N2O Exchange Research Articles

Substantial uptake of nitrous oxide (N2O) by shoots of mature European beech

Similar to soils, tree stems emit and consume nitrous oxide (N2O) from the atmosphere. Although tree leaves dominate tree surface area, they have been completely excluded from field N2O flux measurements and therefore their role in forest N2O exchange remains unknown. We explored the contribution of leaf fluxes to forest N2O exchange. We determined the N2O exchange of mature European beech (Fagus sylvatica) stems and shoots (i.e., terminal branches) and of adjacent forest floor, in a typical temperate upland forest in Germany. The beech stems, and particularly the shoots, acted as net N2O sinks (−0.254 ± 0.827 μg N2O m−2 stem area h−1 and −4.54 ± 1.53 μg N2O m−2 leaf area h−1, respectively), while the forest floor was a net source (2.41 ± 1.08 μg N2O m−2 soil area h−1). The unstudied tree shoots were identified as a significant contributor to the net ecosystem N2O exchange. Moreover, we revealed for the first time that tree leaves act as substantial N2O sinks. Although this is the first study of its kind, it is of global importance for the proper design of future flux studies in forest ecosystems worldwide. Our results demonstrate that excluding tree leaves from forest N2O flux measurements can lead to misinterpretation of tree and forest N2O exchange, and thus global forest greenhouse gas flux inventories.

N2O and CH4 fluxes from intensively managed grassland: The importance of biological and environmental drivers vs. management

Agriculture is the main contributor to anthropogenic nitrous oxide (N2O) and methane (CH4) emissions. Therefore, mitigation options are urgently needed. In contrast to carbon dioxide, eddy covariance measurements of N2O and CH4 fluxes are still scarce, and thus little is known how environmental and biotic drivers as well as management affect the net N2O and CH4 exchange in grasslands. Thus, we investigated the most important drivers of net ecosystem N2O and CH4 fluxes in a temperate grassland, and continued a N2O mitigation experiment (increased clover proportion vs. fertilization with slurry). Random forest gap-filling models were able to capture intermittent emission peaks, performing better for half-hourly N2O than for CH4 fluxes. The unfertilized clover parcel (parcel B) continued to show lower N2O emissions (4.4 and 2.7 kg N2O-N ha−1 yr−1) compared to the fertilized parcel (parcel A; 6.9 and 5.9 kg N2O-N ha−1 yr−1) for 2019 and 2020, respectively. Tier 1 nitrogen (N) emission factors of 2.6 % and 1.9 % were observed at the fertilized parcel during the study period. Lower soil N concentrations indicated a lower N leaching risk at the clover than at the fertilized parcel. Annual CH4 emissions (including periods with sheep grazing) were similar from both parcels, and ranged from 25 to 38.5 kg CH4-C ha−1. The most important drivers of both N2O and CH4 fluxes were lagged precipitation and water filled pore space, but also management (for N2O from parcel B; CH4 from parcel A). Biotic variables such as vegetation height and leaf area index were important predictors for the N2O exchange, while grazing temporarily increased CH4 emissions. Overall, reducing N fertilization and increasing the legume proportion were effective N2O reduction measures. In particular, adjusting N fertilization to plant N demands can help to avoid high N2O emissions from grasslands.

Marine estuaries act as better sink for greenhouse gases during winter in undisturbed mangrove than degraded ones in Sundarban, India

The estuaries provide the key pathway for travelling carbon across the land-ocean interfaces and behave as both source and sink of greenhouse gases (GHGs) in water-atmosphere systems. The sink-source characteristics of estuaries for GHGs vary spatially. The primary driving factors are adjacent ecologies (agriculture, aquaculture, etc.) and proximities to the sea. To study the sink-source characteristics of estuaries for GHGs (methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2)), the water samples were collected from 53 different locations in the estuaries for estimation of dissolved GHGs concentration and air-water GHGs exchanges. The locations represent five zones (Zone I, II, III, IV and V) based on the type and degradation status of mangroves (degraded and undisturbed), anthropogenic activities, and distance from the sea. Zone I, III, V represents to the degraded mangroves far from sea, whereas, Zone II, IV surrounded by undisturbed mangroves and nearer to sea. The average dissolved CH4 concentrations were higher in the estuaries which were adjacent to degraded mangroves (154.4 nmol L−1) than undisturbed mangroves (81.7 nmol L−1). Further, the average dissolved N2O concentrations were 48% higher in the estuaries nearer to degraded mangroves than that of undisturbed ones. Among the degraded mangrove sites, the dissolved CO2 concentrations were higher at Zone I (30.1 μmol L−1) followed by Zone III and IV, whereas in undisturbed sites, it was higher in Zone IV (22.3 μmol L−1) than Zone II (17.6 μmol L−1). Among the 53 locations, 36, 51 and 33 locations acted as a sink (negative value of exchanges) for CH4, N2O and CO2, respectively. The higher sink potential for CH4 was recorded to those estuaries adjacent to undisturbed mangroves (−791.69 μmol m−2 d−1) than the degraded ones (−23.18 μmol m−2 d−1). Similarly, the average air-water N2O and CO2 exchanges were more negative in the estuaries which were nearer to undisturbed mangroves indicating higher sink potential. The pH, and salinity of the estuary water were negatively correlated with air-water CH4 and N2O exchanges, whereas those were positively correlated with CO2 exchanges. Significantly lower dissolved GHGs and air-water GHGs exchange was observed in the estuaries adjacent to the undisturbed mangrove as compared to the degraded mangrove. The reason behind higher sink behaviours of estuaries nearer to undisturbed mangroves are higher intrusion of seawater, less nutrient availability, higher salinity, low carbon contents and alkaline pH compared to estuaries adjacent to degraded mangroves and far from sea.

Mitigation potential and trade-offs for nitrous oxide emissions and carbon balances of irrigated mixed-species and ryegrass-clover pastures

Grazed grasslands are an important source of N2O emissions. Grasslands also store significant amounts of carbon (C), and net changes to these amounts contribute to their net greenhouse gas budgets. We investigated whether net N2O emissions from irrigated pasture grazed by dairy cows can be reduced by planting more diverse species, compared with conventional ryegrass-clover pasture, and whether there are co-benefits for greenhouse gas reduction by net C gains in the ecosystem, or trade-offs through net C losses. Fluxes of CO2 and N2O were measured by eddy covariance, near the boundary of adjacent five-species mixed pasture (MIX) and ryegrass-white clover pasture (RyWC). A split-footprint approach was applied to separate the CO2 and N2O exchange for the two pastures. The CO2 fluxes were gap-filled with marginal distribution sampling (MDS), and the N2O fluxes with k-nearest-neighbour (kNN) regression. To enable calculation of the net ecosystem C balances (NECB) of the two pastures, non-CO2 carbon imports and exports were also determined. Annual N2O emissions from the MIX pasture were 0.14 g N m−2 yr−1 (mean of two years), compared with 0.23 g N m−2 yr−1 from the RyWC pasture. The N2O emissions accounted for 0.34% and 0.61% of the nitrogen inputs to the MIX and RyWC pasture, respectively. Combining all C gains and losses, both pastures recorded net ecosystem C gains in the first year, of 44 and 282 g C m−2 for MIX and RyWC, respectively. In the second year, both recorded losses: −207 g C m−2 for MIX and −86 g C m−2 for RyWC. Thus, in both years, the net C balance gave the RyWC pasture a considerable advantage over the MIX pasture in the net greenhouse gas balance, and this advantage was one magnitude greater than the difference in N2O emissions, when expressed in CO2-equivalent units.

Nitrous Oxide Distributions in the Oxygenated Water Column of the Sargasso Sea

ABSTRACT This study presents dissolved nitrous oxide (N2O) concentrations in the water column at the Bermuda Atlantic Time-series Study (BATS) station and uses a subset of these measurements to estimate air-to-sea flux for four specific time points between September 2018 and June 2019. N2O concentrations at BATS were in the range of 4.0 nmol L−1–16.9 nmol L−1, with vertical profiles which were the mirror inverse of dissolved oxygen. Regardless of season, N2O concentration maxima were found within the oxygen minimum zone (OMZ). The highest maximum N2O values were observed in November and lowest in October. As the water column at BATS remains consistently at dissolved oxygen concentrations greater than 140 µmol L−1, and therefore aerobic, we assume that the bulk of N2O production occurs through nitrification. A nitrification source is supported by a correlation between excess N2O (ΔN2O) below the mixed layer, apparent oxygen utilization (AOU) and nitrate concentrations. We estimate a pooled average yield of 0.027% to 0.038% N2O from nitrification at BATS. Finally, estimates of air–sea exchange of N2O using regional average monthly wind speeds indicated that this region acts as a weak source or a sink of atmospheric N2O, and varies between months.

Effects of Nitrogen and Phosphorus Additions on Soil N2O Emissions and CH4 Uptake in a Phosphorus-Limited Subtropical Chinese Fir Plantation

Increased nitrogen (N) inputs in subtropical forest ecosystems were widely reported. Extra N additions were reported to cause nutrient imbalance and phosphorus (P) limitation in many tropical and subtropical forests, and further result in changes in soil nitrous oxide (N2O) and methane (CH4) fluxes. Here, we conducted experiments with N (high N addition: 15 g N/m2, HN), P (low: 5 g P/m2, LP; high: 15 g P/m2, HP) and their interactive (HNLP and HNHP) treatments to investigate how N and P additions affected CH4 and N2O exchanges in an N-rich Chinese fir plantation (Cunninghamia lanceolata), and further explored the underlying mechanisms through the structural equation model (SEM) analysis. The results indicated that N addition alone (HN) significantly (p < 0.05) increased the soil N2O emissions by 30.15% and 80.47% over annual and 4-month periods, mainly owing to the elevated NH4+-N content. P addition alone (LP and HP) did not significantly affect the soil N2O emissions as compared with the control. The SEM analysis indicated that increased N2O emissions under N addition were primarily explained by the increase in available N and contributed more to the stimulated NH4+-N contents. N and P interactive additions slightly (not significant) stimulated the N2O emissions as compared with that under the N addition alone treatment. High-dose P addition significantly increased the soil CH4 uptake by 15.80% and 16.23% under the HP and HNHP treatments, respectively, while N addition alone and low P addition (LP and HNLP) did not significantly affect CH4 uptake as compared with the control. The increased water-soluble organic carbon and microbial biomass carbon explained the increased CH4 uptake under high P addition. The fertilization effects on N2O emissions and CH4 uptake mainly occurred in the first 4 months and diminished after that. Our results suggested that the direction, magnitude and timing of the N and P addition effects on N2O emissions and CH4 uptake would depend on the soil nutrient status and plant–microbial competition for N and P in subtropical forests.

Nitrous oxide emission from conservation forest of Kampar Peninsula peatland ecosystem

Nitrous oxide (N2O) is a long-lived greenhouse gas with a warming potential of 300 times higher than CO2. Conserving of intact peat swamp forest can hold the natural physical and chemical properties of the soil, such that the N2O emission occurs naturally. To quantify N2O emission from peatland ecosystems, data availability is highly needed. The objectives of this study were to quantify the emission of N2O and determine the main factors controlling N2O emission from peatland conservation forests. This research was conducted from January to December 2020 in the Kampar Peninsula, Pelalawan Regency, Riau Province. This study found that N2O emission at peatland conservation forest was 0.23 ± 0.19 kg-N/ha/year. Substantial changes in soil and environmental factors such as water table, soil temperature, soil moisture, water-filled pore space, NH4-N, and NO3-N significantly affect the exchange of N2O between peatlands and the atmosphere.

Diurnal Tree Stem CH4 and N2O Flux Dynamics from a Riparian Alder Forest

Tree stems play an important role in forest methane (CH4) and nitrous oxide (N2O) flux dynamics. Our paper aimed to determine the unknown diurnal variability of CH4 and N2O exchange in grey alder tree stems. The gas fluxes in tree stems and adjacent soil were measured using manual static and dynamic chamber systems with gas chromatographic and laser-spectroscopic analysis, respectively. The alder trees were predominant emitters of CH4 and N2O; however, N2O emission from stems was negligible. The soil mainly emitted N2O into the atmosphere and was both a source and sink of CH4, depending on environmental conditions. Neither the tree stems nor the riparian forest soil showed significant differences in their CH4 and N2O fluxes between the daytime and nighttime, independently of the exchange rates. In contrast to several previous studies revealing a diurnal variability of greenhouse gas fluxes from tree stems, our investigation did not show any clear daytime–nighttime differences. On the other hand, we found quite clear seasonal dynamics initiated by changing environmental conditions, such as temperature and soil water conditions and tree physiological activity. Our results imply a transport role of tree stems for soil-produced CH4 and N2O rather than the production of these gases in tree tissues, even though this cannot be excluded.

Trees as net sinks for methane (CH4 ) and nitrous oxide (N2 O) in the lowland tropical rain forest on volcanic Réunion Island.

Summary Trees are known to emit methane (CH4) and nitrous oxide (N2O), with tropical wetland trees being considerable CH4 sources. Little is known about CH4 and especially N2O exchange of trees growing in tropical rain forests under nonflooded conditions.We determined CH4 and N2O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400‐yr‐old tropical lowland rain forest situated on a basaltic lava flow (Réunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows.The stems studied were net sinks for atmospheric CH4 and N2O, as were cryptogams, which seemed to be co‐responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH4. Their N2O fluxes were negligible.Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH4 and N2O. The volcanic tropical lowland rain forest appears to be an important CH4 sink, as well as a possible N2O sink.

An urban polluted river as a significant hotspot for water–atmosphere exchange of CH4 and N2O

Polluted urban river systems might be a strong source of atmospheric methane (CH4) and nitrous oxide (N2O), but so far only a few urban river systems have been quantified with regard to their source strength for greenhouse gases (GHGs). In this study, we measured loads of dissolved inorganic nitrogen and organic carbon, dissolved oxygen (DO) concentrations, and fluxes of CH4 and N2O from an urban river in Beijing, China during the course of an entire year. Fluxes calculated using the floating chamber approach or via the diffusion method with measurements of river water GHG concentrations showed comparable temporal variations. However, the flux magnitude based on the diffusion method was found to strongly depend on the underlying parameterization of the gas transfer velocity. In view of the large differences while applying different methodologies to estimate surface water GHG fluxes further studies are still needed to prove and eventually quantify the systematic errors which are likely caused by either the chamber technique or the approaches of individual diffusion models. For both the floating chamber and the diffusion-based flux estimates, strong seasonal variations in CH4 and N2O fluxes from the river surface were observed, with fluxes ranging from 3 to 8374 μg C m−2 h−1 for CH4 and 1–3986 μg N m−2 h−1 for N2O. The CH4 fluxes were strongly negatively correlated with the DO concentration (P < 0.01). The highest N2O fluxes were observed at times with low CH4 fluxes (i.e., in spring and autumn). Annual CH4 and N2O fluxes totaled 19.3–79.4 and 17.4–44.8 kg C (N) ha−1 yr−1, respectively. These high fluxes are in agreement with estimates from the few other studies carried out for urban river systems to date and indicate that urban polluted river systems are a significant regional source of atmospheric GHGs.

Carbon–nitrogen interactions in European forests and semi-natural vegetation – Part 1: Fluxes and budgets of carbon, nitrogen and greenhouse gases from ecosystem monitoring and modelling

Abstract. The impact of atmospheric reactive nitrogen (Nr) deposition on carbon (C) sequestration in soils and biomass of unfertilized, natural, semi-natural and forest ecosystems has been much debated. Many previous results of this dC∕dN response were based on changes in carbon stocks from periodical soil and ecosystem inventories, associated with estimates of Nr deposition obtained from large-scale chemical transport models. This study and a companion paper (Flechard et al., 2020) strive to reduce uncertainties of N effects on C sequestration by linking multi-annual gross and net ecosystem productivity estimates from 40 eddy covariance flux towers across Europe to local measurement-based estimates of dry and wet Nr deposition from a dedicated collocated monitoring network. To identify possible ecological drivers and processes affecting the interplay between C and Nr inputs and losses, these data were also combined with in situ flux measurements of NO, N2O and CH4 fluxes; soil NO3- leaching sampling; and results of soil incubation experiments for N and greenhouse gas (GHG) emissions, as well as surveys of available data from online databases and from the literature, together with forest ecosystem (BASFOR) modelling. Multi-year averages of net ecosystem productivity (NEP) in forests ranged from −70 to 826 g C m−2 yr−1 at total wet + dry inorganic Nr deposition rates (Ndep) of 0.3 to 4.3 g N m−2 yr−1 and from −4 to 361 g C m−2 yr−1 at Ndep rates of 0.1 to 3.1 g N m−2 yr−1 in short semi-natural vegetation (moorlands, wetlands and unfertilized extensively managed grasslands). The GHG budgets of the forests were strongly dominated by CO2 exchange, while CH4 and N2O exchange comprised a larger proportion of the GHG balance in short semi-natural vegetation. Uncertainties in elemental budgets were much larger for nitrogen than carbon, especially at sites with elevated Ndep where Nr leaching losses were also very large, and compounded by the lack of reliable data on organic nitrogen and N2 losses by denitrification. Nitrogen losses in the form of NO, N2O and especially NO3- were on average 27 % (range 6 %–54 %) of Ndep at sites with Ndep &lt; 1 g N m−2 yr−1 versus 65 % (range 35 %–85 %) for Ndep &gt; 3 g N m−2 yr−1. Such large levels of Nr loss likely indicate that different stages of N saturation occurred at a number of sites. The joint analysis of the C and N budgets provided further hints that N saturation could be detected in altered patterns of forest growth. Net ecosystem productivity increased with Nr deposition up to 2–2.5 g N m−2 yr−1, with large scatter associated with a wide range in carbon sequestration efficiency (CSE, defined as the NEP ∕ GPP ratio). At elevated Ndep levels (&gt; 2.5 g N m−2 yr−1), where inorganic Nr losses were also increasingly large, NEP levelled off and then decreased. The apparent increase in NEP at low to intermediate Ndep levels was partly the result of geographical cross-correlations between Ndep and climate, indicating that the actual mean dC∕dN response at individual sites was significantly lower than would be suggested by a simple, straightforward regression of NEP vs. Ndep.

Effect of management and weather variations on the greenhouse gas budget of two grasslands during a 10-year experiment

While many studies have measured the CO2 and N2O exchange of grassland ecosystems and analysed the influence of relevant drivers, only few reported the entire carbon budget over multiple years, which is most relevant for the net greenhouse gas (GHG) source or sink effect. When analysing eddy-covariance-based flux measurements for management and weather related drivers, this is commonly done by comparing either different measurement years at one site or data from different sites distributed regionally or globally. However this procedure makes it usually difficult to clearly attribute observed differences in the carbon exchange to management effects or meteorological drivers.In this study we present results of the carbon and GHG budget of a 10-year paired grassland field experiment in Switzerland comparing intensive and extensive management. We focus on the inter-annual variability of the entire 10-year dataset and especially on the effect of a renovation activity in the intensive field after seven years. By comparing the results of the paired plots, we attempted to disentangle the effects of management including renovation from the effect of seasonal weather conditions on the carbon and greenhouse gas budget. The annual carbon budgets of the two paired fields showed clear systematic differences attributable to the different management. The inter-annual variation (IAV) of the carbon budget was determined as residuals from linear trends. Due to the high correlation of the IAV between the two parallel fields, it was assigned mainly to variations in seasonal weather conditions. The total range of weather attributed variations of annual NECB values was similar in magnitude to the systematic management induced difference between the two fields (100–200 gC m−2 yr-1). In contrast to previous studies on grassland renovation, a large net carbon loss and enhanced N2O emission over 2–3 years was observed here. The excess N2O emission after renovation could be well explained by the IPCC default emission factor approach when considering the additional N input by plant residues and the net soil organic matter mineralisation. Concerning the total GHG budget effect the cumulated GHG uptake by the INT field in the first six years of the experiment was more than compensated by the release in the three post-renovation years.

Seasonal dynamics of stem N2O exchange follow the physiological activity of boreal trees

The role of trees in the nitrous oxide (N2O) balance of boreal forests has been neglected despite evidence suggesting their substantial contribution. We measured seasonal changes in N2O fluxes from soil and stems of boreal trees in Finland, showing clear seasonality in stem N2O flux following tree physiological activity, particularly processes of CO2 uptake and release. Stem N2O emissions peak during the vegetation season, decrease rapidly in October, and remain low but significant to the annual totals during winter dormancy. Trees growing on dry soils even turn to consumption of N2O from the atmosphere during dormancy, thereby reducing their overall N2O emissions. At an annual scale, pine, spruce and birch are net N2O sources, with spruce being the strongest emitter. Boreal trees thus markedly contribute to the seasonal dynamics of ecosystem N2O exchange, and their species-specific contribution should be included into forest emission inventories.

Soil greenhouse gas emissions as impacted by soil moisture and temperature under continuous and holistic planned grazing in native tallgrass prairie

Adaptive multi-paddock (AMP) grazing has demonstrated the potential to substantially improve ecosystems service outcomes relative to the most commonly used grazing management of moderate (MC) and heavy continuous (HC) grazing. We hypothesize that AMP grazing would decrease net soil emissions of CO2, CH4 and N2O exchange between the soil surface and the atmosphere relative to continuous grazing and the management practice options of prescribed fire (AMP-burn), and production of hay (AMP-hay) both managed using AMP grazing. Soil temperature was lower (P < 0.009) and soil moisture higher (P < 0.01) with AMP grazing than with HC and MC grazing. As CO2, CH4 and N2O emissions are less with lower temperatures and increasing soil moisture, they should have declined with AMP grazing. However, AMP grazing had the highest and HC the lowest CO2 emissions, indicating higher levels of soil respiration, an index of soil microbial activity, with AMP. Emissions of N2O were consistent with previous research, being higher under anaerobic conditions and very low under aerobic conditions. AMP, AMP-burn and AMP-hay treatments on average had lower N2O emissions than HC and MC (P ≤ 0.002). Methane (CH4) emissions were negative for most sample dates but were dwarfed by the occasional periods when soils were saturated. These elevations in CH4 emissions occurred on 8 of the 35 dates sampled (rate >0; P ≤ 0.05), 7 times for HC, 4 times for MC, and 3 times for AMP. On the remaining dates sampled (27 of 35), AMP was the strongest CH4 sink ahead of AMP-burn (P = 0.0335), AMP-hay (P = 0.0232) and HC, but was similar to MC (P = 0.17). MC was a stronger sink than HC (P = 0.057). The emissions of CO2 and N2O were decreased with removal of green canopy material at sampling, indicating positive responses could be achieved by adjusting grazing management. Adaptive multi-paddock grazing, but not continuous grazing, can be adjusted to maintain higher proportions of green material, and as this would also benefit energy capture by photosynthesis and livestock diet quality, multiple benefits could accrue from implementing such management. Removal of green material had no influence on CH4 oxidation, which was greatest with AMP grazing. These results are consistent with AMP grazing having a lower intensity ecological impact than continuous grazing.

High N2O consumption potential of weakly disturbed fen mires with dissimilar denitrifier community structure

Peat soils can be strong sources of atmospheric nitrous oxide (N2O), but at the same time act as sinks for the greenhouse gas N2O. However, the role of N2O reduction to dinitrogen (N2) here is still not fully understood. In particular, this applies to pristine or weakly disturbed fen mires. These types of peatland ecosystems are characterised by anoxic soil conditions and special N dynamics restricted to ammonium (NH4+) turnover and very low nitrate (NO3−) availability. N2O and N2 fluxes from intact soil cores from three weakly disturbed fen mire types and two soil habitats (tussocks and hollows) were investigated using the helium (He) incubation approach. Ambient air in headspaces were first substituted with a He-O2 trace gas mixture to quantify N2O and N2 exchanges under prevailing soil oxygen (O2) conditions, and then with an anoxic He trace gas mixture (99.9% He) for establishing the maximum possible denitrification rate. Changing from the He-O2 mixture to a pure He trace gas mixture led to strong increase of N2 fluxes (up to 2916 μg N m−2 h−1) and negative N2O fluxes of up to −72 μg N m−2 h−1. Whilst small differences in N gas fluxes were found between all types of fen mires, an analysis of the denitrifier abundance based on nirK, nirS and nosZ genes indicated respectively more pronounced variation. The structure of denitrifier communities exhibited a strong plot specificity driven by water-filled pore space, soil organic matter and soil pH. This short-term He incubation experiment revealed that weakly disturbed fen mires act as considerable N2O sinks under anoxic conditions and improved our knowledge of the original N dynamics in this peatland ecosystem.

Divergent responses of nitrous oxide, methane and carbon dioxide exchange to pulses of nitrogen addition in a desert in Central Asia

Desert ecosystems are nitrogen-limited, and highly responsive to even small inputs of nitrogen. Recently, desert ecosystems have been affected by increasing levels of nitrogen deposition, which may lead to changes in greenhouse gas efflux. However, the effects of nitrogen deposition on these gases in the desert ecosystems remain poorly understood. To study the dynamics in greenhouse gases under increasing nitrogen, six rates of simulated nitrogen deposition, including 0(N0), 0.5(N0.5), 1.0(N1), 3.0(N3), 6.0(N6), 24.0 (N24) g nitrogen m−2 a−1, were applied in soils of the Gurbantunggut Desert in Central Asia. Efflux of nitrous oxide (N2O), methane (CH4), ecosystem respiration and net carbon ecosystem exchange were measured across two growing seasons for two years. The efflux of the four gases changed greatly in different seasons, which can explain more variation than nitrogen treatments except for N2O. Nitrogen input stimulated N2O emission, especially in the high nitrogen treatments, with N2O emission 1.6 (N6) and 2.6 (N24) times greater than control (N0). However, N2O exchange also depended on the seasons, while only minor changes were found in some seasons (late spring 2010 and late summer 2011). Overall, CH4 uptake (average 31.4 μg m−2 h−1) was not significantly affected by the nitrogen addition. At the high nitrogen treatment levels, ecosystem respiration decreased in the late spring and summer of the second year, with similar responses to nitrogen addition in mid-autumn between two years. Net ecosystem carbon exchange also showed gradual responses to nitrogen addition overall, but did not differ significantly across seasons and treatments. Overall, structural equation models showed that the dynamics in N2O were mostly attributed to variations in nitrogen addition, CH4 to soil moisture (or temperature), ecosystem respiration to plant species richness (or density) and net ecosystem carbon exchange to soil moisture (or temperature). Our study indicates that the changes in greenhouse gas emissions caused by nitrogen deposition in short time scale were small in these desert soils.

Nitrogen deposition induced significant increase of N2O emissions in an dry alpine meadow on the central Qinghai–Tibetan Plateau

Atmospheric concentrations of nitrous oxide (N2O) have increased over the last 150 years due to human activities. Soils are important source of N2O where its production is largely regulated by biological processes. Nitrogen (N) deposition can alter the processes of autotrophic nitrification and denitrification, thus affecting the rate and direction of the soil N2O exchange with the atmosphere. It is obvious that atmospheric N deposition has influenced a dry alpine meadow in the Qinghai–Tibetan Plateau (QTP). Therefore, we investigated the effects of N deposition on the annual N2O emissions in an alpine meadow of the QTP. The N2O flux was measured for 2 years using static chambers and gas chromatography methods at four treatments (N0, background level; N7, add 7 kg N ha−1 yr−1; N20, add 20 kg N ha−1 yr−1; N40, add 40 kg N ha−1 yr−1). We found that high N deposition increased the N2O flux, not only in the growing season but also in winter and the spring thaw period. The average annual N2O fluxes at N0 and N40 plots were 3.1 and 6.1 μg m−2 h−1, respectively. Compared with N0, the average annual N2O fluxes increased at N7, N20, and N40 plots by 13.7%, 47.6%, and 98.7%, respectively, the N2O fluxes in N40 plots increased by 113.6%, 41.6%, and 78.7% during the growing season, winter and spring thaw period, respectively. The emission of N2O during the growing season, winter and spring thaw period accounted for 63.2%, 29.9% and 6.9% of annual total emission of N2O, respectively. The N2O flux correlated significantly with air temperature, soil temperature, soil water content, total nitrogen, total organic carbon, and soil NH4+–N content in the alpine meadow.

CO2, CH4 and N2O flux changes in degraded grassland soil of Inner Mongolia, China

The main purpose of this study was to explore the dynamic changes of greenhouse gas (GHG) from grasslands under different degradation levels during the growing seasons of Inner Mongolia, China. Grassland degradation is associated with the dynamics of GHG fluxes, e.g., CO2, CH4 and N2O fluxes. As one of the global ecological environmental problems, grassland degradation has changed the vegetation productivity as well as the accumulation and decomposition rates of soil organic matter and thus will influence the carbon and nitrogen cycles of ecosystems, which will affect the GHG fluxes between grassland ecosystems and the atmosphere. Therefore, it is necessary to explore how the exchanges of CO2, CH4 and N2O fluxes between soil and atmosphere are influenced by the grassland degradation. We measured the fluxes of CO2, CH4 and N2O in lightly degraded, moderately degraded and severely degraded grasslands in Inner Mongolia of China during the growing seasons from July to September in 2013 and 2014. The typical semi-arid grassland of Inner Mongolia plays a role as the source of atmospheric CO2 and N2O and the sink for CH4. Compared with CO2 fluxes, N2O and CH4 fluxes were relatively low. The exchange of CO2, N2O and CH4 fluxes between the grassland soil and the atmosphere may exclusively depend on the net exchange rate of CO2 in semi-arid grasslands. The greenhouse gases showed a clear seasonal pattern, with the CO2 fluxes of –33.63 –386.36 mg/(m•h), CH4 uptake fluxes of 0.113–0.023 mg/(m•h) and N2O fluxes of –1.68 –19.90 μg/(m•h). Grassland degradation significantly influenced CH4 uptake but had no significant influence on CO2 and N2O emissions. Soil moisture and temperature were positively correlated with CO2 emissions but had no significant effect on N2O fluxes. Soil moisture may be the primary driving factor for CH4 uptake. The research results can be in help to better understand the impact of grassland degradation on the ecological environment.

Cryptogamic stem covers may contribute to nitrous oxide consumption by mature beech trees

Naturally produced by microbial processes in soil, nitrous oxide (N2O) is an important greenhouse gas contributing to climate change. Accordingly, there is a need to accurately quantify the capability of forest ecosystems to exchange N2O with the atmosphere. While N2O emissions from soils have been well studied, trees have so far been overlooked in N2O inventories. Here, we show that stems of mature beech trees (Fagus sylvatica) may act as a substantial sink of N2O from the atmosphere under conditions of soils consuming N2O. Consistent consumption of N2O by all stems investigated (ranging between −2.4 and −3.8 µg m−2 h−1) is a novel finding in contrast to current studies presenting trees as N2O emitters. To understand these fluxes, N2O exchange of photoautotrophic organisms associated with beech bark (lichens, mosses and algae) was quantified under laboratory conditions. All these organisms were net N2O sinks at full rehydration and temperature of 25 °C. The consumption rates were comparable to stem consumption rates measured under field conditions. Cryptogamic stem covers could be a relevant sink of N2O in European beech forests.

Long-term grazing effects on soil-atmosphere exchanges of CO2, CH4 and N2O at different grasslands in Inner Mongolia: A soil core study

Regional greenhouse gas (GHG) budgets in vast grasslands may be changing due to overgrazing and grassland types. However, the comprehensive effects of grazing patterns, environmental factors and grassland types on soil carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) exchanges have been poorly studied. This study investigates the effects of long-term grazing on the soil-atmosphere exchanges of CO 2 , CH 4 and N 2 O in important processes within different grasslands in Inner Mongolia, China. Using manual static chamber and gas chromatography, we measured the fluxes of CO 2 , CH 4 and N 2 O from intact soil cores of paired grazed/ungrazed sites collected from two typical steppes (S tipa grandis and Leymus chinensis ): one wetland in a flood plain and one desert steppe in the region of the Xilin River catchment, Inner Mongolia. Soil gas flux and concentration measurements were conducted in four simulated conditions ( i.e. , drought, dry-wet, intense rainfall and freeze-thaw), which represent important processes in annual GHG exchanges. Extreme drought significantly inhibited CO 2 and N 2 O emissions in all plots but did not change the CH 4 uptake by typical steppes. Dry-wet transition and intense rainfall could remarkably promote soil CO 2 emission pulses at different types, significantly decrease CH 4 uptake by typical steppes, and arouse N 2 O emission pulses at all plots with different times of occurrence. During the freeze-thaw simulation, temperature-induced soil CO 2 emission and CH 4 uptake/emission presented a clear alternative variation, while soil thaw only slightly increased (<15 μg N m −2 h −1 ) in the steppes and sand dunes and was significantly higher in the wetland (11–96 μg N m −2 h −1 ). Long-term grazing significantly inhibited soil respiration rates at all grassland types, significantly decreased CH 4 uptake by the Leymus chinensis steppes, and did not show significant influence on N 2 O emission due to large spatial variations for all types. Compared to the ungrazed Leymus steppes, Stipa steppes, sand dune and wetland, continuously grazed sites were significantly reduced by 22%, 38%, 48% and 47% in total GHG emissions, respectively. Our results indicate that the potential of the steppe soil CH 4 sink function can be offset by N 2 O emission, especially in over-grazed plots. Furthermore, N 2 O emissions should be considered in wetland rangelands with significantly higher N 2 O emission potential (range: 0–343 μg N m −2 h −1 ) more than steppes (range: 0–132 μg N m −2 h −1 ) and sand dunes (range: 0–49 μg N m −2 h −1 ). Nevertheless, comprehensive evaluation of the grazing effect on ecosystem GHG emissions merits consider in both field observation and incubation experiments because soil properties and environmental factors could be changed by vegetation growth in different grazing practices.