD dynamics and the nitrogen content of the topsoil. CH₄ emissions can be well described by WTD and ponding duration during summer. Our results can help both to improve GHG emission reporting and to prioritize and plan emission reduction measures for peat and similar soils at different scales.     

Effects of Elevated CO2 and Temperature on Biomass Growth and Allocation in a Boreal Bioenergy Crop (Phalaris arundinacea L.) from Young and Old Cultivations

An auto-controlled climate system was used to study how a boreal bioenergy crop (reed canary grass, Phalaris arundinacea L., hereafter RCG) responded to a warming climate and elevated CO₂. Over one growing season (April–September of 2009), RCG from young and old cultivations (3 years [3-year] and 10 years [10-year]) was grown in closed chambers under ambient conditions (CON), elevated CO₂ (EC, approximately 700 μmol?mol^?1), elevated temperature (ET, ambient + approximately 3 °C) and elevated temperature and CO₂ (ETC). The treatments were replicated four times. Throughout the growing season, the above-ground (leaf and stem biomass) and below-ground biomasses were measured six times, representing various developmental stages (early stages: the first three stages, and late stages: the last three stages). Compared to the growth observed under CON, EC enhanced RCG biomass growth over the whole growing season (p?<?0.05), whereas ET increased RCG biomass growth in early stages but decreased growth in late stages, regardless of the cultivation age. However, the negative effect of ET later in the growing season was partially mitigated by CO₂ enrichment. Compared to CON plants, the final total biomass was 18 % higher for 3-year plants and 8 % higher for 10-year plants grown under EC. In comparison, for 3-year and 10-year plants, the biomass was 5 and 3 % lower under ET and 7 and 4 % greater under ETC, respectively. Under EC, the below-ground growth contributed more to the total biomass growth compared to the above-ground portion. The opposite situation was observed under ET and ETC. The climate-related changes in biomass growth were smaller in the old cultivation than in the young cultivation due to the lower net assimilation rate and lower specific leaf area in the old cultivation plants.     

Soil Greenhouse Gas Emissions in Response to Corn Stover Removal and Tillage Management Across the US Corn Belt

In-field measurements of direct soil greenhouse gas (GHG) emissions provide critical data for quantifying the net energy efficiency and economic feasibility of crop residue-based bioenergy production systems. A major challenge to such assessments has been the paucity of field studies addressing the effects of crop residue removal and associated best practices for soil management (i.e., conservation tillage) on soil emissions of carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). This regional survey summarizes soil GHG emissions from nine maize production systems evaluating different levels of corn stover removal under conventional or conservation tillage management across the US Corn Belt. Cumulative growing season soil emissions of CO₂, N₂O, and/or CH₄ were measured for 2–5 years (2008–2012) at these various sites using a standardized static vented chamber technique as part of the USDA-ARS’s Resilient Economic Agricultural Practices (REAP) regional partnership. Cumulative soil GHG emissions during the growing season varied widely across sites, by management, and by year. Overall, corn stover removal decreased soil total CO₂ and N₂O emissions by -4 and -7 %, respectively, relative to no removal. No management treatments affected soil CH₄ fluxes. When aggregated to total GHG emissions (Mg CO₂?eq ha^?1) across all sites and years, corn stover removal decreased growing season soil emissions by ?5?±?1 % (mean?±?se) and ranged from -36 % to 54 % (n?=?50). Lower GHG emissions in stover removal treatments were attributed to decreased C and N inputs into soils, as well as possible microclimatic differences associated with changes in soil cover. High levels of spatial and temporal variabilities in direct GHG emissions highlighted the importance of site-specific management and environmental conditions on the dynamics of GHG emissions from agricultural soils.     

Atmospheric impact of bioenergy based on perennial crop (reed canary grass,Phalaris arundinaceae,L.) cultivation on a drained boreal organic soil

Marginal organic soils, abundant in the boreal region, are being increasingly used for bioenergy crop cultivation. Using long‐term field experimental data on greenhouse gas (GHG) balance from a perennial bioenergy crop [reed canary grass (RCG), Phalaris arundinaceae L.] cultivated on a drained organic soil as an example, we show here for the first time that, with a proper cultivation and land‐use practice, environmentally sound bioenergy production is possible on these problematic soil types. We performed a life cycle assessment (LCA) for RCG on this organic soil. We found that, on an average, this system produces 40% less CO₂‐equivalents per MWh of energy in comparison with a conventional energy source such as coal. Climatic conditions regulating the RCG carbon exchange processes have a high impact on the benefits from this bioenergy production system. Under appropriate hydrological conditions, this system can even be carbon‐negative. An LCA sensitivity analysis revealed that net ecosystem CO₂ exchange and crop yield are the major LCA components, while non‐CO₂ GHG emissions and costs associated with crop production are the minor ones. Net bioenergy GHG emissions resulting from restricted net CO₂ uptake and low crop yields, due to climatic and moisture stress during dry years, were comparable with coal emissions. However, net bioenergy emissions during wet years with high net uptake and crop yield were only a third of the coal emissions. As long‐term experimental data on GHG balance of bioenergy production are scarce, scientific data stemming from field experiments are needed in shaping renewable energy source policies.     

Effects of different influent C/N ratios on the performance of various earthworm eco-filter systems: nutrient removal and greenhouse gas emission

In this study, we sought to identify influent carbon-to-nitrogen (C/N) ratios that yield relatively high nutrient removal efficiency with relatively low greenhouse gas (GHG) emissions. The earthworm eco-filter (EE) system, which is composed of earthworms and plants (EP group), was found to be optimal for maximizing nutrient removal while reducing GHG emissions. In this EE system, the optimal influent C/N ratio for nutrient removal and GHG emission under C2N treatment conditions. Nutrient removal efficiency under this condition was 85.19 ± 6.40 % chemical oxygen demand, 71.99 ± 11.28 % total nitrogen, and 77.91 ± 8.51 % total phosphorus, while the CO₂ emission rate was 678.89 ± 201.87 mg m^?2 h^?1. Moreover, the highest nutrient removal and GHG emission rates were both achieved in late summer (August). Thus, carbon variation, season, system variation, as well as synergistic interaction between system variations and seasons, significantly affect nutrient removal efficiencies and GHG emissions.     

Multiyear greenhouse gas balances at a rewetted temperate peatland

Drained peat soils are a significant source of greenhouse gas (GHG) emissions to the atmosphere. Rewetting these soils is considered an important climate change mitigation tool to reduce emissions and create suitable conditions for carbon sequestration. Long‐term monitoring is essential to capture interannual variations in GHG emissions and associated environmental variables and to reduce the uncertainty linked with GHG emission factor calculations. In this study, we present GHG balances: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) calculated for a 5‐year period at a rewetted industrial cutaway peatland in Ireland (rewetted 7 years prior to the start of the study); and compare the results with an adjacent drained area (2‐year data set), and with ten long‐term data sets from intact (i.e. undrained) peatlands in temperate and boreal regions. In the rewetted site, CO₂ exchange (or net ecosystem exchange (NEE)) was strongly influenced by ecosystem respiration (R_eco) rather than gross primary production (GPP). CH₄ emissions were related to soil temperature and either water table level or plant biomass. N₂O emissions were not detected in either drained or rewetted sites. Rewetting reduced CO₂ emissions in unvegetated areas by approximately 50%. When upscaled to the ecosystem level, the emission factors (calculated as 5‐year mean of annual balances) for the rewetted site were (±SD) ?104 ± 80 g CO₂‐C m^?2 yr^?1 (i.e. CO₂ sink) and 9 ± 2 g CH₄‐C m^?2 yr^?1 (i.e. CH₄ source). Nearly a decade after rewetting, the GHG balance (100‐year global warming potential) had reduced noticeably (i.e. less warming) in comparison with the drained site but was still higher than comparative intact sites. Our results indicate that rewetted sites may be more sensitive to interannual changes in weather conditions than their more resilient intact counterparts and may switch from an annual CO₂ sink to a source if triggered by slightly drier conditions.     

Full carbon and greenhouse gas balances of fertilized and nonfertilized reed canary grass cultivations on an abandoned peat extraction area in a dry year

Bioenergy crop cultivation on former peat extraction areas is a potential after‐use option that provides a source of renewable energy while mitigating climate change through enhanced carbon (C) sequestration. This study investigated the full C and greenhouse gas (GHG) balances of fertilized (RCG‐F) and nonfertilized (RCG‐C) reed canary grass (RCG; Phalaris arundinacea) cultivation compared to bare peat (BP) soil within an abandoned peat extraction area in western Estonia during a dry year. Vegetation sampling, static chamber and lysimeter measurements were carried out to estimate above‐ and belowground biomass production and allocation, fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in cultivated strips and drainage ditches as well as the dissolved organic carbon (DOC) export, respectively. Heterotrophic respiration was determined from vegetation‐free trenched plots. Fertilization increased the above‐ to belowground biomass production ratio and the autotrophic to heterotrophic respiration ratio. The full C balance (incl. CO₂, CH₄ and DOC fluxes from strips and ditches) was 96, 215 and 180 g C m^?2 yr^?1 in RCG‐F, RCG‐C and BP, respectively, suggesting that all treatments acted as C sources during the dry year. The C balance was driven by variations in the net CO₂ exchange, whereas the combined contribution of CH₄ and DOC fluxes was <5%. The GHG balances were 3.6, 7.9 and 6.6 t CO₂ eq ha^?1 yr^?1 in RCG‐F, RCG‐C and BP, respectively. The CO₂ exchange was also the dominant component of the GHG balance, while the contributions of CH₄ and N₂O were <1% and 1–6%, respectively. Overall, this study suggests that maximizing plant growth and the associated CO₂ uptake through adequate water and nutrient supply is a key prerequisite for ensuring sustainable high yields and climate benefits in RCG cultivations established on organic soils following drainage and peat extraction.     

Methane and carbon dioxide emissions from inland waters in India – implications for large scale greenhouse gas balances

Inland waters were recently recognized to be important sources of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere, and including inland water emissions in large scale greenhouse gas (GHG) budgets may potentially offset the estimated carbon sink in many areas. However, the lack of GHG flux measurements and well‐defined inland water areas for extrapolation, make the magnitude of the potential offset unclear. This study presents coordinated flux measurements of CH₄ and CO₂ in multiple lakes, ponds, rivers, open wells, reservoirs, springs, and canals in India. All these inland water types, representative of common aquatic ecosystems in India, emitted substantial amounts of CH₄ and a major fraction also emitted CO₂. The total CH₄ flux (including ebullition and diffusion) from all the 45 systems ranged from 0.01 to 52.1 mmol m^?2 d^?1, with a mean of 7.8 ± 12.7 (mean ± 1 SD) mmol m^?2 d^?1. The mean surface water CH₄ concentration was 3.8 ± 14.5 μm (range 0.03–92.1 μm ). The CO₂ fluxes ranged from ?28.2 to 262.4 mmol m^?2 d^?1 and the mean flux was 51.9 ± 71.1 mmol m^?2 d^?1. The mean partial pressure of CO₂ was 2927 ± 3269 μatm (range: 400–11 467 μatm). Conservative extrapolation to whole India, considering the specific area of the different water types studied, yielded average emissions of 2.1 Tg CH₄ yr^?1 and 22.0 Tg CO₂ yr^?1 from India's inland waters. When expressed as CO₂ equivalents, this amounts to 75 Tg CO₂ equivalents yr^?1 (53–98 Tg CO₂ equivalents yr^?1; ± 1 SD)_, with CH₄ contributing 71%. Hence, average inland water GHG emissions, which were not previously considered, correspond to 42% (30–55%) of the estimated land carbon sink of India. Thereby this study illustrates the importance of considering inland water GHG exchange in large scale assessments.     

Conversion of coastal wetlands,riparian wetlands,and peatlands increases greenhouse gas emissions: A global meta‐analysis

Land‐use/land‐cover change (LULCC) often results in degradation of natural wetlands and affects the dynamics of greenhouse gases (GHGs). However, the magnitude of changes in GHG emissions from wetlands undergoing various LULCC types remains unclear. We conducted a global meta‐analysis with a database of 209 sites to examine the effects of LULCC types of constructed wetlands (CWs), croplands (CLs), aquaculture ponds (APs), drained wetlands (DWs), and pastures (PASs) on the variability in CO₂, CH₄, and N₂O emissions from the natural coastal wetlands, riparian wetlands, and peatlands. Our results showed that the natural wetlands were net sinks of atmospheric CO₂ and net sources of CH₄ and N₂O, exhibiting the capacity to mitigate greenhouse effects due to negative comprehensive global warming potentials (GWPs; ?0.9 to ?8.7 t CO₂‐eq ha^?1 year^?1). Relative to the natural wetlands, all LULCC types (except CWs from coastal wetlands) decreased the net CO₂ uptake by 69.7%?456.6%, due to a higher increase in ecosystem respiration relative to slight changes in gross primary production. The CWs and APs significantly increased the CH₄ emissions compared to those of the coastal wetlands. All LULCC types associated with the riparian wetlands significantly decreased the CH₄ emissions. When the peatlands were converted to the PASs, the CH₄ emissions significantly increased. The CLs, as well as DWs from peatlands, significantly increased the N₂O emissions in the natural wetlands. As a result, all LULCC types (except PASs from riparian wetlands) led to remarkably higher GWPs by 65.4%?2,948.8%, compared to those of the natural wetlands. The variability in GHG fluxes with LULCC was mainly sensitive to changes in soil water content, water table, salinity, soil nitrogen content, soil pH, and bulk density. This study highlights the significant role of LULCC in increasing comprehensive GHG emissions from global natural wetlands, and our results are useful for improving future models and manipulative experiments.     

Fed-Batch Cultivation of Arthrospira platensis Using Carbon Dioxide from Alcoholic Fermentation and Urea as Carbon and Nitrogen Sources

To reduce CO₂ emissions from alcoholic fermentation, Arthrospira platensis was cultivated in tubular photobioreactor using either urea or nitrate as nitrogen sources at different light intensities (60 μmol m^?2 s^?1?≤?I?≤?240 μmol m^?2 s^?1). The type of carbon source (pure CO₂ or CO₂ from fermentation) did not show any appreciable influence on the main cultivation parameters, whereas substitution of nitrate for urea increased the nitrogen-to-cell conversion factor (Y _X/N ), and the maximum cell concentration (X _m ) and productivity (P _X ) increased with I. As a result, the best performance using gaseous emissions from alcoholic fermentation (X _m ?=?2,960?±?35 g m^?3, P _X ?=?425?±?5.9 g m^?3 day^?1 and Y _X/N ?=?15?±?0.2 g g^?1) was obtained at I?=?120 μmol m^?2 s^?1 using urea as nitrogen source. The results obtained in this work demonstrate that the combined use of effluents rich in urea and carbon dioxide could be exploited in large-scale cyanobacteria cultivations to reduce not only the production costs of these photosynthetic microorganisms but also the environmental impact associated to the release of greenhouse emissions.     

Effects of seasonality,transport pathway,and spatial structure on greenhouse gas fluxes in a restored wetland

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open‐water diffusion and ebullition fluxes of CO₂, CH₄, and N₂O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy‐covariance measurements of whole‐ecosystem CO₂ and CH₄ exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open‐water and vegetated cover types. Annual open‐water CO₂, CH₄, and N₂O emissions were 915 ± 95 g C‐CO₂ m^?2 yr^?1, 2.9 ± 0.5 g C‐CH₄ m^?2 yr^?1, and 62 ± 17 mg N‐N₂O m^?2 yr^?1, respectively. Diffusion dominated open‐water GHG transport, accounting for >99% of CO₂ and N₂O emissions, and ~71% of CH₄ emissions. Seasonality was minor for CO₂ emissions, whereas CH₄ and N₂O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open‐water fluxes (3.5 ± 0.3 kg CO₂‐eq m^?2 yr^?1) exceeded that of vegetated zones (1.4 ± 0.4 kg CO₂‐eq m^?2 yr^?1) due to high ecosystem respiration. After scaling results to the entire wetland using object‐based cover classification of remote sensing imagery, net uptake of CO₂ (?1.4 ± 0.6 kt CO₂‐eq yr^?1) did not offset CH₄ emission (3.7 ± 0.03 kt CO₂‐eq yr^?1), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO₂‐eq yr^?1. These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.     

Will CO<Subscript>2</Subscript> Emissions from Drained Tropical Peatlands Decline Over Time? Links Between Soil Organic Matter Quality,Nutrients, and C Mineralization Rates

Conversion, drainage, and cultivation of tropical peatlands can change soil conditions, shifting the C balance of these systems, which is important for the global C cycle. We examined the effect of soil organic matter (SOM) quality and nutrients on CO₂ production from peat decomposition using laboratory incubations of Indonesian peat soils from undrained forest in Kalimantan and drained oil palm plantations in Kalimantan and Sumatra. We found that oil palm soils had higher C/N and lower SOM quality than forest soils. Higher substrate quality and nutrient availability, particularly lower ratios of aromatic/aliphatic carbon and C/N, rather than total SOM or carbon, explained the higher rate of CO₂ production by forest soils (10.80 ± 0.23 µg CO₂–C g C h^?1) compared to oil palm soils (5.34 ± 0.26 µg CO₂–C g C h^?1) from Kalimantan. These factors also explained lower rates in Sumatran oil palm (3.90 ± 0.25 µg CO₂–C g C h^?1). We amended peat with nitrogen (N), phosphorus (P), and glucose to further investigate observed substrate and nutrient constraints across the range of observed peat quality. Available N limited CO₂ production, in unamended and amended soils. P addition raised CO₂ production when substrate quality was high and initial P state was low. Glucose addition raised CO₂ production in the presence of added N and P. Our results suggest that decline in SOM quality and nutrients associated with conversion may decrease substrate-driven rates of CO₂ production from peat decomposition over time.     

Carbon and nitrogen mass balance during flue gas treatment with Dunaliella salina cultures

The biotreatment of flue gases with algae cultures is a promising option to sequestrate CO₂, yet the emission of other greenhouse gases (GHG) from the cultures can hamper their environmental benefit. Quantitative data on the sequestration potential for CO₂ and NO _x in relation to the direct production of CH₄ and N₂O are urgently required. The present study assessed the flows of carbon (C) and nitrogen (N) through cultures of the green alga Dunaliella salina, supplied with biodiesel flue gas, by means of mass balancing. D. salina was grown in artificially lighted, field- (42-L bubble column reactor) and laboratory-scale cultures (23 °C, pH 7.5). In the bubble column reactor, algae grew with an average specific growth rate of 0.237 day^?1 under flue gas supplementation (6.3 % (v/v) CO₂, 1.2 ppmv NO _x ), and CO₂ was retained to 39 % in the system. The specific sequestration rate for CO₂ was low, with 0.13 g CO₂ L^?1 day^?1. Cultures emitted up to 13.03 μg CH₄ L^?1 day^?1 and 4261 μg N₂O L^?1 day^?1. The moderate retention of NO _x -N was outweighed by emissions of N₂O-N, and total N in the system decreased by 15.48 % during the 9-day trial. Results suggest that GHG production was mainly the outcome of anaerobic microbial processes and their emission was lower in pre-sterilized cultures. Under the tested conditions, up to six times more CO₂ equivalents were emitted during flue gas treatment. Therefore, the direct GHG emissions of algae culture systems, intended for flue gas treatment (i.e. open ponds) need to be reviewed critically.     

Long‐term impacts of manure amendments on carbon and greenhouse gas dynamics of rangelands

Livestock manure is applied to rangelands as an organic fertilizer to stimulate forage production, but the long‐term impacts of this practice on soil carbon (C) and greenhouse gas (GHG) dynamics are poorly known. We collected soil samples from manured and nonmanured fields on commercial dairies and found that manure amendments increased soil C stocks by 19.0 ± 7.3 Mg C ha^?1 and N stocks by 1.94 ± 0.63 Mg N ha^?1 compared to nonmanured fields (0–20 cm depth). Long‐term historical (1700–present) and future (present–2100) impacts of management on soil C and N dynamics, net primary productivity (NPP), and GHG emissions were modeled with DayCent. Modeled total soil C and N stocks increased with the onset of dairying. Nitrous oxide (N₂O) emissions also increased by ~2 kg N₂O‐N ha^?1 yr^?1. These emissions were proportional to total N additions and offset 75–100% of soil C sequestration. All fields were small net methane (CH₄) sinks, averaging ?4.7 ± 1.2 kg CH₄‐C ha^?1 yr^?1. Overall, manured fields were net GHG sinks between 1954 and 2011 (?0.74 ± 0.73 Mg CO₂ e ha^?1 yr^?1, CO₂e are carbon dioxide equivalents), whereas nonmanured fields varied around zero. Future soil C pools stabilized 40–60 years faster in manured fields than nonmanured fields, at which point manured fields were significantly larger sources than nonmanured fields (1.45 ± 0.52 Mg CO₂e ha^?1 yr^?1 and 0.51 ± 0.60 Mg CO₂e ha^?1 yr^?1, respectively). Modeling also revealed a large background loss of soil C from the passive soil pool associated with the shift from perennial to annual grasses, equivalent to 29.4 ± 1.47 Tg CO₂e in California between 1820 and 2011. Manure applications increased NPP and soil C storage, but plant community changes and GHG emissions decreased, and eventually eliminated, the net climate benefit of this practice.     

Ecosystem carbon response of an Arctic peatland to simulated permafrost thaw

Permafrost peatlands are biogeochemical hot spots in the Arctic as they store vast amounts of carbon. Permafrost thaw could release part of these long‐term immobile carbon stocks as the greenhouse gases (GHGs) carbon dioxide (CO₂) and methane (CH₄) to the atmosphere, but how much, at which time‐span and as which gaseous carbon species is still highly uncertain. Here we assess the effect of permafrost thaw on GHG dynamics under different moisture and vegetation scenarios in a permafrost peatland. A novel experimental approach using intact plant–soil systems (mesocosms) allowed us to simulate permafrost thaw under near‐natural conditions. We monitored GHG flux dynamics via high‐resolution flow‐through gas measurements, combined with detailed monitoring of soil GHG concentration dynamics, yielding insights into GHG production and consumption potential of individual soil layers. Thawing the upper 10–15 cm of permafrost under dry conditions increased CO₂ emissions to the atmosphere (without vegetation: 0.74 ± 0.49 vs. 0.84 ± 0.60 g CO₂–C m^?2 day^?1; with vegetation: 1.20 ± 0.50 vs. 1.32 ± 0.60 g CO₂–C m^?2 day^?1, mean ± SD, pre‐ and post‐thaw, respectively). Radiocarbon dating (¹⁴C) of respired CO₂, supported by an independent curve‐fitting approach, showed a clear contribution (9%–27%) of old carbon to this enhanced post‐thaw CO₂ flux. Elevated concentrations of CO₂, CH₄, and dissolved organic carbon at depth indicated not just pulse emissions during the thawing process, but sustained decomposition and GHG production from thawed permafrost. Oxidation of CH₄ in the peat column, however, prevented CH₄ release to the atmosphere. Importantly, we show here that, under dry conditions, peatlands strengthen the permafrost–carbon feedback by adding to the atmospheric CO₂ burden post‐thaw. However, as long as the water table remains low, our results reveal a strong CH₄ sink capacity in these types of Arctic ecosystems pre‐ and post‐thaw, with the potential to compensate part of the permafrost CO₂ losses over longer timescales.     

High heterotrophic CO<Subscript>2</Subscript> emissions from a Malaysian oil palm plantations during dry-season

Tropical peatlands are currently being rapidly cleared and drained for the establishment of oil palm plantations, which threatens their globally significant carbon sequestration capacity. Large-scale land conversion of tropical peatlands is important in the context of greenhouse gas emission factors and sustainable land management. At present, quantification of carbon dioxide losses from tropical peatlands is limited by our understanding of the relative contribution of heterotrophic and autotrophic respiration to net peat surface CO₂ emissions. In this study we separated heterotrophic and autotrophic components of peat CO₂ losses from two oil palm plantations (one established in ‘2000’ and the other in 1978, then replanted in ‘2006’) using chamber-based emissions sampling along a transect from the rooting to non-rooting zones on a peatland in Selangor, Peninsular Malaysia over the course of 3 months (June–August, 2014). Collar CO₂ measurements were compared with soil temperature and moisture at site and also accompanied by depth profiles assessing peat C and bulk density. The soil respiration decreased exponentially with distance from the palm trunks with the sharpest decline found for the plantation with the younger palms with overall fluxes of 1341 and 988 mg CO₂ m^?2 h^?1, respectively, at the 2000 and 2006 plantations, respectively. The mean heterotrophic flux was 909 ± SE 136 and 716 ± SE 201 mg m^?2 h^?1 at the 2000 and 2006 plantations, respectively. Autotrophic emissions adjacent to the palm trunks were 845 ± SE 135 and 1558 ± SE 341 mg m^?2 h^?1 at the 2000 and 2006 plantations, respectively. Heterotrophic CO₂ flux was positively related to peat soil moisture, but not temperature. Total peat C stocks were 60 kg m^?2 (down to 1 m depth) and did not vary among plantations of different ages but SOC concentrations declined significantly with depth at both plantations but the decline was sharper in the second generation 2006 plantation. The CO₂ flux values reported in this study suggest a potential for very high carbon (C) loss from drained tropical peats during the dry season. This is particularly concerning given that more intense dry periods related to climate change are predicted for SE Asia. Taken together, this study highlights the need for careful management of tropical peatlands, and the vulnerability of their carbon storage capability under conditions of drainage.     

Carbon balance of rewetted and drained peat soils used for biomass production: a mesocosm study

Rewetting of drained peatlands has been recommended to reduce CO₂ emissions and to restore the carbon sink function of peatlands. Recently, the combination of rewetting and biomass production (paludiculture) has gained interest as a possible land use option in peatlands for obtaining such benefits of lower CO₂ emissions without losing agricultural land. This study quantified the carbon balance (CO₂, CH₄ and harvested biomass C) of rewetted and drained peat soils under intensively managed reed canary grass (RCG) cultivation. Mesocosms were maintained at five different groundwater levels (GWLs), that is 0, 10, 20 cm below the soil surface, representing rewetted peat soils, and 30 and 40 cm below the soil surface, representing drained peat soils. Net ecosystem exchange (NEE) of CO₂ and CH₄ emissions was measured during the growing period of RCG (May to September) using transparent and opaque closed chamber methods. The average dry biomass yield was significantly lower from rewetted peat soils (12 Mg ha^?1) than drained peat soils (15 Mg ha^?1). Also, CO₂ fluxes of gross primary production (GPP) and ecosystem respiration (ER) from rewetted peat soils were significantly lower than from drained peat soils, but net uptake of CO₂ was higher from rewetted peat soils. Cumulative CH₄ emissions were negligible (0.01 g CH₄ m^?2) from drained peat soils but were significantly higher (4.9 g CH₄ m^?2) from rewetted peat soils during measurement period (01 May–15 September 2013). The extrapolated annual C balance was 0.03 and 0.68 kg C m^?2 from rewetted and drained peat soils, respectively, indicating that rewetting and paludiculture can reduce the loss of carbon from peatlands.     

Understory vegetation management affected greenhouse gas emissions and labile organic carbon pools in an intensively managed Chinese chestnut plantation

Background and aims

The impact of understory vegetation control or replacement with selected plant species, which are common forest plantation management practices, on soil C pool and greenhouse gas (GHG, including CO₂, CH₄ and N₂O) emissions are poorly understood. The objective of this paper was to investigate the effects of understory vegetation management on the dynamics of soil GHG emissions and labile C pools in an intensively managed Chinese chestnut (Castanea mollissima Blume) plantation in subtropical China.

Methods

A 12-month field experiment was conducted to study the dynamics of soil labile C pools and GHG emissions in a Chinese chestnut plantation under four different understory management practices: control (Control), understory removal (UR), replacement of understory vegetation with Medicago sativa L. (MS), and replacement with Lolium perenne L. (LP). Soil GHG emissions were determined using the static chamber/GC technique.

Results

Understory management did not change the seasonal pattern of soil GHG emissions; however, as compared with the Control, the UR treatment increased soil CO₂ and N₂O emissions and CH₄ uptake, and the MS and LP treatments increased CO₂ and N₂O emissions and reduced CH₄ uptake (P?<?0.05 for all treatment effects, same below). The total global warming potential (GWP) of GHG emissions in the Control, UR, MS, and LP treatments were 36.56, 39.40, 42.36, and 42.99 Mg CO₂ equivalent (CO₂-e) ha^?1 year^?1, respectively, with CO₂ emission accounting for more than 95 % of total GWP regardless of the understory management treatment. The MS and LP treatments increased soil organic C (SOC), total N (TN), soil water soluble organic C (WSOC) and microbial biomass C (MBC), while the UR treatment decreased SOC, TN and NO₃ ^?-N but had no effect on WSOC and MBC. Soil GHG emissions were correlated with soil temperature and WSOC across the treatments, but had no relationship with soil moisture content and MBC.

Conclusions

Although replacing competitive understory vegetation with legume or less competitive non-legume species increased soil GHG emissions and total GWP, such treatments also increased soil C and N pools and are therefore beneficial for increasing soil C storage, maintaining soil fertility, and enhancing the productivity of Chinese chestnut plantations.