Impacts of climate and land use on N2O and CH4 fluxes from tropical ecosystems in the Mt. Kilimanjaro region,Tanzania

In this study, we quantify the impacts of climate and land use on soil N₂O and CH₄ fluxes from tropical forest, agroforest, arable and savanna ecosystems in Africa. To do so, we measured greenhouse gases (GHG) fluxes from 12 different ecosystems along climate and land‐use gradients at Mt. Kilimanjaro, combining long‐term in situ chamber and laboratory soil core incubation techniques. Both methods showed similar patterns of GHG exchange. Although there were distinct differences from ecosystem to ecosystem, soils generally functioned as net sources and sinks for N₂O and CH₄ respectively. N₂O emissions correlated positively with soil moisture and total soil nitrogen content. CH₄ uptake rates correlated negatively with soil moisture and clay content and positively with SOC. Due to moderate soil moisture contents and the dominance of nitrification in soil N turnover, N₂O emissions of tropical montane forests were generally low (<1.2 kg N ha^?1 year^?1), and it is likely that ecosystem N losses are driven instead by nitrate leaching (~10 kg N ha^?1 year^?1). Forest soils with well‐aerated litter layers were a significant sink for atmospheric CH₄ (up to 4 kg C ha^?1 year^?1) regardless of low mean annual temperatures at higher elevations. Land‐use intensification significantly increased the soil N₂O source strength and significantly decreased the soil CH₄ sink. Compared to decreases in aboveground and belowground carbon stocks enhanced soil non‐CO₂ GHG emissions following land‐use conversion from tropical forests to homegardens and coffee plantations were only a small factor in the total GHG budget. However, due to lower ecosystem carbon stock changes, enhanced N₂O emissions significantly contributed to total GHG emissions following conversion of savanna into grassland and particularly maize. Overall, we found that the protection and sustainable management of aboveground and belowground carbon and nitrogen stocks of agroforestry and arable systems is most crucial for mitigating GHG emissions from land‐use change.     

Fluxes of N2O and CH4 from soils of savannas and seasonally-dry ecosystems

Aim Savannas and seasonally‐dry ecosystems cover a significant part of the world's land surface. If undisturbed, these ecosystems might be expected to show a net uptake of methane (CH₄) and a limited emission of nitrous oxide (N₂O). Land management has the potential to change dramatically the characteristics and gas exchange of ecosystems. The present work investigates the contribution of warm climate seasonally‐dry ecosystems to the atmospheric concentration of nitrous oxide and methane, and analyses the impact of land‐use change on N₂O and CH₄ fluxes from the ecosystems in question. Location Flux data reviewed here were collected from the literature; they come from savannas and seasonally‐dry ecosystems in warm climatic regions, including South America, India, Australasia and Mediterranean areas. Methods Data on gas fluxes were collected from the literature. Two factors were considered as determinants of the variation in gas fluxes: land management and season. Land management was grouped into: (1) control, (2) ‘burned only’ and (3) managed ecosystems. The season was categorized as dry or wet. In order to avoid the possibility that the influence of soil properties on gas fluxes might confound any differences caused by land management, sites were grouped in homogeneous clusters on the basis of soil properties, using multivariate analyses. Inter‐ and intra‐cluster analysis of gas fluxes were performed, taking into account the effects of season, land management and main vegetation types. Results Soils were often acid and nutrient‐poor, with low water retention. N₂O emissions were generally very low (median flux 0.32 mg N₂O m^?2 day^?1), and no significant differences were observed between woodland savannas and managed savannas. The highest fluxes (up to 12.9 mg N₂O m^?2 day^?1) were those on relatively fertile soils with high air‐filled porosity and water retention. The effect of season on N₂O production was evident only when sites were separated in homogeneous groups on the basis of soil properties. CH₄ fluxes varied over a wide range (?22.9 to 3.15 mg CH₄ m^?2 day^?1, where the negative sign denotes removal of gas from the atmosphere), with an annual average daily flux of ?0.48 ± 0.96 (SD) mg CH₄ m^?2 day^?1 in undisturbed (control) sites. Land‐use change dramatically reduced this CH₄ sink. Managed sites were weak sinks of CH₄ in the dry season and became sources of CH₄ in the wet season. This was particularly evident for pastures. Burning alone did not reduce soil net CH₄ oxidation, but decreased N₂O production. Main conclusions Despite the low potential for N₂O production, both in natural and managed conditions, tropical seasonally‐dry ecosystems represent a significant source of N₂O (4.4 Tg N₂O year^?1) on a global scale, as a consequence of the large area they occupy. The same environments represent a potential CH₄ sink of 5.17 Tg CH₄ year^?1. However, assuming that c. 30% of the tropical land is converted to different uses, the sink would be reduced to 3.2 Tg CH₄ year^?1. The limited information on fluxes from Mediterranean ecosystems does not allow a meaningful scaling up.     

Atmospheric methane uptake by tropical montane forest soils and the contribution of organic layers

Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH₄), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH₄ consumption (6.2 Tg CH₄ year^?1), limited data are available on CH₄ exchange from tropical montane forests. We present the results of an extensive study on CH₄ exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH₄ sinks, with decreasing annual uptake rates from 5.9 kg CH₄–C ha^?1 year^?1 at 1,000 m to 0.6 kg CH₄–C ha^?1 year^?1 at 3,000 m. Topography had no effect on soil atmospheric CH₄ uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH₄ uptake rates, mineral nitrogen content of the mineral soil and with CO₂ emissions indicated that the largest CH₄ uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH₄ uptake was N limited instead of inhibited by NH₄ ⁺. Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH₄ budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests.     

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.     

Water, temperature, and vegetation regulation of methyl chloride and methyl bromide fluxes from a shortgrass steppe ecosystem

Temperate grasslands are considered to be a significant sink for CH₃Br, although large uncertainties exist about the magnitude of this sink because of a paucity of field measurements. Here, we report the results of a combined field and laboratory study that investigated the effects of water, temperature, and plant community composition on CH₃Cl and CH₃Br fluxes in a semiarid temperate grassland. A novel stable isotope tracer technique was also employed to deconvolute simultaneous production and oxidation of CH₃Cl and CH₃Br. Net and gross fluxes were measured from different landforms (ridges, floodplains) and cover types (grass‐dominated, shrub‐dominated) to capture a representative range of hydrologic regimes, temperatures, and plant communities. In field experiments, net CH₃Cl and CH₃Br uptake was observed at all grass‐dominated sites (?400±77 nmol CH₃Cl m^?2 day^?1 and ?3.4±0.9 nmol CH₃Br m^?2 day^?1), while net CH₃Cl emission (439±58 nmol CH₃Cl m^?2 day^?1) was observed at sites dominated by the shrub Atriplex canescens, indicating that this plant is a strong CH₃Cl producer. Gross CH₃Cl and CH₃Br oxidation were comparable with estimates from other dryland ecosystems (507±115 nmol CH₃Cl m^?2 day^?1 and 9.1±2.2 nmol CH₃Br m^?2 day^?1), although CH₃Br oxidation rates were at least five times lower than those observed in more mesic temperate grasslands. We suggest that estimates of the temperate grassland CH₃Br sink should be reduced by ≥19% (≥1.8 Gg yr^?1) to account for the weaker sink strength of semiarid environments. Identification of A. canescens as a ‘new’ CH₃Cl source may have important ramifications for the global atmospheric budget of CH₃Cl, given the global distribution of this plant and its congeners and their widespread presence in many dryland ecosystems. Laboratory experiments revealed that soil water was the chief regulator of CH₃Cl and CH₃Br oxidation, while temperature had no observed effect between 14 and 26 °C. Oxidation rates rose most rapidly between 0.4% and 5% volumetric water content, suggesting that methyl halide‐oxidizing bacteria respond strongly to small inputs of water under the very driest conditions. Soil drying and rewetting experiments did not appear to affect the oxidation of CH₃Cl and CH₃Br by soil microorganisms, which are presumably adapted to frequent wet/dry cycles.     

Annual methane uptake by temperate semiarid steppes as regulated by stocking rates,aboveground plant biomass and topsoil air permeability

Overgrazing‐induced degradation of temperate semiarid steppes may affect the soil sink for atmospheric methane (CH₄). However, previous studies have primarily focused on the growing season and on single grazing patterns. Thus, the response of annual CH₄ uptake by steppes compared with various grazing practices is uncertain. In this study, we investigated the effects of grazing on the annual CH₄ uptake by two typical Eurasian semiarid steppes (the Stipa grandis steppe and the Leymus chinensis steppe) located in Inner Mongolia, China. The CH₄ fluxes were measured year‐round using static chambers and gas chromatography at 12 field sites that differed primarily in grazing intensities. Our results indicated that steppe soils were CH₄ sinks throughout the year. The annual CH₄ uptake correlated with stocking rates, whereas the seasonality of CH₄ uptake was primarily dominated by temperature. The annual CH₄ uptake at all sites averaged 3.7±0.7 kg C ha^?1 yr^?1 (range: 2.3–4.5), where approximately 35% (range: 23–40%) occurred during the nongrowing season. Light‐to‐moderate grazing (stocking rate≤1 sheep ha^?1 yr^?1) did not significantly change the annual CH₄ uptake compared with ungrazed steppes, but heavy grazing reduced annual CH₄ uptake significantly (by 24–31%, P<0.05). These findings imply that easing the pressure of heavily grazed steppes (e.g. moving to light or moderate stocking rates) would help restore steppe soil sinks for atmospheric CH₄. The empirical equations based on the significant relationships between annual CH₄ uptake and stocking rates, aboveground plant biomass and topsoil air permeability (P<0.01) could provide simple approaches for the estimation of regional CH₄ uptake by temperate semiarid steppes.     

Landscape controls of CH4 fluxes in a catchment of the forest tundra ecotone in northern Siberia

Terrestrial ecosystems in northern high latitudes exchange large amounts of methane (CH₄) with the atmosphere. Climate warming could have a great impact on CH₄ exchange, in particular in regions where degradation of permafrost is induced. In order to improve the understanding of the present and future methane dynamics in permafrost regions, we studied CH₄ fluxes of typical landscape structures in a small catchment in the forest tundra ecotone in northern Siberia. Gas fluxes were measured using a closed‐chamber technique from August to November 2003 and from August 2006 to July 2007 on tree‐covered mineral soils with and without permafrost, on a frozen bog plateau, and on a thermokarst pond. For areal integration of the CH₄ fluxes, we combined field observations and classification of functional landscape structures based on a high‐resolution Quickbird satellite image. All mineral soils were net sinks of atmospheric CH₄. The magnitude of annual CH₄ uptake was higher for soils without permafrost (1.19 kg CH₄ ha⁻¹ yr⁻¹) than for soils with permafrost (0.37 kg CH₄ ha⁻¹ yr⁻¹). In well‐drained soils, significant CH₄ uptake occurred even after the onset of ground frost. Bog plateaux, which stored large amounts of frozen organic carbon, were also a net sink of atmospheric CH₄ (0.38 kg CH₄ ha⁻¹ yr⁻¹). Thermokarst ponds, which developed from permafrost collapse in bog plateaux, were hot spots of CH₄ emission (approximately 200 kg CH₄ ha⁻¹ yr⁻¹). Despite the low area coverage of thermokarst ponds (only 2.1% of the total catchment area), emissions from these sites resulted in a mean catchment CH₄ emission of 3.8 kg CH₄ ha⁻¹ yr⁻¹. Export of dissolved CH₄ with stream water was insignificant. The results suggest that mineral soils and bog plateaux in this region will respond differently to increasing temperatures and associated permafrost degradation. Net uptake of atmospheric CH₄ in mineral soils is expected to gradually increase with increasing active layer depth and soil drainage. Changes in bog plateaux will probably be much more rapid and drastic. Permafrost collapse in frozen bog plateaux would result in high CH₄ emissions that act as positive feedback to climate warming.     

Land-Use Change and Biogeochemical Controls of Methane Fluxes in Soils of Eastern Amazonia

Tropical soils account for 10%–20% of the 15–35 Tg of atmospheric methane (CH₄) consumed annually by soils, although tropical deforestation could be changing the soil sink. The objectives of this study were (a) to quantify differences in soil CH₄ fluxes among primary forest, secondary forest, active pasture, and degraded pasture in eastern Amazonia; and (b) to investigate controlling mechanisms of CH₄ fluxes, including N availability, gas-phase transport, and soil respiration. At one ranch, Fazenda Vitória, annual uptake estimates (kg CH₄ha⁻¹ y⁻¹) based on monthly measurements were: primary forest, 2.1; secondary forest, 1.0; active pasture, 1.3; degraded pasture, 3.1. The lower annual uptake in the active pasture compared with the primary forest was due to CH₄ production during the wet season in the pasture soils, which is consistent with findings from other studies. In contrast, the degraded pasture was never a CH₄ source. Expressing uptake as a negative flux and emission as a positive flux, CH₄ fluxes were positively correlated with CO₂ fluxes, indicating that root and microbial respiration in the productive pastures, and to a lesser extent in the primary forest, contributed to the formation of anaerobic microsites where CH₄ was produced, whereas this productivity was absent in the degraded pasture. In all land uses, uptake rates of atmospheric CH₄ were greater in the dry season than in the wet season, indicating the importance of soil water content and gas transport on CH₄ fluxes. These clay soils had low annual uptake rates relative to reported rates on sandy soils, which also is consistent with gas transport within the soil being a limiting factor. Nitrogen availability indices did not correlate with CH₄ fluxes, indicating that inhibition of CH₄ oxidation was not an important mechanism explaining differences among land uses. At another ranch, Fazenda Agua Parada, no significant effect of pasture age was observed along a chronosequence of pasture ages. We conclude that land-use change can either increase or decrease the soil sink of CH₄, depending on the duration of wet and dry seasons, the effects of seasonal precipitation on gas-phase transport, and the phenology and relative productivity of the vegetation in each land use.     

Methane uptake rates in Japanese forest soils depend on the oxidation ability of topsoil, with a new estimate for global methane uptake in temperate forest

To clarify the reason for the higher CH₄ uptake rate in Japanese forest soils, twenty-seven sites were established for CH₄ flux measurement. The first order rate constant for CH₄ uptake was also determined using soil core incubation at 14 sites. The CH₄ uptake rate had a seasonal fluctuation, high in summer and low in winter, and the rate correlated with soil temperature at 17 sites. The annual CH₄ uptake rates ranged from 2.7 to 24.8 kg CH₄ ha⁻¹ y⁻¹ (the average of these rates was 9.7 or 10.9 kg CH₄ ha⁻¹ y⁻¹, depending on method of calculation), which is somewhat higher than the uptake rates reported in previous literature. The averaged CH₄ uptake rate correlated closely with the CH₄ oxidation rate of the topsoil (0–5 cm) in the study sites. The CH₄ oxidation constant of the topsoil was explained by a multiple regression model using total pore volume of the soil, nitrate content, and C/N ratio (p < 0.05, R ² = 0.684). This result and comparison with literature data suggest that the high CH₄ uptake rate in Japanese forest soils depends on the high porosity probably due to volcanic ash parent materials. According to our review of the literature, the CH₄ uptake rate in temperate forests in Europe is significantly different from that in Asia and North America. A new global CH₄ uptake rate in temperate forests was estimated to be 5.4 Tg y⁻¹ (1 SE is 1.1 Tg y⁻¹) on a continental basis.     

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.     

Pristine New Zealand forest soil is a strong methane sink

Methanotrophic bacteria oxidize methane (CH₄) in forest soils that cover ～30% of Earth's land surface. The first measurements for a pristine Southern Hemisphere forest are reported here. Soil CH₄ oxidation rate averaged 10.5±0.6 kg CH₄ ha^?1 yr^?1, with the greatest rates in dry warm soil (up to 17 kg CH₄ ha^?1 yr^?1). Methanotrophic activity was concentrated beneath the organic horizon at 50–100 mm depth. Water content was the principal regulator of (r²=0.88) from the most common value of field capacity to less than half of this when the soil was driest. Multiple linear regression analysis showed that soil temperature was not very influential. However, inverse co‐variability confounded the separation of soil water and temperature effects in situ. Fick's law explained the role of water content in regulating gas diffusion and substrate supply to the methanotrophs and the importance of pore size distribution and tortuosity. This analysis also showed that the chambers used in the study did not affect the oxidation rate measurements. The soil was always a net sink for atmospheric CH₄ and no net CH₄ (or nitrous oxide, N₂O) emissions were measured over the 17‐month long study. For New Zealand, national‐scale extrapolation of our data suggested the potential to offset 13% of CH₄ emissions from ca. 90 M ruminant animals. Our average was about 6.5 times higher than rates reported for most Northern Hemisphere forest soils. This very high was attributed to the lack of anthropogenic disturbance for at least 3000–5000 years and the low rate of atmospheric nitrogen deposition. Our truly baseline data could represent a valid preagricultural, preindustrial estimate of the soil sink for temperate latitudes.     

The effect of termite biomass and anthropogenic disturbance on the CH4 budgets of tropical forests in Cameroon and Borneo

The exchange of CH₄ between tropical forests and the atmosphere was determined by simultaneously measuring the net CH₄ flux at the soil surface and assessing the flux contribution from soil-feeding termite biomass, both within the soil profile and in mounds. In Cameroon the flux of CH₄ ranged from a net emission of 40.7 ng m^–2 s^–1 to a net CH₄ oxidation of –53.0 ng m^–2 s^–1. Soil-inhabiting termite biomass was significantly correlated with CH₄ flux. Termite mounds emitted up to 2000 ng s^–1 mound^–1. Termite-derived CH₄ emission reduced the soil sink strength by up to 28%. Disturbance also had a strong effect on the soil sink strength, with the average rate of CH₄ oxidation, at – 17.5 ng m^–2 s^–1, being significantly smaller (≈ 36%) at the secondary forest site than the –27.2 ng m^–2 s^–1, observed at the primary forest site. CH₄ budgets calculated for each site indicated that both forests were net sinks for CH₄ at – 6.1 kg ha^–1 y^–1 in the near-primary forest and – 3.1 kg ha^–1 y^–1 in the secondary forest. In Borneo, three forest sites representing a disturbance gradient were examined. CH₄ oxidation rates ranged from 0 to – 32.1 ng m^–2s^–1 and a significant correlation between the net flux and termite biomass was observed only in an undisturbed primary forest, although the biomass was insufficient to cause net emission of CH₄. Rates of CH₄ oxidation were not significantly different across the disturbance gradient but were, however, larger in the primary forest (averaging – 15.4 ng m^–2 s^–1) than in an old-growth secondary forest (–13.9 ng m^–2s^–1) and a young secondary re-growth (– 10.8 ng m^–2s^–1). CH₄ flux from termite mounds ranged from net oxidation in an abandoned mound to a maximum emission of 468 ng s^–1 mound^–1. CH₄ budgets calculated for each site indicated that CH₄ flux from termite mounds had an insignificant effect on the budget of CH₄ at the regional scale at all three forest sites. Annual oxidation rates were – 4.8, – 4.2 and – 3.4 kg ha^–1 y^–1 in the primary, secondary and young secondary forests, respectively.     

Urbanization can accelerate climate change by increasing soil N2O emission while reducing CH4 uptake

Urban land-use change has the potential to affect local to global biogeochemical carbon (C) and nitrogen (N) cycles and associated greenhouse gas (GHG) fluxes. We conducted a meta-analysis to (1) assess the effects of urbanization-induced land-use conversion on soil nitrous oxide (N₂O) and methane (CH₄) fluxes, (2) quantify direct N₂O emission factors (EF_d) of fertilized urban soils used, for example, as lawns or forests, and (3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N₂O emissions by 153%, to 3.0 kg N ha⁻¹ year⁻¹, while rates of soil CH₄ uptake are reduced by 50%, to 2.0 kg C ha⁻¹ year⁻¹. The global mean annual N₂O EF_d of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N₂O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N₂O emission by 0.46 Tg N₂O-N year⁻¹ and decreased soil CH₄ uptake by 0.58 Tg CH₄-C year⁻¹. Urbanization driven changes in soil N₂O emission and CH₄ uptake are associated with changes in soil properties (bulk density, pH, total N content, and C/N ratio), increased temperature, and management practices, especially fertilizer use. Overall, our meta-analysis shows that urbanization increases soil N₂O emissions and reduces the role of soils as a sink for atmospheric CH₄. These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and by restoring native ecosystems in urban landscapes.     

Climatic role of terrestrial ecosystem under elevated CO2: a bottom‐up greenhouse gases budget

The net balance of greenhouse gas (GHG) exchanges between terrestrial ecosystems and the atmosphere under elevated atmospheric carbon dioxide (CO₂) remains poorly understood. Here, we synthesise 1655 measurements from 169 published studies to assess GHGs budget of terrestrial ecosystems under elevated CO₂. We show that elevated CO₂ significantly stimulates plant C pool (NPP) by 20%, soil CO₂ fluxes by 24%, and methane (CH₄) fluxes by 34% from rice paddies and by 12% from natural wetlands, while it slightly decreases CH₄ uptake of upland soils by 3.8%. Elevated CO₂ causes insignificant increases in soil nitrous oxide (N₂O) fluxes (4.6%), soil organic C (4.3%) and N (3.6%) pools. The elevated CO₂‐induced increase in GHG emissions may decline with CO₂ enrichment levels. An elevated CO₂‐induced rise in soil CH₄ and N₂O emissions (2.76 Pg CO₂‐equivalent year^?1) could negate soil C enrichment (2.42 Pg CO₂ year^?1) or reduce mitigation potential of terrestrial net ecosystem production by as much as 69% (NEP, 3.99 Pg CO₂ year^?1) under elevated CO₂. Our analysis highlights that the capacity of terrestrial ecosystems to act as a sink to slow climate warming under elevated CO₂ might have been largely offset by its induced increases in soil GHGs source strength.     

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.     

Carbon budget and methane and nitrous oxide emissions over the growing season in a Miscanthus sinensis grassland in Tomakomai,Hokkaido, Japan

Species in the Miscanthus genus have been proposed as biofuel crops that have potential to mitigate elevated atmospheric carbon dioxide (CO₂) levels and nitrous oxide (N₂O) and methane (CH₄) emissions. Miscanthus sinensis is widespread throughout Japan and has been used for biomass production for centuries. We assessed the carbon (C) budget and N₂O and CH₄ emissions over the growing season for 2 years in a M. sinensis‐dominated grassland that was naturally established around 1972 in Tomakomai, Hokkaido, Japan, which is near the northern limit for M. sinensis grassland establishment on Andisols. Average C budget was ?0.31 Mg C ha^?1, which indicates C was released from the grassland ecosystem to the atmosphere. Dominant components in the C budget appeared to be aboveground net primary production of plants (1.94–2.80 Mg C ha^?1) and heterotrophic respiration (2.27–3.11 Mg C ha^?1). The measurement of belowground net primary production (BNPP) of plants in the M. sinensis grassland was extremely variable, thus only an approximate value could be calculated. Mean C budget calculated with the approximated BNPP value was 1.47 and ?0.23 Mg C ha^?1 for 2008 and 2009, respectively. Given belowground biomass (9.46–9.86 Mg C ha^?1) was 3.1–6.5 times higher than that of aboveground biomass may provide additional evidence suggesting this grassland represents a C sink. Average CH₄ emissions across years of ?1.34 kg C ha^?1 would indicate this grassland acts as an atmospheric CH₄ sink. Furthermore, average N₂O emissions across years were 0.22 kg N ha^?1. While the site may contribute N₂O to the atmosphere, this value is lower compared with other grassland types. Global warming potential calculated with the approximated BNPP value was ?5.40 and 0.95 Mg CO₂ Eq ha^?1 for 2008 and 2009, respectively, and indicates this grassland could contribute to mitigation of global warming.     

Methane emissions in two drained peat agro-ecosystems with high and low agricultural intensity

Methane (CH₄) emissions were compared for an intensively and extensively managed agricultural area on peat soils in the Netherlands to evaluate the effect of reduced management on the CH₄ balance. Chamber measurements (photoacoustic methods) for CH₄ were performed for a period of three years in the contributing landscape elements in the research sites. Various factors influencing CH₄ emissions were evaluated and temperature of water and soil was found to be the main driver in both sites. For upscaling of CH₄ fluxes to landscape scale, regression models were used which were specific for each of the contributing landforms. Ditches and bordering edges were emission hotspots and emitted together between 60% and 70% of the total terrestrial CH₄ emissions. Annual terrestrial CH₄ fluxes were estimated to be 203 (±48%), 162 (±60%) and 146 (±60%) kg CH₄ ha^?1 and 157 (±63%), 180 (±54%) and 163 (±59%) kg CH₄ ha^?1 in the intensively managed site and extensively managed site, for 2006, 2007 and 2008 respectively. About 70% of the CH₄ was emitted in the summer period. Farm based emissions caused per year an additional 257 kg CH₄ ha^?1 and 172 kg CH₄ ha^?1 for the intensively managed site and extensively managed site, respectively. To further evaluate the effect of agricultural activity on the CH₄ balance, the annual CH₄ fluxes of the two managed sites were also compared to the emissions of a natural peat site with no management and high ground water levels. By comparing the terrestrial and additional farm based emissions of the three sites, we finally concluded that transformation of intensively managed agricultural land to nature development will lead to an increase in terrestrial CH₄ emission, but will not by definition lead to a significant increase in CH₄ emission when farm based emissions are included.     

Does dissolved organic carbon regulate biological methane oxidation in semiarid soils?

In humid ecosystems, the rate of methane (CH₄) oxidation by soil‐dwelling methane‐oxidizing bacteria (MOB) is controlled by soil texture and soil water holding capacity, both of which limit the diffusion of atmospheric CH₄ into the soil. However, it remains unclear whether these same mechanisms control CH₄ oxidation in more arid soils. This study was designed to measure the proximate controls of potential CH₄ oxidation in semiarid soils during different seasons. Using a unique and well‐constrained 3‐million‐year‐old semiarid substrate age gradient, we were able to hold state factors constant while exploring the relationship between seasonal potential CH₄ oxidation rates and soil texture, soil water holding capacity, and dissolved organic carbon (DOC). We measured unexpectedly higher rates of potential CH₄ oxidation in the wet season than the dry season. Although other studies have attributed low CH₄ oxidation rates in dry soils to desiccation of MOB, we present several lines of evidence that this may be inaccurate. We found that soil DOC concentration explained CH₄ oxidation rates better than soil physical factors that regulate the diffusion of CH₄ from the atmosphere into the soil. We show evidence that MOB facultatively incorporated isotopically labeled glucose into their cells, and MOB utilized glucose in a pattern among our study sites that was similar to wet‐season CH₄ oxidation rates. This evidence suggests that DOC, which is utilized by MOB in other environments with varying effects on CH₄ oxidation rates, may be an important regulator of CH₄ oxidation rates in semiarid soils. Our collective understanding of the facultative use of DOC by MOB is still in its infancy, but our results suggest it may be an important factor controlling CH₄ oxidation in soils from dry ecosystems.     

Methane emissions from rice paddies natural wetlands,lakes in China: synthesis new estimate

Sources of methane (CH₄) become highly variable for countries undergoing a heightened period of development due to both human activity and climate change. An urgent need therefore exists to budget key sources of CH₄, such as wetlands (rice paddies and natural wetlands) and lakes (including reservoirs and ponds), which are sensitive to these changes. For this study, references in relation to CH₄ emissions from rice paddies, natural wetlands, and lakes in China were first reviewed and then reestimated based on the review itself. Total emissions from the three CH₄ sources were 11.25 Tg CH₄ yr^?1 (ranging from 7.98 to 15.16 Tg CH₄ yr^?1). Among the emissions, 8.11 Tg CH₄ yr^?1 (ranging from 5.20 to 11.36 Tg CH₄ yr^?1) derived from rice paddies, 2.69 Tg CH₄ yr^?1 (ranging from 2.46 to 3.20 Tg CH₄ yr^?1) from natural wetlands, and 0.46 Tg CH₄ yr^?1 (ranging from 0.33 to 0.59 Tg CH₄ yr^?1) from lakes (including reservoirs and ponds). Plentiful water and warm conditions, as well as its large rice paddy area make rice paddies in southeastern China the greatest overall source of CH₄, accounting for approximately 55% of total paddy emissions. Natural wetland estimates were slightly higher than the other estimates owing to the higher CH₄ emissions recorded within Qinghai‐Tibetan Plateau peatlands. Total CH₄ emissions from lakes were estimated for the first time by this study, with three quarters from the littoral zone and one quarter from lake surfaces. Rice paddies, natural wetlands, and lakes are not constant sources of CH₄, but decreasing ones influenced by anthropogenic activity and climate change. A new progress‐based model used in conjunction with more observations through model‐data fusion approach could help obtain better estimates and insights with regard to CH₄ emissions deriving from wetlands and lakes in China.     

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.