Soil‐derived trace gas fluxes from different energy crops – results from a field experiment in Southwest Germany

Willow coppice, energy maize and Miscanthus were evaluated regarding their soil‐derived trace gas emission potential involving a nonfertilized and a crop‐adapted slow‐release nitrogen (N) fertilizer scheme. The N application rate was 80 kg N ha^?1 yr^?1 for the perennial crops and 240 kg N ha^?1 yr^?1 for the annual maize. A replicated field experiment was conducted with 1‐year measurements of soil fluxes of CH₄, CO₂ and N₂O in weekly intervals using static chambers. The measurements revealed a clear seasonal trend in soil CO₂ emissions, with highest emissions being found for the N‐fertilized Miscanthus plots (annual mean: 50 mg C m^?² h^?1). Significant differences between the cropping systems were found in soil N₂O emissions due to their dependency on amount and timing of N fertilization. N‐fertilized maize plots had highest N₂O emissions by far, which accumulated to 3.6 kg N₂O ha^?1 yr^?1. The contribution of CH₄ fluxes to the total soil greenhouse gas subsumption was very small compared with N₂O and CO₂. CH₄ fluxes were mostly negative indicating that the investigated soils mainly acted as weak sinks for atmospheric CH₄. To identify the system providing the best ratio of yield to soil N₂O emissions, a subsumption relative to biomass yields was calculated. N‐fertilized maize caused the highest soil N₂O emissions relative to dry matter yields. Moreover, unfertilized maize had higher relative soil N₂O emissions than unfertilized Miscanthus and willow. These results favour perennial crops for bioenergy production, as they are able to provide high yields with low N₂O emissions in the field.     

Soil nitrous oxide and methane fluxes are low from a bioenergy crop (canola) grown in a semi-arid climate

Understanding nitrous oxide (N₂O) and methane (CH₄) fluxes from agricultural soils in semi‐arid climates is necessary to fully assess greenhouse gas emissions from bioenergy cropping systems, and to improve our knowledge of global terrestrial gaseous exchange. Canola is grown globally as a feedstock for biodiesel production, however, resulting soil greenhouse gas fluxes are rarely reported for semi‐arid climates. We measured soil N₂O and CH₄ fluxes from a rain‐fed canola crop in a semi‐arid region of south‐western Australia for 1 year on a subdaily basis. The site included N fertilized (75 kg N ha^?1 yr^?1) and nonfertilized plots. Daily N₂O fluxes were low (?1.5 to 4.7 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 128 g N₂O‐N ha^?1 (standard error, 12 g N₂O‐N ha^?1) from N fertilized soil and 80 g N₂O‐N ha^?1 (standard error, 11 g N₂O‐N ha^?1) from nonfertilized soil. Daily CH₄ fluxes were also low (?10.3 to 11.9 g CH₄‐C ha^?1 day^?1), and did not differ with treatments, with an average annual net emission of 6.7 g CH₄–C ha^?1 (standard error, 20 g CH₄–C ha^?1). Greatest daily N₂O fluxes occurred when the soil was fallow, and following a series of summer rainfall events. Summer rainfall increased soil water contents and available N, and occurred when soil temperatures were >25 °C, and when there was no active plant growth to compete with soil microorganisms for mineralized N; conditions known to promote N₂O production. The proportion of N fertilizer emitted as N₂O, after correction for emissions from the no N fertilizer treatment, was 0.06%; 17 times lower than IPCC default value for the application of synthetic N fertilizers to land (1.0%). Soil greenhouse gas fluxes from bioenergy crop production in semi‐arid regions are likely to have less influence on the net global warming potential of biofuel production than in temperate climates.     

Nitrous oxide emissions during establishment of eight alternative cellulosic bioenergy cropping systems in the North Central United States

Greenhouse gas (GHG) emissions from soils are a key sustainability metric of cropping systems. During crop establishment, disruptive land‐use change is known to be a critical, but under reported period, for determining GHG emissions. We measured soil N₂O emissions and potential environmental drivers of these fluxes from a three‐year establishment‐phase bioenergy cropping systems experiment replicated in southcentral Wisconsin (ARL) and southwestern Michigan (KBS). Cropping systems treatments were annual monocultures (continuous corn, corn–soybean–canola rotation), perennial monocultures (switchgrass, miscanthus, and poplar), and perennial polycultures (native grass mixture, early successional community, and restored prairie) all grown using best management practices specific to the system. Cumulative three‐year N₂O emissions from annuals were 142% higher than from perennials, with fertilized perennials 190% higher than unfertilized perennials. Emissions ranged from 3.1 to 19.1 kg N₂O‐N ha^?1 yr^?1 for the annuals with continuous corn > corn–soybean–canola rotation and 1.1 to 6.3 kg N₂O‐N ha^?1 yr^?1 for perennials. Nitrous oxide peak fluxes typically were associated with precipitation events that closely followed fertilization. Bayesian modeling of N₂O fluxes based on measured environmental factors explained 33% of variability across all systems. Models trained on single systems performed well in most monocultures (e.g., R² = 0.52 for poplar) but notably worse in polycultures (e.g., R² = 0.17 for early successional, R² = 0.06 for restored prairie), indicating that simulation models that include N₂O emissions should be parameterized specific to particular plant communities. Our results indicate that perennial bioenergy crops in their establishment phase emit less N₂O than annual crops, especially when not fertilized. These findings should be considered further alongside yield and other metrics contributing to important ecosystem services.     

Soil-atmospheric exchange of CO2, CH4, and N2O in three subtropical forest ecosystems in southern China

The magnitude, temporal, and spatial patterns of soil‐atmospheric greenhouse gas (hereafter referred to as GHG) exchanges in forests near the Tropic of Cancer are still highly uncertain. To contribute towards an improvement of actual estimates, soil‐atmospheric CO₂, CH₄, and N₂O fluxes were measured in three successional subtropical forests at the Dinghushan Nature Reserve (hereafter referred to as DNR) in southern China. Soils in DNR forests behaved as N₂O sources and CH₄ sinks. Annual mean CO₂, N₂O, and CH₄ fluxes (mean±SD) were 7.7±4.6 Mg CO₂‐C ha^?1 yr^?1, 3.2±1.2 kg N₂O‐N ha^?1 yr^?1, and 3.4±0.9 kg CH₄‐C ha^?1 yr^?1, respectively. The climate was warm and wet from April through September 2003 (the hot‐humid season) and became cool and dry from October 2003 through March 2004 (the cool‐dry season). The seasonality of soil CO₂ emission coincided with the seasonal climate pattern, with high CO₂ emission rates in the hot‐humid season and low rates in the cool‐dry season. In contrast, seasonal patterns of CH₄ and N₂O fluxes were not clear, although higher CH₄ uptake rates were often observed in the cool‐dry season and higher N₂O emission rates were often observed in the hot‐humid season. GHG fluxes measured at these three sites showed a clear increasing trend with the progressive succession. If this trend is representative at the regional scale, CO₂ and N₂O emissions and CH₄ uptake in southern China may increase in the future in light of the projected change in forest age structure. Removal of surface litter reduced soil CO₂ effluxes by 17–44% in the three forests but had no significant effect on CH₄ absorption and N₂O emission rates. This suggests that microbial CH₄ uptake and N₂O production was mainly related to the mineral soil rather than in the surface litter layer.     

Long‐term nutrient addition increases respiration and nitrous oxide emissions in a New England salt marsh

Salt marshes may act either as greenhouse gas (GHG) sources or sinks depending on hydrological conditions, vegetation communities, and nutrient availability. In recent decades, eutrophication has emerged as a major driver of change in salt marsh ecosystems. An ongoing fertilization experiment at the Great Sippewissett Marsh (Cape Cod, USA) allows for observation of the results of over four decades of nutrient addition. Here, nutrient enrichment stimulated changes to vegetation communities that, over time, have resulted in increased elevation of the marsh platform. In this study, we measured fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in dominant vegetation zones along elevation gradients of chronically fertilized (1,572 kg N ha^?1 year^?1) and unfertilized (12 kg N ha^?1 year^?1) experimental plots at Great Sippewissett Marsh. Flux measurements were performed using darkened chambers to focus on community respiration and excluded photosynthetic CO₂ uptake. We hypothesized that N‐replete conditions in fertilized plots would result in larger N₂O emissions relative to control plots and that higher elevations caused by nutrient enrichment would support increased CO₂ and N₂O and decreased CH₄ emissions due to the potential for more oxygen diffusion into sediment. Patterns of GHG emission supported our hypotheses. Fertilized plots were substantially larger sources of N₂O and had higher community respiration rates relative to control plots, due to large emissions of these GHGs at higher elevations. While CH₄ emissions displayed a negative relationship with elevation, they were generally small across elevation gradients and nutrient enrichment treatments. Our results demonstrate that at decadal scales, vegetation community shifts and associated elevation changes driven by chronic eutrophication affect GHG emission from salt marshes. Results demonstrate the necessity of long‐term fertilization experiments to understand impacts of eutrophication on ecosystem function and have implications for how chronic eutrophication may impact the role that salt marshes play in sequestering C and N.     

Initial nitrous oxide,carbon dioxide,and methane costs of converting conservation reserve program grassland to row crops under no‐till vs. conventional tillage

Around 4.4 million ha of land in USDA Conservation Reserve Program (CRP) contracts will expire between 2013 and 2018 and some will likely return to crop production. No‐till (NT) management offers the potential to reduce the global warming costs of CO₂, CH₄, and N₂O emissions during CRP conversion, but to date there have been no CRP conversion tillage comparisons. In 2009, we converted portions of three 9–21 ha CRP fields in Michigan to conventional tillage (CT) or NT soybean production and reserved a fourth field for reference. Both CO₂ and N₂O fluxes increased following herbicide application in all converted fields, but in the CT treatment substantial and immediate N₂O and CO₂ fluxes occurred after tillage. For the initial 201‐day conversion period, average daily N₂O fluxes (g N₂O‐N ha^?1 d^?1) were significantly different in the order: CT (47.5 ± 6.31, n = 6) ? NT (16.7 ± 2.45, n = 6) ? reference (2.51 ± 0.73, n = 4). Similarly, soil CO₂ fluxes in CT were 1.2 times those in NT and 3.1 times those in the unconverted CRP reference field. All treatments were minor sinks for CH₄ (?0.69 ± 0.42 to ?1.86 ± 0.37 g CH₄–C ha^?1 d^?1) with no significant differences among treatments. The positive global warming impact (GWI) of converted soybean fields under both CT (11.5 Mg CO₂e ha^?1) and NT (2.87 Mg CO₂e ha^?1) was in contrast to the negative GWI of the unconverted reference field (?3.5 Mg CO₂e ha^?1) with on‐going greenhouse gas (GHG) mitigation. N₂O contributed 39.3% and 55.0% of the GWI under CT and NT systems with the remainder contributed by CO₂ (60.7% and 45.0%, respectively). Including foregone mitigation, we conclude that NT management can reduce GHG costs by ~60% compared to CT during initial CRP conversion.     

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 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.     

Molecular mechanisms of water table lowering and nitrogen deposition in affecting greenhouse gas emissions from a Tibetan alpine wetland

Rapid climate change and intensified human activities have resulted in water table lowering (WTL) and enhanced nitrogen (N) deposition in Tibetan alpine wetlands. These changes may alter the magnitude and direction of greenhouse gas (GHG) emissions, affecting the climate impact of these fragile ecosystems. We conducted a mesocosm experiment combined with a metagenomics approach (GeoChip 5.0) to elucidate the effects of WTL (?20 cm relative to control) and N deposition (30 kg N ha^?1 yr^?1) on carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes as well as the underlying mechanisms. Our results showed that WTL reduced CH₄ emissions by 57.4% averaged over three growing seasons compared with no‐WTL plots, but had no significant effect on net CO₂ uptake or N₂O flux. N deposition increased net CO₂ uptake by 25.2% in comparison with no‐N deposition plots and turned the mesocosms from N₂O sinks to N₂O sources, but had little influence on CH₄ emissions. The interactions between WTL and N deposition were not detected in all GHG emissions. As a result, WTL and N deposition both reduced the global warming potential (GWP) of growing season GHG budgets on a 100‐year time horizon, but via different mechanisms. WTL reduced GWP from 337.3 to ?480.1 g CO₂‐eq m^?2 mostly because of decreased CH₄ emissions, while N deposition reduced GWP from 21.0 to ?163.8 g CO₂‐eq m^?2, mainly owing to increased net CO₂ uptake. GeoChip analysis revealed that decreased CH₄ production potential, rather than increased CH₄ oxidation potential, may lead to the reduction in net CH₄ emissions, and decreased nitrification potential and increased denitrification potential affected N₂O fluxes under WTL conditions. Our study highlights the importance of microbial mechanisms in regulating ecosystem‐scale GHG responses to environmental changes.     

Aboveground plant biomass,carbon, and nitrogen dynamics before and after burning in a seminatural grassland of Miscanthus sinensis in Kumamoto,Japan

Although fire has been used for several thousand years to maintain Miscanthus sinensis grasslands in Japan, there is little information about the nutrient dynamics in these ecosystems immediately after burning. We investigated the loss of aboveground biomass; carbon (C) and nitrogen (N) dynamics; surface soil C change before and after burning; and carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes 2 h after burning in a M. sinensis grassland in Kumamoto, Japan. We calculated average C and N accumulation rates within the soil profile over the past 7300 years, which were 58.0 kg C ha^?1 yr^?1 and 2.60 kg N ha^?1 yr^?1, respectively. After burning, 98% of aboveground biomass and litter were consumed. Carbon remaining on the field, however, was 102 kg C ha^?1. We found at least 43% of C was possibly lost due to decomposition. However, remaining C, which contained ash and charcoal, appeared to contribute to C accumulation in soil. There was no difference in the amount of 0–5 cm surface soil C before and after burning. The amount of remaining litter on the soil surface indicated burning appeared not to have caused a reduction in soil C nor did it negatively impact the sub‐surface vegetative crown of M. sinensis. Also, nearly 50 kg N ha^?1 of total aboveground biomass and litter N was lost due to burning. Compared with before the burning event, postburning CO₂ and CH₄ fluxes from soil appeared not to be directly affected by burning. However, it appears the short time span of measurements of N₂O flux after burning sufficiently characterized the pattern of increasing N₂O fluxes immediately after burning. These findings indicate burning did not cause significant reductions in soil C nor did it result in elevated CO₂ and CH₄ emissions from the soil relative to before the burning event.     

Nitrous oxide emissions from a cropped soil in a semi-arid climate

Understanding nitrous oxide (N₂O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N₂O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha^?1 yr^?1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N₂O using gas chromatography. Daily N₂O emissions were low (?1.8 to 7.3 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 0.11 kg N₂O‐N ha^?1 from N‐fertilized soil and 0.09 kg N₂O‐N ha^?1 from nonfertilized soil. Over half (55%) the annual N₂O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N₂O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N₂O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N₂O emission, demonstrating that a proportion of N₂O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N₂O emissions.     

Environmental factors controlling temporal and spatial variability in the soil‐atmosphere exchange of CO2, CH4 and N2O from an Australian subtropical rainforest

The temporal variations in CO₂, CH₄ and N₂O fluxes were measured over two consecutive years from February 2007 to March 2009 from a subtropical rainforest in south‐eastern Queensland, Australia, using an automated sampling system. A concurrent study using an additional 30 manual chambers examined the spatial variability of emissions distributed across three nearby remnant rainforest sites with similar vegetation and climatic conditions. Interannual variation in fluxes of all gases over the 2 years was minimal, despite large discrepancies in rainfall, whereas a pronounced seasonal variation could only be observed for CO₂ fluxes. High infiltration, drainage and subsequent high soil aeration under the rainforest limited N₂O loss while promoting substantial CH₄ uptake. The average annual N₂O loss of 0.5 ± 0.1 kg N₂O‐N ha^?1 over the 2‐year measurement period was at the lower end of reported fluxes from rainforest soils. The rainforest soil functioned as a sink for atmospheric CH₄ throughout the entire 2‐year period, despite periods of substantial rainfall. A clear linear correlation between soil moisture and CH₄ uptake was found. Rates of uptake ranged from greater than 15 g CH₄‐C ha^?1 day^?1 during extended dry periods to less than 2–5 g CH₄‐C ha^?1 day^?1 when soil water content was high. The calculated annual CH₄ uptake at the site was 3.65 kg CH₄‐C ha^?1 yr^?1. This is amongst the highest reported for rainforest systems, reiterating the ability of aerated subtropical rainforests to act as substantial sinks of CH₄. The spatial study showed N₂O fluxes almost eight times higher, and CH₄ uptake reduced by over one‐third, as clay content of the rainforest soil increased from 12% to more than 23%. This demonstrates that for some rainforest ecosystems, soil texture and related water infiltration and drainage capacity constraints may play a more important role in controlling fluxes than either vegetation or seasonal variability.     

Soil carbon dioxide and methane fluxes as affected by tillage and N fertilization in dryland conditions

Background and aims

The effects of tillage and N fertilization on CO₂ and CH₄ emissions are a cause for concern worldwide. This paper quantifies these effects in a Mediterranean dryland area.

Methods

CO₂ and CH₄ fluxes were measured in two field experiments. A long-term experiment compared two types of tillage (NT, no-tillage, and CT, conventional intensive tillage) and three N fertilization rates (0, 60 and 120 kg N ha^?1). A short-term experiment compared NT and CT, three N fertilization doses (0, 75 and 150 kg N ha^?1) and two types of fertilizer (mineral N and organic N with pig slurry). Aboveground and root biomass C inputs, soil organic carbon stocks and grain yield were also quantified.

Results

The NT treatment showed a greater mean CO₂ flux than the CT treatment in both experiments. In the long-term experiment CH₄ oxidation was greater under NT, whereas in the short-term experiment it was greater under CT. The fertilization treatments also affected CO₂ emissions in the short-term experiment, with the greatest fluxes when 75 and 150 kg organic N ha^?1 was applied. Overall, the amount of CO₂ emitted ranged between 0.47 and 6.0 kg CO₂?equivalent kg grain^?1. NT lowered yield-scaled emissions in both experiments, but these treatment effects were largely driven by an increase in grain yield.

Conclusions

In dryland Mediterranean agroecosystems the combination of NT and medium rates of either mineral or organic N fertilization can be an appropriate strategy for optimizing CO₂ and CH₄ emissions and grain yield.     

Net annual global warming potential and greenhouse gas intensity in Chinese double rice‐cropping systems: a 3‐year field measurement in long‐term fertilizer experiments

The impact of agricultural management on global warming potential (GWP) and greenhouse gas intensity (GHGI) is not well documented. A long‐term fertilizer experiment in Chinese double rice‐cropping systems initiated in 1990 was used in this study to gain an insight into a complete greenhouse gas accounting of GWP and GHGI. The six fertilizer treatments included inorganic fertilizer [nitrogen and phosphorus fertilizer (NP), nitrogen and potassium fertilizer (NK), and balanced inorganic fertilizer (NPK)], combined inorganic/organic fertilizers at full and reduced rate (FOM and ROM), and no fertilizer application as a control. Methane (CH₄) and nitrous oxide (N₂O) fluxes were measured using static chamber method from November 2006 through October 2009, and the net ecosystem carbon balance was estimated by the changes in topsoil (0–20 cm) organic carbon (SOC) density over the 10‐year period 1999–2009. Long‐term fertilizer application significantly increased grain yields, except for no difference between the NK and control plots. Annual topsoil SOC sequestration rate was estimated to be 0.96 t C ha^?1 yr^?1 for the control and 1.01–1.43 t C ha^?1 yr^?1 for the fertilizer plots. Long‐term inorganic fertilizer application tended to increase CH₄ emissions during the flooded rice season and significantly increased N₂O emissions from drained soils during the nonrice season. Annual mean CH₄ emissions ranged from 621 kg CH₄ ha^?1 for the control to 1175 kg CH₄ ha^?1 for the FOM plots, 63–83% of which derived from the late‐rice season. Annual N₂O emission averaged 1.15–4.11 kg N₂O–N ha^?1 in the double rice‐cropping systems. Compared with the control, inorganic fertilizer application slightly increased the net annual GWPs, while they were remarkably increased by combined inorganic/organic fertilizer application. The GHGI was lowest for the NP and NPK plots and highest for the FOM and ROM plots. The results of this study suggest that agricultural economic viability and GHGs mitigation can be simultaneously achieved by balanced fertilizer application.     

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.     

Exotic Spartina alterniflora invasion alters ecosystem–atmosphere exchange of CH4 and N2O and carbon sequestration in a coastal salt marsh in China

Coastal salt marshes are sensitive to global climate change and may play an important role in mitigating global warming. To evaluate the impacts of Spartina alterniflora invasion on global warming potential (GWP) in Chinese coastal areas, we measured CH₄ and N₂O fluxes and soil organic carbon sequestration rates along a transect of coastal wetlands in Jiangsu province, China, including open water; bare tidal flat; and invasive S. alterniflora, native Suaeda salsa, and Phragmites australis marshes. Annual CH₄ emissions were estimated as 2.81, 4.16, 4.88, 10.79, and 16.98 kg CH₄ ha^?1 for open water, bare tidal flat, and P. australis, S. salsa, and S. alterniflora marshes, respectively, indicating that S. alterniflora invasion increased CH₄ emissions by 57–505%. In contrast, negative N₂O fluxes were found to be significantly and negatively correlated (P < 0.001) with net ecosystem CO₂ exchange during the growing season in S. alterniflora and P. australis marshes. Annual N₂O emissions were 0.24, 0.38, and 0.56 kg N₂O ha^?1 in open water, bare tidal flat and S. salsa marsh, respectively, compared with ‐0.51 kg N₂O ha^?1 for S. alterniflora marsh and ?0.25 kg N₂O ha^?1 for P. australis marsh. The carbon sequestration rate of S. alterniflora marsh amounted to 3.16 Mg C ha^?1 yr^?1 in the top 100 cm soil profile, a value that was 2.63‐ to 8.78‐fold higher than in native plant marshes. The estimated GWP was 1.78, ?0.60, ?4.09, and ?1.14 Mg CO₂eq ha^?1 yr^?1 in open water, bare tidal flat, P. australis marsh and S. salsa marsh, respectively, but dropped to ?11.30 Mg CO₂eq ha^?1 yr^?1 in S. alterniflora marsh. Our results indicate that although S. alterniflora invasion stimulates CH₄ emissions, it can efficiently mitigate increases in atmospheric CO₂ and N₂O along the coast of China.     

Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, China

Aims

A 3-year field experiment (October 2004–October 2007) was conducted to quantify N₂O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

Methods

Static chamber-GC techniques were used to measure soil N₂O fluxes in three treatments (three replicates per treatment): CK (no fertilizer); N150 (300 kg N fertilizer ha^?1 yr^?1 or 150 kg N?ha^?1 per crop); N250 (500 kg N fertilizer ha^?1 yr^?1 kg or 250 kg N?ha^?1 per crop). Nitrate (NO ₃ ^? ) leaching losses were measured at nearby sites using free-drained lysimeters.

Results

The annual N₂O fluxes from the N fertilized treatments were in the range of 1.9 to 6.7 kg N?ha^?1 yr^?1 corresponding to an N₂O emission factor ranging from 0.12 % to 1.06 % (mean value: 0.61 %). The relationship between monthly soil N₂O fluxes and NO ₃ ^- leaching losses can be described by a significant exponential decaying function.

Conclusions

The N₂O emission factor obtained in our study was somewhat lower than the current IPCC default emission factor (1 %). Nitrate leaching, through removal of topsoil NO ₃ ^? , is an underrated regulating factor of soil N₂O fluxes from cropland, especially in the regions where high NO ₃ ^- leaching losses occur.     

Ecosystem–atmosphere exchange of CH4 and N2O and ecosystem respiration in wetlands in the Sanjiang Plain, Northeastern China

Natural wetlands are critically important to global change because of their role in modulating atmospheric concentrations of CO₂, CH₄, and N₂O. One 4‐year continuous observation was conducted to examine the exchanges of CH₄ and N₂O between three wetland ecosystems and the atmosphere as well as the ecosystem respiration in the Sanjiang Plain in Northeastern China. From 2002 to 2005, the mean annual budgets of CH₄ and N₂O, and ecosystem respiration were 39.40 ± 6.99 g C m^?2 yr^?1, 0.124 ± 0.05 g N m^?2 yr^?1, and 513.55 ± 8.58 g C m^?2 yr^?1 for permanently inundated wetland; 4.36 ± 1.79 g C m^?2 yr^?1, 0.11 ± 0.12 g N m^?2 yr^?1, and 880.50 ± 71.72 g C m^?2 yr^?1 for seasonally inundated wetland; and 0.21 ± 0.1 g C m^?2 yr^?1, 0.28 ± 0.11 g N m^?2 yr^?1, and 1212.83 ± 191.98 g C m^?2 yr^?1 for shrub swamp. The substantial interannual variation of gas fluxes was due to the significant climatic variability which underscores the importance of long‐term continuous observations. The apparent seasonal pattern of gas emissions associated with a significant relationship of gas fluxes to air temperature implied the potential effect of global warming on greenhouse gas emissions from natural wetlands. The budgets of CH₄ and N₂O fluxes and ecosystem respiration were highly variable among three wetland types, which suggest the uncertainties in previous studies in which all kinds of natural wetlands were treated as one or two functional types. New classification of global natural wetlands in more detailed level is highly expected.     

Soil-atmosphere exchange of nitrous oxide and methane in New Zealand terrestrial ecosystems and their mitigation options: a review

The two non-CO₂ greenhouse gases (GHGs) nitrous oxide (N₂O) and methane (CH₄) comprise 54.8% of total New Zealand emissions. Nitrous oxide is mainly generated from mineral N originating from animal dung and urine, applied fertiliser N, biologically fixed N₂, and mineralisation of soil organic N. Even though about 96% of the anthropogenic CH₄ emitted in New Zealand is from ruminant animals (methanogenesis), methane uptake by aerobic soils (methanotrophy) can significantly contribute to the removal of CH₄ from the atmpsphere, as the global estimates confirm. Both the net uptake of CH₄ by soils and N₂O emissions from soils are strongly influenced by changes in land use and land management. Quantitative information on the fluxes of these two non-CO₂ GHGs is required for a range of land-use and land-management ecosystems to determine their contribution to the national emissions inventory, and for assessing the potential of mitigation options. Here we report soil N₂O fluxes and CH₄ uptake for a range of land-use and land-management systems collated from published and unpublished New Zealand studies. Nitrous oxide emissions are highest in dairy-grazed pastures (10–12 kg N₂O–N ha^?1 year^{? 1}), intermediate in sheep-grazed pastures, (4–6 kg N₂O–N ha^?1 year^?1), and lowest in forest, shrubland and ungrazed pasture soils (1–2 kg N₂O–N ha^?1 year^?1). N deposited in the form of animal urine and dung, and N applied as fertiliser, are the principal sources of N₂O production. Generally, N₂O emissions from grazed pasture soils are high when the soil water-filled pore-space is above field capacity, and net CH₄ uptake is low or absent. Although nitrification inhibitors have shown some promise in reducing N₂O emissions from grazed pasture systems, their efficacy as an integral part of farm management has yet to be tested. Methane uptake was highest for a New Zealand Beech forest soil (10–11 kg CH₄ ha^?1 year^?1), intermediate in some pine forest soils (4–6 kg CH₄ ha^?1 year^?1), and lowest in most pasture (<1 kg CH₄ ha^?1 year^?1) and cropped soils (1.5 kg CH₄ ha^?1 year^?1). Afforestation /reforestation of pastures results in increases in soil CH₄ uptake, largely as a result of increases in soil aeration status and changes in the population and activities of methanotrophs. Soil CH₄ uptake is also seasonally dependent, being about two to three times higher in a dry summer and autumn than in a wet winter. There are no practical ways yet available to reduce CH₄ emissions from agricultural systems. The mitigation options to reduce gaseous emissions are discussed and future research needs identified.     

Climate- and crop-responsive emission factors significantly alter estimates of current and future nitrous oxide emissions from fertilizer use

The current Intergovernmental Panel on Climate Change (IPCC) default methodology (tier 1) for calculating nitrous oxide (N₂O) emissions from nitrogen applied to agricultural soils takes no account of either crop type or climatic conditions. As a result, the methodology omits factors that are crucial in determining current emissions, and has no mechanism to assess the potential impact of future climate and land‐use change. Scotland is used as a case study to illustrate the development of a new methodology, which retains the simple structure of the IPCC tier 1 methodology, but incorporates crop‐ and climate‐dependent emission factors (EFs). It also includes a factor to account for the effect of soil compaction because of trampling by grazing animals. These factors are based on recent field studies in Scotland and elsewhere in the UK. Under current conditions, the new methodology produces significantly higher estimates of annual N₂O emissions than the IPCC default methodology, almost entirely because of the increased contribution of grazed pasture. Total emissions from applied fertilizer and N deposited by grazing animals are estimated at 10 662 t N₂O‐N yr^?1 using the newly derived EFs, as opposed to 6 796 t N₂O‐N yr^?1 using the IPCC default EFs. On a spatial basis, emission levels are closer to those calculated using field observations and detailed soil modelling than to estimates made using the IPCC default methodology. This can be illustrated by parts of the western Ayrshire basin, which have previously been calculated to emit 8–9 kg N₂O‐N ha^?1 yr^?1 and are estimated here as 6.25–8.75 kg N₂O‐N ha^?1 yr^?1, while the IPCC default methodology gives a maximum emission level of only 3.75 kg N₂O‐N ha^?1 yr^?1 for the whole area. The new methodology is also applied in conjunction with scenarios for future climate‐ and land‐use patterns, to assess how these emissions may change in the future. The results suggest that by 2080, Scottish N₂O emissions may increase by up to 14%, depending on the climate scenario, if fertilizer and land management practices remain unchanged. Reductions in agricultural land use, however, have the potential to mitigate these increases and, depending on the replacement land use, may even reduce emissions to below current levels.