Average N2O Emissions Research Articles

Nitrous oxide emission from a flooded tropical wetland across a vegetation and land use gradient

Abstract This study investigated, using the closed chamber method, the impact of (1) vegetation community type (Typha latifolia, Cyperus papyrus and Phragmites mauritianus) in a natural tropical freshwater marsh wetland (marsh) and (2) conversion of a natural tropical freshwater marsh into a rice paddy wetland (rice paddy), on nitrous oxide (N2O) emission. Both the marsh and the rice paddy were continuously flooded, while the rice paddy was unfertilized. Average N2O emission from the marsh did not vary significantly (p > 0.05) among the vegetation communities, ranging from 0.5 to 0.6 μg m−2 h−1. Similarly, these N2O emission rates were not significantly different (p > 0.05) from those recorded in the rice paddy (0.7 ± 2.8 [SE] μg m−2 h−1). There was no significant correlation (p > 0.05) between environmental parameters and N2O emission. We concluded that vegetation community type does not affect N2O emission from natural tropical freshwater marshes under continuous flooding. Further, converting natural tropical freshwater marshes into continuously flooded and unfertilized rice paddies does not affect N2O emission but instead enhances carbon emission, as was depicted by the significantly lower (p > 0.05) soil organic carbon content in the rice paddy. In view of climate change mitigation, therefore, wetland management should give priority to the conservation/protection of natural wetlands.

Methane and Nitrous Oxide Emissions from a Temperate Peatland under Simulated Enhanced Nitrogen Deposition

Nitrogen (N) deposition has increased in recent years and is significantly affected by global change and human activities. Wetlands are atmospheric CH4 and N2O sources and may be affected by changes in N deposition. To reveal the effects of increased N deposition on peatland greenhouse gas exchange, we observed the CH4 and N2O emissions from controlled microcosms collected from a temperate peatland in the Xiaoxing’an mountains, Northeast China. We found that the moss biomass did not change, but the total herb biomass increased by 94% and 181% with 5 and 10-times-higher N deposition, respectively. However, there were no significant changes in CH4 emissions from the microcosms with N addition. The unchanged CH4 emissions were mainly caused by the opposite effect of increased nitrate and ammonium concentrations on soil CH4 production and the increased plant biomass on CH4 emission. We also found that the manipulated microcosms with 5 and 10-times-higher N deposition had 8 and 20-times-higher seasonal average N2O emissions than the control microcosms, respectively. The increased N2O emissions were mainly caused by short-term (≤7 d) pulse emissions after N addition. The pulse N2O emission peaks were up to 1879.7 and 3836.5 μg m−2 h−1 from the microcosms with 5 and 10-times-higher N deposition, respectively. Nitrate and ammonium concentrations increasing in the soil pore water were the reason for the N2O emissions enhanced by N addition. Our results indicate that the increase in N deposition had no effects on the CH4 emissions but increased the N2O emissions of the temperate peatland. Moreover, pulse emissions are very important for evaluating the effect of N addition on N2O emissions.

Interactive effects of irrigation system and level on grain yield, crop water use, and greenhouse gas emissions of summer maize in North China Plain

Irrigation management is one of most critical factors influencing soil N2O and CO2 emissions in dryland agriculture. To explore the effects of irrigation systems and levels on the mitigation of N2O and CO2 emissions from maize fields and to determine the balance among greenhouse gases (GHG) emission, water-saving and grain yield, a two–year field experiment was conducted in the North China Plain (NCP) during the growing seasons of 2018 and 2019. Two irrigation systems (i.e., flood irrigation, FI, and drip irrigation, DI) were adopted with four irrigation levels in each system, including 65 mm/event (sufficient irrigation, CK), 50 mm/event (decreased by 23 %), 35 mm/event (by 46 %) and 20 mm/event (by 69 %), respectively. The results showed that both irrigation systems and levels had significant effects on soil N2O and CO2 emissions (P < 0.05). Nitrous oxide (N2O) and CO2 emissions peaked following irrigation or irrigation + fertilization events during sowing to early filling stage (R1), with the peak values increasing with irrigation levels. Meanwhile, peak values from FI were higher than those from DI at 50 mm and 65 mm irrigation levels. The average cumulative N2O and CO2 emissions of DI treatments were 14.9 % and 6.23 % lower than those of FI treatments (P < 0.05), respectively. Soil moisture was identified as one of the most crucial factors influencing N2O and CO2 fluxes. Deficit irrigation efficiently deceased cumulative N2O and CO2 emissions, but moderate to severe deficit irrigation brought significant reduction in grain yield. Drip irrigation with a slight deficit irrigation level (decreased by 23 %) obtained the best economic and environmental benefits, which achieved the dual goal of lower GHG emissions but higher WUE without sacrificing grain yield.

GHG Emissions from Drainage Ditches in Peat Extraction Sites and Peatland Forests in Hemiboreal Latvia

We determined the magnitude of instantaneous greenhouse gas (GHG) emissions from drainage ditches in hemiboreal peatlands in Latvia during the frost-free period of 2021 and evaluated the main affecting factors. In total, 10 research sites were established in drained peatlands in Latvia, including active and abandoned peat extraction sites and peatland forests. Results demonstrated that in terms of global warming potential, the contribution of CO2 emissions to the total budget of GHG emissions from drainage ditches can exceed the CH4 contribution. The average CO2 and N2O emissions from drainage ditches in peatland forests were significantly higher than those from ditches in peat extraction sites, while there was no difference in average CH4 emissions from ditches between peatland forests and peat extraction sites. Emissions from ditches of all GHGs increased with increasing temperature. In addition, CO2 and N2O emissions from drainage ditches increased with decreasing groundwater (GW) level. They were also negatively correlated with water level in ditches, but positively with potassium (K) and total nitrogen (TN) concentrations in water. By contrast, CH4 emissions from drainage ditches increased with increasing GW level and water level in ditches but were negatively correlated with K and TN concentrations in water.

The effect of chamber placement site on N2O emission under different fertilizer regimes from maize field

Static chamber method has been used to monitor greenhouse gas (GHG) emission from agricultural soils. When monitoring GHG emission in rice or wheat systems, all the plant stems can be covered in the top chamber. Under this situation, both nitrous oxide (N2O) emission from the root free soils and the root area soils can be estimated. However, in maize field, most of the studies placed the chamber within the maize row due to the difficulty to cover the whole maize stalk. N2O emission from the root area was not estimated since the majority of maize roots were excluded in the chamber. This may lead to a under estimation of global N2O emission from maize cultivation. To evaluate the spatial variability of N2O emission from maize field, a two-year field experiment was conducted. N2O emissions from three sampling sites including inter-rows, within-rows and root area were monitored under five different fertilization regimes. Results showed that chamber placement site had a significant effect on the N2O emission during the maize growing season. Placing the chamber in inter-rows either overestimated or underestimated N2O emission. In 2018, N2O emission rate in the inter-rows was 69 % higher than the root area on average of the five treatments; whereas, in 2020, N2O emission rate in the inter-rows was 2.5 times lower than the root area. N fertilization regimes had a significant effect on N2O emission. On average of chamber sites, N2O emission under N fertilizer broadcasting was around 2 times lower compared to nest-applied. The highest N2O emission rate was observed when both basal and dressing N fertilizer were applied in a nest, which was 1.1–3.2 times higher than the other N fertilizer placement regimes. The N2O emission rate from the within-rows was close to the average value of the three sampling sites. Our results suggested that placing chamber in the center of two neighboring maize plants could represent the average area-scaled N2O emission in maize field.

Estimation of nitrous oxide emissions from rice paddy fields using the DNDC model: a case study of South Korea.

The Denitrification-Decomposition (DNDC)-Rice is a mechanistic model which is widely used for the simulation and estimation of greenhouse gas emissions [nitrous oxide (N2O)] from soils under rice cultivation. N2O emissions from paddy fields in South Korea are of high importance for their cumulative effect on climate. The objective of this study was to estimate the N2O emissions and biogeochemical factors involved in N2O emissions such as ammonium (NH4+) and nitrate (NO3-) using the DNDC model in the rice-growing regions of South Korea. N2O emission was observed at every application of fertilizer and during end-season drainage at different rice-growing regions in South Korea. Maximum NH4+ and NO3- were observed at 0-10 cm depth of soil. NH4+ increased at each fertilizer application and no change in NO3- was observed during flooding. NH4+ decreased and NO3- increased simultaneously at end-season drainage. Minimum and maximum cumulative N2O emissions were observed at Chungcheongbuk-do and Jeju-do regions of South Korea, respectively. The simulated average cumulative N2O emission in rice paddies of South Korea was 1.37 kg N2O-N ha-1 season-1. This study will help in calculating the total nitrogen emissions from agriculture land of South Korea and the World.

Nitrous Oxide Emissions and Carbon Footprint of Decentralized Urine Fertilizer Production by Nitrification and Distillation

Combining partial nitrification, granular activated carbon (GAC) filtration, and distillation is a well-studied approach to convert urine into a fertilizer. To evaluate the environmental sustainability of a technology, the operational carbon footprint and therefore nitrous oxide (N2O) emissions should be known, but N2O emissions from urine nitrification have not been assessed yet. Therefore, N2O emissions of a decentralized urine nitrification reactor were monitored for 1 month. During nitrification, 0.4–1.2% of the total nitrogen load was emitted as N2O-N with an average N2O emission factor (EFN2O) of 0.7%. Additional N2O was produced during anoxic storage between nitrification and GAC filtration with an estimated EFN2O of 0.8%, resulting in an EFN2O of 1.5% for the treatment chain. N2O emissions during nitrification can be mitigated by 60% by avoiding low dissolved oxygen or anoxic conditions and nitrite concentrations above 5 mg-N L–1. Minimizing the hydraulic retention time between nitrification and GAC filtration can reduce N2O formation during intermediate storage by 100%. Overall, the N2O emissions accounted for 45% of the operational carbon footprint of 14 kg-CO2,equiv kg-N–1 for urine fertilizer production. Using electricity from renewable sources and applying the proposed N2O mitigation strategies could potentially lower the carbon footprint by 85%.

Suburban agriculture increased N levels but decreased indirect N2O emissions in an agricultural-urban gradient river

The effects of land use on riverine N2O emissions are not well understood, especially in suburban zones between urban and rural with distinct anthropogenic perturbations. Here, we investigated in situ riverine N2O emissions among suburban, urban, and rural sections of a typical agricultural-urban gradient river, the Qinhuai River of Southeastern China from June 2010 to September 2012. Our results showed that suburban agriculture greatly increased riverine N concentration compared to traditional agricultural rivers (TAR). The mean total dissolved nitrogen (TDN) concentration was 8.18 mg N L−1 in the suburban agricultural rivers (SUAR), which was almost the same as that in the urban rivers (UR, of 8.50 mg N L−1), compared to that in TAR (0.92 mg N L−1). However, the annual average indirect N2O flux from the SUAR was only 27.15 μg N2O-N m−2 h−1, which was slightly higher than that from the TAR (13.14 μg N2O-N m−2 h−1) but much lower than that from the UR (131.10 μg N2O-N m−2 h−1). Moreover, the average N2O emission factor (EF5r, N2O-N/DIN-N) in the SUAR (0.0002) was significantly lower than those in the TAR (0.0028) and UR (0.0004). The limited indirect N2O fluxes from the SUAR are best explained by the high riverine dissolved organic carbon (DOC) and low dissolved oxygen, which probably reduced the denitrification source N2O by favoring complete denitrification to produce N2 and inhibited the nitrification source N2O, respectively. An exponential decrease model incorporating dissolved inorganic nitrogen and DOC could greatly improve our EF5r predictions in the agricultural-urban gradient river. Given the unprecedented suburban agriculture in the world, more studies in suburban agricultural rivers are needed to further refine the EF5r and better reveal the mechanisms behind indirect N2O emissions as influenced by suburban agriculture.

Nitrous oxide emissions from wastewater treatment - Revisiting the IPCC 2019 refinement guidelines

Many entities (such as water utilities and government agencies) that operate or manage wastewater treatment plants (WWTPs) are seeking to estimate and reduce greenhouse gas (GHG) emissions. Nitrous oxide (N2O) is a major GHG emitted directly from WWTPs with nitrification/ nitrogen removal processes. These N2O emissions are fugitive, highly variable and relatively difficult to measure. In the absence of direct emissions measurements, using typical GHG accounting guidelines (e.g., those published by the Intergovernmental Panel on Climate Change, IPCC), the calculation methods for estimating emissions apply one or more emission factor(s) (EF). The EFs reference parameters in wastewater treatment that can be measured or estimated reasonably reliably (e.g., influent total nitrogen (TN) load). For GHG estimation and reporting purposes, most operating entities are reliant on default EF values for wastewater N2O listed in GHG accounting guidelines (e.g., IPCC). Such default factors have historically been derived from expert opinion, using limited literature references, and based on actual measurements from wastewater treatment plants. When compared to the older IPCC (2006) Guidelines, the latest default EF for wastewater treatment N2O in the IPCC 2019 Refinement represents an increase of approximately 32-fold from 0.05% to 1.6% of influent TN load. This increase was derived from a literature review, as described in Volume 5 (Chapter 6, Annex 6A) of the IPCC (2019) Refinement to the 2006 Guidelines. In this study, the literature data cited by IPCC (2019), were checked and the same linear regression approach used to reproduce the derivation of an ‘average’ (default) EF for N2O from full-scale WWTPs. Several apparent literature citation errors were identified in the relevant annex that appear to have resulted in an over-estimation of the ‘average’ (default) EF in the IPCC (2019) Guidelines. After correction of these errors to align with the source literature data, the outcome of this study is a suggested revised default EF of 1.1% (±0.16%, α = 0.05) of WWTP influent TN load emitted as N2O. This revised default value is supported by inclusion of datapoints from newer literature references (2018-2021), in addition to those cited by IPCC (2019). It also aligns more closely with the weighted average N2O emission factors published by the Danish Environmental Protection Agency in 2020.

Simulation of N2O emissions from greenhouse vegetable production under different management systems in North China

Quantifying greenhouse gas emissions from greenhouse vegetable production systems (GVPS) is helpful to direct carbon sequestration and greenhouse gas emissions reduction. The objectives of this study were to (i) evaluate the performance of the improved WHCNS-Veg model (soil Water Heat Carbon Nitrogen Simulation for Vegetable) in simulating nitrous oxide (N2O) emissions under different GVPS and (ii) analyze the impact of different management systems on N2O emissions from the GVPS. Greenhouse vegetable experiments were carried out under different management systems, including conventional (CON), integrated (INT) and organic (ORG), in Quzhou County, Hebei province from 2013 to 2015. Field observations of soil water content, soil nitrate concentration and N2O emissions were used to calibrate and validate the WHCNS-Veg model. The sensitivity analysis results showed that the effects of soil hydraulic parameters on N2O emissions were higher than those of N transformation and vegetable genetic parameters. The improved WHCNS-Veg model simulated soil water content, soil nitrate concentration and N2O emissions well under different GVPS. The average N2O emissions in the spring-summer season (16.3 kg N ha−1) were about 2.5 times higher than the autumn-winter season (6.4 kg N ha−1). Among the four seasons, the average N2O emissions of CON, ORG and INT were 13.5, 11.3 and 9.3 kg N ha−1, respectively, accounting for 1.4–1.7% of the total fertilizer input. The N2O emissions of the INT and ORG systems were 31.1% and 16.3% lower than the CON treatment, respectively. The results indicated that the improved WHCNS-Veg model can be used to simulate and analyze N2O emissions under different management systems and substituting organic manure for chemical fertilizer could effectively reduce the N2O emissions in the GVPS.

Long-term trends of direct nitrous oxide emission from fuel combustion in South Asia

An increasing concentration of nitrous oxide (N2O) in the global atmosphere can perturb the ecological balance, affecting the climate and human life. South Asia, one of the world’s most populous regions, is a hotspot for N2O emission. Although agriculture traditionally dominated the region, economic activities are rapidly shifting towards industry and energy services. These activites may become the largest emitters of N2O in future. Yet, few attempts have been made to estimate long-term direct N2O emission from fuel combustion for the different energy-consuming sectors in the South Asian region. Therefore, the present study developed a comprehensive sectoral N2O emission inventory for South Asian countries for the time period of 1990–2017, with projections till 2041. It revealed that the average N2O emission from fuel combustion in the South Asia region is about 40.96 Gg yr−1 with a possible uncertainty of ±12 Gg yr−1, showing an increase of more than 100% from 1990 to 2017. Although India is the major contributor, with an average of 34 Gg yr−1 of N2O emissions, in terms of growth, small countries like Bhutan and Maldives are dominating other South Asian countries. Sector-wise, the residential sector contributed a maximum emission of 14.52 Gg yr−1 of N2O but this is projected to reduce by more than 50% by 2041. This is because of the successful promotion of cleaner fuels like liquefied petroleum gas over more polluting fuelwood. Power generation contributed 9.43 Gg yr−1of N2O emissions, exhibiting a maximum growth of 395%, followed by road transport (289%) and industry (231%). Future N2O emissions from transport, power and industry are projected to rise by 2.8, 3.3, and 23.9 times their 2017 estimates, respectively, due to the incapability of current policies to combat rising fossil fuel consumption. Mitigation options, such as replacing diesel and compressed natural gas vehicles with electricity-driven vehicles, can decelerate N2O emissions to 45% by 2041 for road transport. A 41% reduction is possible by displacing coal with renewables in the power and industry sectors. Overall, the South Asian contribution to global N2O emissions has enlarged from 2.7% in 1990 to 5.7% in 2007–2016, meaning there is an urgent need for N2O emission mitigation in the region.

No-Tillage Improvement of Nitrogen Absorption and Utilization in a Chinese Mollisol Using 15N-Tracing Method

To better understand the mechanism of nitrogen (N) distribution, absorption, utilization and loss in fertilizer under different tillage practices, a study was conducted to quantitatively explore the fate of fertilizer N in the soil–plant–atmosphere using the 15N labelling technique under the long-term conservation tillage experiment in Northeast China. The test crop used was corn. This study compared the residual amount of 15N fertilizer in soil, the content of 15N fertilizer N in particle organic nitrogen (PON), light fraction organic matter nitrogen (LFOMN) and heavy fraction organic matter N (HFOMN) under different tillage practices. In addition, N uptake, utilization and distribution by corn, the emission of N2O and the gas loss of fertilizer N, and the fertilizer N utilization rate were also taken into account. The results showed that no tillage (NT) had a significantly lower amount of residual 15N fertilizer than a moldboard plow (MP) (p &lt; 0.05). In general, the content under NT at the 0–30 cm soil layer was 7.85% lower than that of MP. NT led to significantly greater PON and LFOMN of soil organic N compared to MP (p &lt; 0.05). 15N from N uptake, fertilizer absorption and utilization under NT were significantly higher than that under MP (p &lt; 0.05), the soil N absorbed by plants under NT or MP was greater than 70%. The distribution of 15N from N fertilizer in each corn part increases in this order: seed &gt; leave &gt; sheath &gt; stem &gt; bract &gt; ear; about 57.91–64.92% of 15N is distributed in the grain. NT resulted in significantly lower average and cumulative N2O emissions than those from MP based on the static closed chamber approach (p &lt; 0.05). The average and cumulative emissions of soil fertilizer 15N-N2O under MP were also significantly greater than that of NT. Among the N2O emissions, 15.3% and 22.98% came from fertilizer N under NT and MP, respectively. On average, 0.1–0.16% of fertilizer N was lost in the form of N2O. There was a significant difference in fertilizer utilization between NT and MP, and NT was 4.23% larger than MP (p &lt; 0.05). These one year findings suggest that NT plays a positive role in improving the N absorption and utilization of fertilizer in a Chinese mollisol and long-term effects need to be further studied.

Driving Factors on Greenhouse Gas Emissions in Permafrost Region of Daxing'an Mountains, Northeast China

AbstractPermafrost regions are an important source of greenhouse gases. However, the effects of different permafrost wetland types on greenhouse gas emissions and the driving factors are still unclear in the permafrost region. Here, we selected three typical permafrost wetlands from the Daxing'an Mountains to investigate the effects of permafrost wetland types on greenhouse gas emissions. The cumulative N2O, CO2, and CH4 emissions were 84–122, 657,942–1,446,121, and 173–16,924 kg km−2, respectively. The linear mixed effects model indicated that N2O emissions were significantly affected by the NO3−‐N content, whereas CO2 emissions were mainly driven by soil temperature, water table level, and NO3−‐N content. CH4 emissions were affected by soil temperature and water table level. Permafrost wetland types significantly affected the average and cumulative N2O, CO2, and CH4 emissions. The cumulative N2O emissions were highest in the Larix gmelinii ‐ Carex appendiculata (LC) wetland and lowest in the Betula fruticosa Pall. (B) wetland, driven by NO3−‐N content. The cumulative CO2 emissions were highest in the B wetland and lowest in the L. gmelinii ‐ Ledum palustre var. dilatatum (LL) wetland. The cumulative CH4 emissions from B wetland were significantly higher than those from LL and LC wetlands. The differences in cumulative CO2 and CH4 emissions were driven by the water table level. Our findings indicate that NO3−‐N content affect the spatial‐temporal variation of N2O emissions, whereas water table level influence the spatial‐temporal variation of CO2 and CH4 emissions in the permafrost region of the Daxing'an Mountains.

Dynamic modelling of N2 O emissions from a full-scale granular sludge partial nitritation-anammox reactor.

Partial nitration-anammox is a resource-efficient pathway for nitrogen removal from wastewater. However, the advantages of this nitrogen removal technology may be counter-acted by the emission of N2 O, a potent greenhouse gas. In this study, mathematical modelling was applied to analyse N2 O formation and emission dynamics and to develop N2 O mitigation strategies for a one-stage partial nitritation-anammox granular sludge reactor. Dynamic model calibration for such a full-scale reactor was performed, applying a one-dimensional biofilm model and including several N2 O formation pathways. Simultaneous calibration of liquid phase concentrations and N2 O emissions leads to improved model fit compared to their consecutive calibration. The model could quantitatively predict the average N2 O emissions and qualitatively characterize the N2 O dynamics, adjusting only seven parameter values. The model was validated with N2 O data from an independent data set at different aeration conditions. Nitrifier nitrification was identified as the dominating N2 O formation pathway. Off-gas recirculation as a potential N2 O emission reduction strategy was tested by simulation and showed indeed some improvement, be it at the cost of higher aeration energy consumption.

Estimating natural nitrous oxide emissions from the Qinghai–Tibetan Plateau using a process-based model: Historical spatiotemporal patterns and future trends

Fluctuations in natural nitrous oxide emissions (N2O) are not fully understood, especially on the Qinghai–Tibetan Plateau. In order to characterize the differences in historical N2O variations among different regions of the Qinghai-Tibetan Plateau and investigate how a warmer and wetter climate change would affect the N2O fluxes over the Qinghai-Tibetan Plateau, a process-based model (TRIPLEX-GHG) has been validated using 14 sites with annual N2O emission fluxes, and is used to estimate the spatiotemporal patterns of natural N2O fluxes, and predict its future trend for the Qinghai-Tibetan Plateau (the temperature increasing by 1.5 °C and the precipitation increasing by 10%). Considering the model performance evaluation indicators (R2=0.83, rRMSE=44.6%, PBIAS=7.18%), the performance of the model for simulating annual N2O fluxes is “moderate”. The estimated average total natural N2O emissions of the Qinghai-Tibetan Plateau from 1970 to 2017 were 0.313 Tg N yr−1, ranging from 0.305 Tg N yr−1 (1983) to 0.325 Tg N yr−1 (2006). The estimated total natural N2O emissions of the Qinghai-Tibetan Plateau slightly increased from 1970 to 2017, with a mean increase of 0.0002 Tg N yr−1. Between the 1970s and 2010s, most increases in N2O flux were observed in the northern and southeastern regions of the Qinghai-Tibetan Plateau. The area in which the rate of N2O released was greater than 15 µg N m−2 h−1 was mainly found in plateau temperate regions. Regions with relatively high temperatures released more N2O, which indicates that temperature is a dominant factor for N2O emissions in high-altitude regions. Natural N2O emissions of the Qinghai-Tibetan Plateau will increase up to 0.335 Tg N yr−1 (by approximately 6.13%) by 2050 if the climate continues to become warmer and wetter.

Biochar rather than organic fertilizer mitigated the global warming potential in a saline-alkali farmland

A 3-year field study was conducted to examine the effects of continuous application of biochar and organic fertilizer on global warming potential (GWP) and greenhouse gas intensity (GHGI) as well as crop yield in a saline-alkali farmland of northern China. Six treatments were included: 1) control (only chemical fertilizer with N200 kg ha−1 yr–1 and P2O5 120 kg ha−1 yr–1, CK); 2) biochar at 5.0 t ha–1 yr–1 (C1); 3) biochar at 10.0 t ha–1 yr–1 (C2); 4) biochar at 20.0 t ha–1 yr–1 (C3); 5) organic fertilizer at 7.5 t ha–1 yr–1 (M1); and 6) organic fertilizer at 10.0 t ha–1 yr–1 (M2). Biochar and organic fertilizer treatments were adjusted to the same N and P level as the control by supplemented urea and diammonium phosphate. The results showed that both biochar and organic fertilizer reduced the annual average emissions of CH4, N2O and ecosystem respiration (Re), but the emissions from the biochar treatments were lower than those from the organic fertilizer treatments. Biochar but not organic fertilizer increased the maize and wheat yields. Both biochar and organic fertilizer increased the soil carbon (C) storage and the net ecosystem carbon budget (NECB). The ΔSOCs of biochar and organic fertilizer treatments were increased by 9.1%–16.5% and 5.0%–5.8%, respectively, compared with the control (1.21 t C ha−1 yr−1). The mean annual net GWPs of biochar and organic fertilizer treatments were decreased by 282.5%–365.0% and 89.2%–110.0%, respectively, compared with the control (1.2 t CO2-eq ha−1 yr−1). The GHGIs were decreased by 266.7%–344.4% and 88.9%–110.0%, respectively, compared with the control (0.09 t CO2-eq ha−1 yr−1). The net GWPs and GHGIs were significantly lower in the C2 and C3 than other treatments, but there was no significant different between C2 and C3. The N2O emissions and ΔSOC contributed greatly to the net GWP, therefore, controlling soil N2O emissions and increasing soil C storage are key components of reducing net GWP and GHGI. In conclusion, application of biochar is an effective agricultural practice to improve soil quality, increase crop yield and reduce greenhouse gases (GHGs) emissions in the saline-alkali soil of northern China.

Three Gorges Dam: friend or foe of riverine greenhouse gases?

Dams are often regarded as greenhouse gas (GHG) emitters. However, our study indicated that the world's largest dam, the Three Gorges Dam (TGD), has caused significant drops in annual average emissions of CO2, CH4 and N2O over 4300 km along the Yangtze River, accompanied by remarkable reductions in the annual export of CO2 (79%), CH4 (50%) and N2O (9%) to the sea. Since the commencement of its operation in 2003, the TGD has altered the carbonate equilibrium in the reservoir area, enhanced methanogenesis in the upstream, and restrained methanogenesis and denitrification via modifying anoxic habitats through long-distance scouring in the downstream. These findings suggest that ‘large-dam effects’ are far beyond our previous understanding spatiotemporally, which highlights the fundamental importance of whole-system budgeting of GHGs under the profound impacts of huge dams.

Soil surface management of legume cover has the potential to mitigate nitrous oxide emissions from the fallow season during wheat production

Soil surface management, i.e., mulch by film, straw or cover crop, is very important to water availability in soil on drylands worldwide, especially during the fallow season, when there is a high concentration of soil nitrate nitrogen (N) to produce nitrous oxide (N2O). To determine whether soil surface management affects N2O emissions during the fallow season, we conducted an experiment to compare N2O emissions from a wheat field that received different surface soil management strategies: control (CK), straw mulch and incorporation (SR), planting legume green manure and incorporation (GM), SR plus GM (SR + GM), and plastic film mulch (FM). The results showed that the average hourly N2O emissions during the fallow season were in the order SR (7.4 μg N m−2 h−1), GM (10.7 μg N m−2 h−1), SR + GM (11.7 μg N m−2 h−1), FM (15.5 μg N m−2 h−1), and CK (16.4 μg N m−2 h−1). Correspondingly, reduced total N2O emissions were observed in the SR, GM and SR + GM treatments, with an average reduction of 39.0% (from 302 to 184 g N ha−1) while increased N2O emissions were from the GM and SR + GM treatments in the wheat growing season. Additionally, N2O emissions were related to soil nitrate N content, microbial biomass and moisture. Overall, considering N2O emissions, C and N inputs by plant residues and grain yield, the management of GM has the potential to reduce greenhouse gas emissions and improve soil C sequestration and soil fertility. These results emphasized the importance of legume green manure to wheat-fallow cropping systems.

Reduction in net greenhouse gas emissions through a combination of pig manure and reduced inorganic fertilizer application in a double-rice cropping system: Three-year results

Manure amendment in croplands is common practice for soil carbon sequestration, and may also reduce greenhouse gas emissions. Few studies focus on the effects of manure application on the net greenhouse gas emissions (NGHGE, the global warming impacts of soil carbon sequestration and CH4 and N2O emissions) in double-rice cropping fields. Herein, a field experiment was conducted to analyze the effects of pig manure application in combination with reduced chemical fertilizers on the NGHGE, soil properties, and yields in a double-rice paddy field in 2012–2015. Four treatments were included: 0 N (no nitrogen fertilizer); 1/2 N (chemical nitrogen fertilizer reduced by 50%); N (100% chemical nitrogen fertilizer); and 1/2 N + PM (pig manure complemented with chemical fertilizer application). The average annual CH4 emissions for 1/2 N + PM were 53%, 50%, and 32% higher than those for 0 N, 1/2 N, and N treatments, respectively (p < 0.05). The soil organic carbon sequestration rates (SOCSR) for 1/2 N + PM were 224%, 208%, and 192% higher than those for 0 N, 1/2 N, and N treatments, respectively (p < 0.05). The average annual N2O emissions from 1/2 N + PM were 51% lower than those from the N treatment. Compared to 0 N, 1/2 N and N treatments, the average NGHGE for 1/2 N + PM decreased by 41%, 41%, and 52%, and the average greenhouse gas intensity (GHGI, the yield-scaled NGHGE) from 1/2 N + PM reduced by 67%, 52%, and 53%, respectively. The decreases in NGHGE and GHGI were predominantly due to increased SOCSR (contributions of 187–308% and 81–325%, respectively) in 1/2 N + PM. The average soil nitrate, microbial biomass carbon and nitrogen, soil organic carbon contents, and pH value for 1/2 N + PM treatment were higher than those for the 1/2 N and N treatments (p < 0.05). Compared to 0 N and 1/2 N treatments, 1/2 N + PM treatment significantly increased the average rice yield. However, no significant difference in average yield was observed between the 1/2 N + PM and N treatments. Gross margin analysis showed that the economic profit for 1/2 N + PM was higher than that for the other three treatments. Thus, the combined application of reduced chemical fertilizers and pig manure is an effective and economic way to neutralize greenhouse gas emissions and increase soil fertility in double-rice cropping systems.

Seasonal Nitrous Oxide Emissions From Hydroponic Tomato and Cucumber Cultivation in a Commercial Greenhouse Company

Nitrous oxide (N2O) is considered as the most critical greenhouse gas (GHG) emitted by agricultural and horticultural food production. Hydroponic vegetable cultivation in greenhouse systems has a high potential for N2O emissions due to the intense application of nitrogen-containing fertilizers. Previous studies on model hydroponic systems indicate that N2O emissions per unit area can be several times higher than typically found during field cultivation. However, reliable data from production-scale hydroponic systems is missing. Here we report our findings from monitoring the N2O emissions in a commercial production greenhouse, located in the east of Germany, over a period of 1 year. We used the static chamber method to estimate N2O fluxes in the root zones of hydroponic tomato and cucumber cultures on rock wool growing bags with drip fertigation. Regular sampling intervals (weekly-biweekly) were used to calculate whole season cumulative N2O emissions and N2O emission factors (EFs) based on the amount of nitrogen fertilizer applied. Our results indicate that the seasonal N2O emissions from hydroponic greenhouse cultivation are considerably smaller than expected from previous studies. In total, we estimated average cumulative N2O emissions of 2.3 and 1.5 kg N2O–N ha−1 yr−1 for tomato and cucumber cultures, respectively. Average EFs were 0.31% for tomato cultivation with drain re-use (closed hydroponic system), and 0.13% for cucumber cultivation without drain re-use (open hydroponic system). These values lie below the general EF for N2O from agricultural soils, noted with 1% by the intergovernmental panel on climate change (IPCC). In conclusion, considering the high yield of greenhouse cultivation, hydroponic systems provide a way of producing vegetables climate-friendly, in terms of direct GHG emissions. Further attention should be given to reducing energy inputs, e.g., by using regenerative sources or thermal discharge from industrial processes, and to increasing circularity, e.g., by using recycling fertilizers derived from waste streams. Especially in urban and peri-urban areas, the use of hydroponics is promising to increase local and sustainable food production.