CH4 and N2O emissions from smallholder agricultural systems on tropical peatlands in Southeast Asia

There are limited data for greenhouse gas (GHG) emissions from smallholder agricultural systems in tropical peatlands, with data for non-CO₂ emissions from human-influenced tropical peatlands particularly scarce. The aim of this study was to quantify soil CH₄ and N₂O fluxes from smallholder agricultural systems on tropical peatlands in Southeast Asia and assess their environmental controls. The study was carried out in four regions in Malaysia and Indonesia. CH₄ and N₂O fluxes and environmental parameters were measured in cropland, oil palm plantation, tree plantation and forest. Annual CH₄ emissions (in kg CH₄ ha⁻¹ year⁻¹) were: 70.7 ± 29.5, 2.1 ± 1.2, 2.1 ± 0.6 and 6.2 ± 1.9 at the forest, tree plantation, oil palm and cropland land-use classes, respectively. Annual N₂O emissions (in kg N₂O ha⁻¹ year⁻¹) were: 6.5 ± 2.8, 3.2 ± 1.2, 21.9 ± 11.4 and 33.6 ± 7.3 in the same order as above, respectively. Annual CH₄ emissions were strongly determined by water table depth (WTD) and increased exponentially when annual WTD was above −25 cm. In contrast, annual N₂O emissions were strongly correlated with mean total dissolved nitrogen (TDN) in soil water, following a sigmoidal relationship, up to an apparent threshold of 10 mg N L⁻¹ beyond which TDN seemingly ceased to be limiting for N₂O production. The new emissions data for CH₄ and N₂O presented here should help to develop more robust country level ‘emission factors’ for the quantification of national GHG inventory reporting. The impact of TDN on N₂O emissions suggests that soil nutrient status strongly impacts emissions, and therefore, policies which reduce N-fertilisation inputs might contribute to emissions mitigation from agricultural peat landscapes. However, the most important policy intervention for reducing emissions is one that reduces the conversion of peat swamp forest to agriculture on peatlands in the first place.     

Annual nitrous oxide emissions from open-air and greenhouse vegetable cropping systems in China

Aims

A field experiment was conducted to quantify annual nitrous oxide (N₂O) fluxes from control and fertilized plots under open-air and greenhouse vegetable cropping systems in southeast China. We compiled the reported global field annual N₂O flux measurements to estimate the emission factor of N fertilizer for N₂O and its background emissions from vegetable fields.

Methods

Fluxes of N₂O were measured using static chamber-GC techniques over the 2010–2011 annual cycle with multiple cropping seasons.

Results

The mean annual N₂O fluxes from the controls were 46.1?±?2.3 μg N₂O-N m^?2 hr^?1 and 68.3?±?4.1 μg N₂O-N m^?2 hr^?1 in the open-air and greenhouse vegetable systems, respectively. For the plots receiving 900 kg?N?ha^?1, annual N₂O emissions averaged 90.6?±?8.9 μg N₂O-N m^?2 hr^?1 and 106.4?±?6.6 μg N₂O-N?m^?2 hr^?1 in the open-air and greenhouse vegetable systems, respectively. By pooling published field N₂O flux measurements taken over or close to a full year, the N₂O emission factor for N fertilizer averaged 0.63?±?0.09 %, with a background emission of 2.67?±?0.80 kg N₂O-N ha^?1 in Chinese vegetable fields. Annual N₂O emissions from Chinese vegetable systems were estimated to be 84.7 Gg N₂O-N yr^?1, consisting of 72.5 Gg N₂O-N yr^?1 and 12.2 Gg N₂O-N yr^?1 in the open-air and greenhouse vegetable systems, respectively.

Conclusions

While N₂O emissions from the greenhouse vegetable cropping system tended to be slightly higher compared to the open-air system in our experiment, the synthesis of literature data suggests that N₂O emissions would be greater at low N-rates but smaller at high N-rates in greenhouse systems than in open-air vegetable cropping systems. The estimates of this study suggest that vegetable cropping systems covering 11.4 % in national total cropping area, contributed 21–25 % to the total N₂O emission from Chinese croplands.     

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.     

Greenhouse gas fluxes over managed grasslands in Central Europe

Central European grasslands are characterized by a wide range of different management practices in close geographical proximity. Site‐specific management strategies strongly affect the biosphere–atmosphere exchange of the three greenhouse gases (GHG) carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). The evaluation of environmental impacts at site level is challenging, because most in situ measurements focus on the quantification of CO₂ exchange, while long‐term N₂O and CH₄ flux measurements at ecosystem scale remain scarce. Here, we synthesized ecosystem CO₂, N₂O, and CH₄ fluxes from 14 managed grassland sites, quantified by eddy covariance or chamber techniques. We found that grasslands were on average a CO₂ sink (−1,783 to −91 g CO₂ m⁻² year⁻¹), but a N₂O source (18–638 g CO₂‐eq. m⁻² year⁻¹), and either a CH₄ sink or source (−9 to 488 g CO₂‐eq. m⁻² year⁻¹). The net GHG balance (NGB) of nine sites where measurements of all three GHGs were available was found between −2,761 and −58 g CO₂‐eq. m⁻² year⁻¹, with N₂O and CH₄ emissions offsetting concurrent CO₂ uptake by on average 21 ± 6% across sites. The only positive NGB was found for one site during a restoration year with ploughing. The predictive power of soil parameters for N₂O and CH₄ fluxes was generally low and varied considerably within years. However, after site‐specific data normalization, we identified environmental conditions that indicated enhanced GHG source/sink activity (“sweet spots”) and gave a good prediction of normalized overall fluxes across sites. The application of animal slurry to grasslands increased N₂O and CH₄ emissions. The N₂O‐N emission factor across sites was 1.8 ± 0.5%, but varied considerably at site level among the years (0.1%–8.6%). Although grassland management led to increased N₂O and CH₄ emissions, the CO₂ sink strength was generally the most dominant component of the annual GHG budget.     

Effect of stand age on greenhouse gas fluxes from a Sitka spruce [Picea sitchensis (Bong.) Carr.] chronosequence on a peaty gley soil

The influence of forest stand age in a Picea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs – CO₂, CH₄ and N₂O) and (2) overall net ecosystem global warming potential (GWP), was investigated in a 2‐year study. The objective was to isolate the effect of forest stand age on soil edaphic characteristics (temperature, water table and volumetric moisture) and the consequent influence of these characteristics on the GHG fluxes. Fluxes were measured in a chronosequence in Harwood, England, with sites comprising 30‐ and 20‐year‐old second rotation forest and a site clearfelled (CF) some 18 months before measurement. Adjoining unforested grassland (UN) acted as a control. Comparisons were made between flux data, soil temperature and moisture data and, at the 30‐year‐old and CF sites, eddy covariance data for net ecosystem carbon (C) exchange (NEE). The main findings were: firstly, integrated CO₂ efflux was the dominant influence on the GHG budget, contributing 93–94% of the total GHG flux across the chronosequence compared with 6–7% from CH₄ and N₂O combined. Secondly, there were clear links between the trends in edaphic factors as the forest matured, or after clearfelling, and the emission of GHGs. In the chronosequence sites, annual fluxes of CO₂ were lower at the 20‐year‐old (20y) site than at the 30‐year‐old (30y) and CF sites, with soil temperature the dominant control. CH₄ efflux was highest at the CF site, with peak flux 491±54.5 μg m⁻² h⁻¹ and maximum annual flux 18.0±1.1 kg CH₄ ha⁻¹ yr⁻¹. No consistent uptake of CH₄ was noted at any site. A linear relationship was found between log CH₄ flux and the closeness of the water table to the soil surface across all sites. N₂O efflux was highest in the 30y site, reaching 108±38.3 μg N₂O‐N m⁻² h⁻¹ (171 μg N₂O m⁻² h⁻¹) in midsummer and a maximum annual flux of 4.7±1.2 kg N₂O ha⁻¹ yr⁻¹ in 2001. Automatic chamber data showed a positive exponential relationship between N₂O flux and soil temperature at this site. The relationship between N₂O emission and soil volumetric moisture indicated an optimum moisture content for N₂O flux of 40–50% by volume. The relationship between C : N ratio data and integrated N₂O flux was consistent with a pattern previously noted across temperate and boreal forest soils.     

Methane and nitrous oxide fluxes in annual and perennial land-use systems of the irrigated areas in the Aral Sea Basin

Land use and agricultural practices can result in important contributions to the global source strength of atmospheric nitrous oxide (N₂O) and methane (CH₄). However, knowledge of gas flux from irrigated agriculture is very limited. From April 2005 to October 2006, a study was conducted in the Aral Sea Basin, Uzbekistan, to quantify and compare emissions of N₂O and CH₄ in various annual and perennial land-use systems: irrigated cotton, winter wheat and rice crops, a poplar plantation and a natural Tugai (floodplain) forest. In the annual systems, average N₂O emissions ranged from 10 to 150 μg N₂O-N m⁻² h⁻¹ with highest N₂O emissions in the cotton fields, covering a similar range of previous studies from irrigated cropping systems. Emission factors (uncorrected for background emission), used to determine the fertilizer-induced N₂O emission as a percentage of N fertilizer applied, ranged from 0.2% to 2.6%. Seasonal variations in N₂O emissions were principally controlled by fertilization and irrigation management. Pulses of N₂O emissions occurred after concomitant N-fertilizer application and irrigation. The unfertilized poplar plantation showed high N₂O emissions over the entire study period (30 μg N₂O-N m⁻² h⁻¹), whereas only negligible fluxes of N₂O (<2 μg N₂O-N m⁻² h⁻¹) occurred in the Tugai. Significant CH₄ fluxes only were determined from the flooded rice field: Fluxes were low with mean flux rates of 32 mg CH₄ m⁻² day⁻¹ and a low seasonal total of 35.2 kg CH₄ ha⁻¹. The global warming potential (GWP) of the N₂O and CH₄ fluxes was highest under rice and cotton, with seasonal changes between 500 and 3000 kg CO₂ eq. ha⁻¹. The biennial cotton–wheat–rice crop rotation commonly practiced in the region would average a GWP of 2500 kg CO₂ eq. ha⁻¹ yr⁻¹. The analyses point out opportunities for reducing the GWP of these irrigated agricultural systems by (i) optimization of fertilization and irrigation practices and (ii) conversion of annual cropping systems into perennial forest plantations, especially on less profitable, marginal lands.     

Riparian nitrogen dynamics in two geomorphologically distinct tropical rain forest watersheds: nitrous oxide fluxes

Fluxes of N₂O at the soil surface, dissolved N₂O in near-surface groundwater, and potential N₂O production rates were measured across riparian catenas in two rain forest watersheds in Puerto Rico. In the Icacos watershed, mean N₂O fluxes were highest at topographic breaks in the landscape (≃ 40–300 μg N₂O-N m⁻² h⁻¹). At other locations in the riparian zone and hillslope, fluxes were lower (⩽ 2 μg N₂O-N m⁻² h⁻¹). This pattern of surface N₂O fluxes was persistent. In the Bisley watershed, mean suface N₂O fluxes were lower (<40 μg N₂O-N m⁻² h⁻¹) and no identifiable spatial or temporal pattern. Although the spatial patterns and intensities of N₂O emissions differed between the two watersheds, surface soils from both sites had a high potential to reduce NO₃ to N₂O (and perhaps N₂). This potential declined sharply with depth as did soil %C, %N, and potential N-mineralization. Simple controls on denitrification (i.e. aeration, nitrate, and carbon) explained characteristics of potential N₂O production in surface and deep soils from riparian and upslope locations. In the field, spatial patterns in these controlling variables were defined by geomorphological differences between the two watersheds, which then explained the spatial patterns of observed N₂O flux     

A comparison between measured and modeled N2O emissions from Inner Mongolian semi-arid grassland

The objectives of this study were (1) to determine the effect of land use on N₂O emissions from Inner Mongolian semi-arid grasslands of China and (2) to evaluate the process-based DNDC model to extrapolate our field measurements from a limited number of sites to a larger temporal and spatial scale. The results suggest the following. Rainfall event was the dominant controlling factor for the seasonal variations of the N₂O fluxes. The seven selected sites exhibited a similar seasonal trend in N₂O emission, despite their different vegetation, land use and textures. In the typical steppe, N₂O fluxes generally decrease with decreasing soil organic C (SOC) and total N content, indicating that soil C and N pools are very important in determining the spatial magnitude of the N₂O flux. N₂O emissions were very small during the entire growing season, averaging only 0.76 g N₂O-N ha^–1 day^–1 for the five typical steppe sites, 0.35 g N₂O-N ha^–1 day^–1 for the mown meadow steppe site, and 0.83 g N₂O-N ha^–1 day^–1 from the cropped meadow steppe site. No enhanced effect due to overgrazing was observed for the N₂O emission from the semi-arid grasslands. This was mainly results from the decreased SOC content due to overgrazing, which may have reduced the promoting effect of increased soil bulk density by trampling and animal excreta. Except for the mown steppe site, the model predictions of the N₂O flux for the six different sites agree well with the observed values (r ² ranging from 0.35 to 0.68). It would be concluded that the DNDC model captured the key driving process for N₂O emission. Nitrification was the predominant process, contributing 64–88% to the N₂O emission. However, in terms of the magnitude of the N₂O emission, further modifications should focus on the underestimated N₂O flux during the spring and autumn periods (nitrification, freeze/thaw cycles) and the effect of topography and the mowing on N₂O emission.     

Impacts of monoculture cropland to alley cropping agroforestry conversion on soil N2O emissions

Monoculture croplands are a major source of global anthropogenic emissions of nitrous oxide (N₂O), a potent greenhouse gas that contributes to ozone depletion. Agroforestry has the potential to reduce N₂O emissions. Presently, there is no systematic comparison of soil N₂O emissions between cropland agroforestry and monoculture systems in Central Europe. We investigated the effects of converting the monoculture cropland system into the alley cropping agroforestry system on soil N₂O fluxes at three sites (each site has paired agroforestry and monoculture) in Germany, where agroforestry combined crop rows and poplar short-rotation coppice (SRC). We measured soil N₂O fluxes monthly over 2 years (March 2018–January 2020) using static vented chambers. Annual soil N₂O emissions from agroforestry ranged from 0.21 to 2.73 kg N ha⁻¹ year⁻¹, whereas monoculture N₂O emissions ranged from 0.34 to 3.00 kg N ha⁻¹ year⁻¹. During the rotation of corn crop, with high fertilization rates, agroforestry reduced soil N₂O emissions by 9% to 56% compared to monocultures. This was mainly caused by low soil N₂O emissions from the unfertilized agroforestry tree rows. Soil N₂O fluxes were predominantly controlled by soil mineral N in both agroforestry and monoculture systems. Our findings suggest that optimized fertilizer input will further enhance the potential of agroforestry for mitigating N₂O emissions.     

Diurnal fluctuations of dissolved nitrous oxide (N2O) concentrations and estimates of N2O emissions from a spring-fed river: implications for IPCC methodology

There is uncertainty in the estimates of indirect nitrous oxide (N₂O) emissions as defined by the Intergovernmental Panel on Climate Change (IPCC). The uncertainty is due to the challenge and dearth of in situ measurements. Recent work in a subtropical stream system has shown the potential for diurnal variability to influence the downstream N transfer, N form, and estimates of in‐stream N₂O production. Studies in temperate stream systems have also shown diurnal changes in stream chemistry. The objectives of this study were to measure N₂O fluxes and dissolved N₂O concentrations from a spring‐fed temperate river to determine if diurnal cycles were occurring. The study was performed during a 72 h period, over a 180 m reach, using headspace chamber methodology. Significant diurnal cycles were observed in radiation, river temperature and chemistry including dissolved N₂O‐N concentrations. These data were used to further assess the IPCC methodology and experimental methodology used. River NO₃‐N and N₂O‐N concentrations averaged 3.0 mg L⁻¹ and 1.6 μg L⁻¹, respectively, with N₂O saturation reaching a maximum of 664%. The N₂O‐N fluxes, measured using chamber methodology, ranged from 52 to 140 μg m⁻² h⁻¹ while fluxes predicted using the dissolved N₂O concentration ranged from 13 to 25 μg m⁻² h⁻¹. The headspace chamber methodology may have enhanced the measured N₂O flux and this is discussed. Diurnal cycles in N₂O% saturation were not large enough to influence downstream N transfer or N form with variability in measured N₂O fluxes greater and more significant than diurnal variability in N₂O% saturation. The measured N₂O fluxes, extrapolated over the study reach area, represented only 6 × 10⁻⁴% of the NO₃‐N that passed through the study reach over a 72 h period. This is only 0.1% of the IPCC calculated flux.     

Drought-induced nitrous oxide flux dynamics in an enclosed tropical forest

El Niño–La Niña cycles strongly influence dry and wet seasons in the tropics and consequently nitrous oxide (N₂O) emissions from tropical rainforest soils. We monitored whole‐system and soil chamber N₂O fluxes during 5‐month‐long droughts in the Biosphere 2 tropical forest to determine how rainfall changes N₂O production. A consistent pattern of N₂O flux changes during drought and subsequent wetting emerged from our experiments. Soil surface drying during the first days of drought, presumably increased gas transport out of the soil, which increased N₂O fluxes. Subsequent drying caused an exponential decrease in whole‐system (4.0±0.1% day^?1) and soil chamber N₂O flux (8.9±0.8% day^?1; south chamber; and 13.7±1.1% day^?1; north chamber), which was highly correlated with soil moisture content. Soil air N₂O concentration ([N₂O]) and flux measurements revealed that surface N₂O production persisted during drought. The first rainfall after drought triggered a N₂O pulse, which amounted to 25% of drought‐associated reduction in N₂O flux and 1.3±0.4% of annual N₂O emissions. Physical displacement of soil air by water and soil chemistry changes during drought could not account for the observed N₂O pulse. We contend that osmotic stress on the microbial biomass must have supplied the N source for pulse N₂O, which was produced at the litter–soil interface. After the pulse, N₂O fluxes were consistently 90% of predrought values. Nitrate change rate, nutrient, [N₂O], and flux analyses suggested that nitrifiers dominated N₂O production during the pulse and denitrifiers during wet conditions. N₂O flux measurements in Biosphere 2, especially during the N₂O pulse, demonstrate that large‐scale integration methods, such as flux towers, are essential for improving ecosystem N₂O flux estimates.     

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.     

Estimation of annual nitrous oxide emissions from a transitional grassland-forest region in Saskatchewan, Canada

The increasing atmospheric N₂O concentration and the imbalance in its global budget have triggered the interest in quantifying N₂O fluxes from various ecosystems. This study was conducted to estimate the annual N₂O emissions from a transitional grassland-forest region in Saskatchewan, Canada. The study region was stratified according to soil texture and land use types, and we selected seven landscapes (sites) to cover the range of soil texture and land use characteristics in the region. The study sites were, in turn, stratified into distinguishable spatial sampling units (i.e., footslope and shoulder complexes), which reflected the differences in soils and soil moisture regimes within a landscape. N₂O emission was measured using a sealed chamber method. Our results showed that water-filled pore space (WFPS) was the variable most correlated to N₂O fluxes. With this finding, we estimated the total N₂O emissions by using regression equations that relate WFPS to N₂O emission, and linking these regression equations with a soil moisture model for predicting WFPS. The average annual fluxes from fertilized cropland, pasture/hay land, and forest areas were 2.00, 0.04, and 0.02 kg N₂O-N ha^–1 yr^–1, respectively. The average annual fluxes for the medium- to fine-textured and sandy-textured areas were 1.40 and 0.04 kg N₂O-N ha^–1 yr^–1, respectively. The weighted-average annual flux for the study region is 0.95 kg N₂O-N ha^–1yr^–1. The fertilized cropped areas covered only 47% of the regional area but contributed about 98% of the regional flux. We found that in the clay loam, cropped site, 2% and 3% of the applied fertilizer were emitted as N₂O on the shoulders and footslopes, respectively.Contribution no. R824 of Saskatchewan Center for Soil Research, Saskatoon, Saskatchewan, Canada     

Microbial N Turnover and N-Oxide (N<Subscript>2</Subscript>O/NO/NO<Subscript>2</Subscript>) Fluxes in Semi-arid Grassland of Inner Mongolia

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N₂O, NO and NO₂ were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1 mg N kg⁻¹ soil dry weight day⁻¹. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N₂O and NO _x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N₂O fluxes varied between 0.39 and 1.60 μg N₂O-N m⁻² h⁻¹, equivalent to 0.03–0.14 kg N₂O-N ha⁻¹ y⁻¹, and were considerably lower than previously reported for the same region. NO _x flux rates ranged between 0.16 and 0.48 μg NO _x -N m⁻² h⁻¹, equivalent to 0.01–0.04 kg NO _x -N ha⁻¹ y⁻¹, respectively. N₂O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance.     

Methane and nitrous oxide exchange in differently fertilised grassland in southern Germany

We examined the effect of fertilisation (200 kg cattle slurry-N ha^–1 year^–1) on the exchange of N₂O and CH₄ in the soil–plant system of meadow agroecosystems in southern Germany. From 1996 to 1998, we regularly determined the gas fluxes (closed chamber method) and associated environmental parameters. N₂O and CH₄ fluxes were not significantly affected by fertilisation. N₂O fluxes at the unfertilised and fertilised plots were small, generally between 50 and –20 g N m^–2 h^–1. We identified some incidents of N₂O uptake. CH₄-C fluxes ranged from 1.3 to –0.2 mg m^–2 h^–1 and were not significantly different from 0 at both plots. We budgeted an annual net emission of 15.5 and 29.6 mg m^–2 N₂O-N and an annual CH₄-C net emission of 184.2 and 122.7 mg m^–2 at the unfertilised and fertilised plots, respectively. Apparently, rapid N mineralization and uptake in the densely rooted topsoil prevents N losses and the inhibition of CH₄ oxidation.     

Measurement of field scale N2O emission fluxes from a wheat crop using micrometeorological techniques

Measurements of N₂O emission fluxes from a 3 ha field of winter wheat were measured using eddy covariance and relaxed eddy accumulation continuously over 10 days during April 1994. The measurements averaged fluxes over approximately 10⁵ m² of the field, which was fertilised with NH₄NO₃ at a rate of 43 kg N ha^-1 at the beginning of the measurements. The emission fluxes became detectable after the first heavy rainfall, which occured 4 days after fertiliser application. Emissions of N₂O increased rapidly during the day following the rain to a maximum of 280 ng N m^-2s^-1 and declined over the following week. During the period of significant emission fluxes, a clear diurnal cycle in N₂O emission was observed, with the daytime maximum coinciding with the soil temperature maximum at 12 cm depth. The temperature dependence of the N₂O emission was equivalent to an activation energy for N₂O production of 108 kJ mol^-1. The N₂O fluxes measured using relaxed eddy accumulation, averaged over 30 to 270 min, were in agreement with those of the eddy covariance system within 60%. The total emission of N₂O over the period of continuous measurement (10 days) was equivalent to about 10 kg N₂O-N, or 0.77% of the N fertiliser applied.     

Carbon storage and fluxes in ponderosa pine forests at different developmental stages

We compared carbon storage and fluxes in young and old ponderosa pine stands in Oregon, including plant and soil storage, net primary productivity, respiration fluxes, eddy flux estimates of net ecosystem exchange (NEE), and Biome‐BGC simulations of fluxes. The young forest (Y site) was previously an old‐growth ponderosa pine forest that had been clearcut in 1978, and the old forest (O site), which has never been logged, consists of two primary age classes (50 and 250 years old). Total ecosystem carbon content (vegetation, detritus and soil) of the O forest was about twice that of the Y site (21 vs. 10 kg C m^?2 ground), and significantly more of the total is stored in living vegetation at the O site (61% vs. 15%). Ecosystem respiration (R_e) was higher at the O site (1014 vs. 835 g C m^?2 year^?1), and it was largely from soils at both sites (77% of R_e). The biological data show that above‐ground net primary productivity (ANPP), NPP and net ecosystem production (NEP) were greater at the O site than the Y site. Monte Carlo estimates of NEP show that the young site is a source of CO₂ to the atmosphere, and is significantly lower than NEP(O) by c. 100 g C m^?2 year^?1. Eddy covariance measurements also show that the O site was a stronger sink for CO₂ than the Y site. Across a 15‐km swath in the region, ANPP ranged from 76 g C m^?2 year^?1 at the Y site to 236 g C m^?2 year^?1 (overall mean 158 ± 14 g C m^?2 year^?1). The lowest ANPP values were for the youngest and oldest stands, but there was a large range of ANPP for mature stands. Carbon, water and nitrogen cycle simulations with the Biome‐BGC model suggest that disturbance type and frequency, time since disturbance, age‐dependent changes in below‐ground allocation, and increasing atmospheric concentration of CO₂ all exert significant control on the net ecosystem exchange of carbon at the two sites. Model estimates of major carbon flux components agree with budget‐based observations to within ± 20%, with larger differences for NEP and for several storage terms. Simulations showed the period of regrowth required to replace carbon lost during and after a stand‐replacing fire (O) or a clearcut (Y) to be between 50 and 100 years. In both cases, simulations showed a shift from net carbon source to net sink (on an annual basis) 10–20 years after disturbance. These results suggest that the net ecosystem production of young stands may be low because heterotrophic respiration, particularly from soils, is higher than the NPP of the regrowth. The amount of carbon stored in long‐term pools (biomass and soils) in addition to short‐term fluxes has important implications for management of forests in the Pacific North‐west for carbon sequestration.     

N2O, CH4 and CO2 emissions from seasonal tropical rainforests and a rubber plantation in Southwest China

The main focus of this study was to evaluate the effects of soil moisture and temperature on temporal variation of N₂O, CO₂ and CH₄ soil-atmosphere exchange at a primary seasonal tropical rainforest (PF) site in Southwest China and to compare these fluxes with fluxes from a secondary forest (SF) and a rubber plantation (RP) site. Agroforestry systems, such as rubber plantations, are increasingly replacing primary and secondary forest systems in tropical Southwest China and thus effect the N₂O emission in these regions on a landscape level. The mean N₂O emission at site PF was 6.0 ± 0.1 SE μg N m⁻² h⁻¹. Fluxes of N₂O increased from <5 μg N m⁻² h⁻¹ during dry season conditions to up to 24.5 μg N m⁻² h⁻¹ with re-wetting of the soil by the onset of first rainfall events. Comparable fluxes of N₂O were measured in the SF and RP sites, where mean N₂O emissions were 7.3 ± 0.7 SE μg N m⁻² h⁻¹ and 4.1 ± 0.5 SE μg N m⁻² h⁻¹, respectively. The dependency of N₂O fluxes on soil moisture levels was demonstrated in a watering experiment, however, artificial rainfall only influenced the timing of N₂O emission peaks, not the total amount of N₂O emitted. For all sites, significant positive correlations existed between N₂O emissions and both soil moisture and soil temperature. Mean CH₄ uptake rates were highest at the PF site (−29.5 ± 0.3 SE μg C m⁻² h⁻¹), slightly lower at the SF site (−25.6 ± 1.3 SE μg C m⁻² h⁻¹) and lowest for the RP site (−5.7 ± 0.5 SE μg C m⁻² h⁻¹). At all sites, CH₄ uptake rates were negatively correlated with soil moisture, which was also reflected in the lower uptake rates measured in the watering experiment. In contrast to N₂O emissions, CH₄ uptake did not significantly correlate with soil temperature at the SF and RP sites, and only weakly correlated at the PF site. Over the 2 month measurement period, CO₂ emissions at the PF site increased significantly from 50 mg C m⁻² h⁻¹ up to 100 mg C m⁻² h⁻¹ (mean value 68.8 ± 0.8 SE mg C m⁻² h⁻¹), whereas CO₂ emissions at the SF and RP site where quite stable and varied only slightly around mean values of 38.0 ± 1.8 SE mg C m⁻² h⁻¹ (SF) and 34.9 ± 1.1 SE mg C m⁻² h⁻¹ (RP). A dependency of soil CO₂ emissions on changes in soil water content could be demonstrated for all sites, thus, the watering experiment revealed significantly higher CO₂ emissions as compared to control chambers. Correlation of CO₂ emissions with soil temperature was significant at the PF site, but weak at the SF and not evident at the RP site. Even though we demonstrated that N and C trace gas fluxes significantly varied on subdaily and daily scales, weekly measurements would be sufficient if only the sink/ source strength of non-managed tropical forest sites needs to be identified.     

Soil–atmosphere exchange of greenhouse gases in a Eucalyptus marginata woodland, a clover-grass pasture, and Pinus radiata and Eucalyptus globulus plantations

Soils provide the largest terrestrial carbon store, the largest atmospheric CO₂ source, the largest terrestrial N₂O source and the largest terrestrial CH₄ sink, as mediated through root and soil microbial processes. A change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease. We measured soil–atmosphere exchange of CO₂, N₂O and CH₄ in four adjacent land‐use systems (native eucalypt woodland, clover‐grass pasture, Pinus radiata and Eucalyptus globulus plantation) for short, but continuous, periods between October 2005 and June 2006 using an automated trace gas measurement system near Albany in southwest Western Australia. Mean N₂O emission in the pasture was 26.6 μg N m⁻² h⁻¹, significantly greater than in the natural and managed forests (< 2.0 μg N m⁻² h⁻¹). N₂O emission from pasture soil increased after rainfall events (up to 100 μg N m⁻² h⁻¹) and as soil water content increased into winter, whereas no soil water response was detected in the forest systems. Gross nitrification through ¹⁵N isotope dilution in all land‐use systems was small at water holding capacity < 30%, and under optimum soil water conditions gross nitrification ranged between < 0.1 and 1.0 mg N kg⁻¹ h⁻¹, being least in the native woodland/eucalypt plantation < pine plantation < pasture. Forest soils were a constant CH₄ sink, up to −20 μg C m⁻² h⁻¹ in the native woodland. Pasture soil was an occasional CH₄ source, but weak CH₄ sink overall (−3 μg C m⁻² h⁻¹). There were no strong correlations (R < 0.4) between CH₄ flux and soil moisture or temperature. Soil CO₂ emissions (35–55 mg C m⁻² h⁻¹) correlated with soil water content (R < 0.5) in all but the E. globulus plantation. Soil N₂O emissions from improved pastures can be considerable and comparable with intensively managed, irrigated and fertilised dairy pastures. In all land uses, soil N₂O emissions exceeded soil CH₄ uptake on a carbon dioxide equivalent basis. Overall, afforestation of improved pastures (i) decreases soil N₂O emissions and (ii) increases soil CH₄ uptake.     

Effects of Litter Inputs on N<Subscript>2</Subscript>O Emissions from a Tropical Rainforest in Southwest China

Litter inputs are expected to have a strong impact on soil N₂O efflux. This study aimed to assess the effects of the litter decomposition process and nutrient efflux from litter to soil on soil N₂O efflux in a tropical rainforest. A paired study with a control (L) treatment and a litter-removed (NL) treatment was followed for 2 years, continuously monitoring the effects of these treatments on soil N₂O efflux, fresh litter input, decomposed litter carbon (LCI) and nitrogen (LNI), soil nitrate (NO₃ ^?–N), ammonium (NH₄ ⁺–N), dissolved organic carbon (DOC), and dissolved nitrogen (DN). Soil N₂O flux was 0.48 and 0.32 kg N₂O–N ha^?1 year^?1 for the L and NL treatments, respectively. Removing the litter caused a decrease in the annual soil N₂O emission by 33%. The flux values from the litter layer were higher in the rainy season as compared to the dry season (2.10 ± 0.28 vs. 1.44 ± 0.35 μg N m^?2 h^?1). The N₂O fluxes were significantly correlated with the soil NO₃ ^?–N contents (P < 0.05), indicating that the N₂O emission was derived mainly from denitrification as well as other NO₃ ^? reduction processes. Suitable soil temperature and moisture sustained by rainfall were jointly attributed to the higher soil N₂O fluxes of both treatments in the rainy season. The N₂O fluxes from the L were mainly regulated by LCI, whereas those from the NL were dominated jointly by soil NO₃ ^? content and temperature. The effects of LCI and LNI on the soil N₂O fluxes were the greatest in the 2 months after litter decomposition. Our results show that litter may affect not only the variability in the quantity of N₂O emitted, but also the mechanisms that govern N₂O production. However, further studies are still required to elucidate the impacting mechanisms of litter decomposition on N₂O emission from tropical forests.