Average N2O Emissions Research Articles

Spatial and temporal variations of N2O emissions from global forest and grassland ecosystems

A process-based dynamic ecosystem model of TRIPLEX-GHG was used to estimate the spatial and temporal patterns of N2O fluxes from global forest and grassland ecosystems under the effects of global warming and elevated CO2 concentrations. From 1992 to 2015, the estimated average N2O emissions from forests and grasslands were 3.62 ± 0.16 Tg N yr−1 and 1.40 ± 0.03 Tg N yr−1, respectively. Tropical regions made large contributions (83.9% for forests and 74% for grasslands) to the total N2O budgets, which were due to the larger N2O flux values and large natural forest and grassland areas. The regional variations in N2O emissions mainly resulted from the differences in the spatial distributions of climate characteristics, especially the precipitation patterns. In addition, anomalous years when N2O emissions were relatively low/high were mainly due to the changes in climate patterns, which may have been induced by El Niño/La Niña events with different strengths and frequencies. Soil N2O emissions from forests showed a positive effect on the atmospheric N2O concentrations during June to November (R2: 0.14˜0.28), while those from grasslands showed a positive effect during the growing seasons (R2: 0.17˜0.28). Although natural N2O sources (forests and grasslands in this study) showed slightly increasing trends, with 9.9 Gg N increment per year for forests and 2.1 Gg N increment per year for grasslands, they were not the main contributors to the elevated N2O concentrations.

Coregulation of nitrous oxide emissions by nitrogen and temperature in China's third largest freshwater lake (Lake Taihu)

AbstractNitrous oxide (N2O) is a potent greenhouse gas and contributes to the loss of stratospheric ozone. However, the role of inland waterbodies in the dynamics of atmospheric N2O is poorly understood. We investigated N2O fluxes and their controlling factors in Lake Taihu, a large and shallow (2400 km2, 1.9 m depth) eutrophic lake in eastern China. Long‐term measurements (2011–2016) revealed spatial and temporal variations in the lake surface N2O fluxes. The mean N2O flux from the lake was 3.5 ± 1.8 (mean ± SD) μmol m−2 d−1, with an annual N2O budget of 134.4 ± 69.8 Mg (106 g) yr−1. The highest N2O fluxes occurred in the eutrophic zone with significant anthropogenic N inputs, and the lowest fluxes occurred in the noneutrophic zone with no external N inflow. A seasonal pattern in N2O fluxes was observed only in the noneutrophic zone and was strongly correlated with water temperature. No seasonality in the N2O fluxes was observed in the eutrophic zone with high N concentrations in the water, indicating that N concentrations play a dominant role in regulating N2O fluxes compared to water temperature. The average N2O emission factor in Lake Taihu was 0.18%, with temporal and spatial variations negatively associated with N concentration but positively associated with the mass ratio of dissolved organic carbon to dissolved inorganic nitrogen. Our results suggest that anthropogenic activities strongly affect N2O fluxes in freshwater lakes.

Nitrous oxide emissions from soils under traditional cropland and apple orchard in the semi-arid Loess Plateau of China

The conversions of cropland to forest, or other tree-based systems, are considered to be important processes affecting regional and global greenhouse gas budgets, especially for nitrous oxide (N2O). From April 2007 to March 2009, in the rain-fed semi-arid climate of the Loess Plateau, China, soil N2O emissions were measured using static chambers from a winter wheat field and an apple orchard, which had been established in part of the wheat field 23 years earlier.Annual average N2O emissions from the apple orchard (2.40 kg N2O ha−1yr−1) were 12.15% higher than those in the wheat field (2.14 kg N2O ha−1yr−1). Seasonal rainfall, in combination with higher nitrogen fertilization, had a promoting effect on N2O emissions in the apple orchard compared with the wheat field. The amounts and patterns of summer rainfall and winter snowfall were the principal controllers of seasonal and annual N2O fluxes in these rain-fed semi-arid regions, likely through their influence on soil moisture content. Since there may be a moisture threshold associated with summer rainfall and winter snowfall triggering higher N2O emissions, climatic regimes should be taken into account when assessing the effects of land use on N2O emissions in the Loess Plateau.

Emissions of nitrous oxide (N2O) from soil surfaces and their historical changes in East Asia: a model-based assessment

This study assessed historical changes in emissions of nitrous oxide (N2O), a potent greenhouse gas and stratospheric ozone-depleting substance, from the soils of East Asia to the atmosphere. A process-based terrestrial ecosystem model (VISIT) was used to simulate the nitrogen cycle and associated N2O emissions as a function of climate, land use, atmospheric deposition, and agricultural inputs from 1901 to 2016. The mean regional N2O emission rate in the 2000s was estimated to be 2.03 Tg N2O year−1 (1.29 Tg N year−1; approximately one-third from natural ecosystems and two-thirds from croplands), more than triple the rate in 1901. A sensitivity analysis suggested that the increase of N2O emissions was primarily attributable to the increase of agricultural inputs from fertilizer and manure. The simulated N2O emissions showed a clear seasonal cycle and interannual variability, primarily in response to meteorological conditions and nitrogen inputs. The spatial pattern of the simulated N2O emissions revealed hot spots in agricultural areas of China, South Korea, and Japan. The average N2O emission factor (emission per unit nitrogen input) was estimated to be 1.38%, a value comparable to previous estimates. These biogeochemical modeling results will facilitate identifying ways to mitigate global warming and manage agricultural practices in this region.

The effect of temperature shifts on N2O and NO emissions from a partial nitritation reactor treating reject wastewater

Temperature has a known effect on ammonia oxidizing bacteria (AOB) activities, reducing its ammonia oxidizing rate (AOR) when temperature is lowered. However, little is known concerning its effect on N2O and NO emissions which are produced during ammonia oxidation having a greenhouse effect. To study this, an AOB enriched partial nitrification sequencing batch reactor (PN-SBR) was operated within a two step-wise feed under 5 different temperatures (30-25-20-15-10 °C). A decrease on the specific AOR (sAOR) was detected when decreasing the temperature. N2O emissions were also affected by the temperature but only the ones produced during the first aeration of the cycle, when AOBs shifted from a period of low activity to a period of high activity. N2O emission factors (%) detected during the second aerobic phase were similar among all temperatures tested and lower than the emissions detected during the first aerated phase. The average N2O emission factor was in the range of 0.15–0.70% N2O-N/NH4+-N oxidized in the first aeration phase and 0.14–0.15% N2O-N/NH4+-N-oxidized in the second aeration phase at 10 to 30 °C, respectively. On the other hand, NO emissions were very similar under all temperatures resulting in 0.03–0.06% of NH4+-N oxidized.

Benefits of integrated nutrient management on N2O and NO mitigations in water-saving ground cover rice production systems

To cope with challenges of food security and water scarcity in rice production, water-saving ground cover rice production systems (GCRPSs) are increasingly adopted in China and globally. Reduced soil moisture as well as increased soil aeration and temperature under GCRPSs may promote soil N transformations, and in turn give rise to environmental challenges. These include emissions of the potent greenhouse gas nitrous oxide (N2O) and atmospheric pollutant nitric oxide (NO). Using conventional flooding rice cultivation as a reference, a three-year field experiment was conducted to investigate the performances of GCRPSs under inorganic (urea) or integrated nutrient management (a combination of synthetic and organic fertilizers), with regards to soil N2O and NO emissions as well as grain yields. N2O and NO emissions in GCRPSs exhibited high seasonal and interannual variations along with changes in soil inorganic N content and rainfall. When urea alone was applied, the average N2O and NO emissions from GCRPSs were 4.11 and 0.14 kg N ha−1, respectively. These emissions were significantly higher than those of conventional rice cultivation, with 1.47 and 0.052 kg N ha−1 for N2O and NO, respectively. When integrated nutrient management was performed for GCRPSs, N2O and NO emissions were reduced by approximately 77% and 50%, respectively, i.e., the emission magnitude comparable with N-trace gas losses from conventional rice cultivation. Moreover, GCRPSs with integrated nutrient management resulted in optimal grain yields, and thus, the yield-scaled N2O + NO emissions were the lowest compared to other treatments. Averaged over 3 years, the direct emission factors of N2O and NO for GCRPSs with urea alone were 2.58% and 0.064%, respectively. Those for GCRPSs with integrated nutrient management were 0.48% and 0.016%, respectively. The results of this study suggest that GCRPS with integrated nutrient management is an eco-friendly strategy for optimizing crop yields while mitigating N2O and NO emissions.

N2O and CO2 emissions following repeated application of organic and mineral N fertiliser from a vegetable crop rotation

Accounting for nitrogen (N) release from organic amendments (OA) can reduce the use of synthetic N-fertiliser, sustain crop production, and potentially reduce soil borne greenhouse gases (GHG) emissions. However, it is difficult to assess the GHG mitigation potential for OA as a substitute of N-fertiliser over the long term due to only part of the organic N added to soil is being released in the first year after application. High-resolution nitrous oxide (N2O) and carbon dioxide (CO2) emissions monitored from a horticultural crop rotation over 2.5 years from conventional urea application rates were compared to treatments receiving an annual application of raw and composted chicken manure combined with conventional and reduced N-fertiliser rates. The repeated application of composted manure did not increase annual N2O emissions while the application of raw manure resulted in N2O emissions up to 35.2 times higher than the zero N fertiliser treatment and up to 4.7 times higher than conventional N-fertiliser rate due to an increase in C and N availability following the repeated application of raw OA. The main factor driving N2O emissions was the incorporation of organic material accompanied by high soil moisture while the application of synthetic N-fertiliser induced only short-term N2O emission pulse. The average annual N2O emission factor calculated accounting for the total N applied including OA was equal to 0.27 ± 0.17%, 3.7 times lower than the IPCC default value. Accounting for the estimated N release from OA only enabled a more realistic N2O emission factor to be defined for organically amended field that was equal to 0.48 ± 0.3%. This study demonstrated that accounting for the N released from repeated application of composted rather than raw manure can be a viable pathway to reduce N2O emissions and maintain soil fertility.

Correction: Five-year study of the effects of simulated nitrogen deposition levels and forms on soil nitrous oxide emissions from a temperate forest in northern China.

[This corrects the article DOI: 10.1371/journal.pone.0189831.].

No-tillage reduces long-term yield-scaled soil nitrous oxide emissions in rainfed Mediterranean agroecosystems: A field and modelling approach

There is a strong need to identify agricultural management practices that maintain agronomic productivity while diminishing soil N2O emissions. The yield-scaled N2O emissions (YSNE) indicator can help to evaluate the adequacy of a given agricultural practice under both aspects. Long-term (18-yr) soil water and mineral N dynamics, crop biomass and yields, and 2011–2012 soil N2O emissions and ancillary variables were measured on barley (Hordeum vulgare L.) production in a tillage (conventional tillage, CT; no-tillage, NT) and N rate (0, 60 and 120 kg N ha−1) combination under rainfed Mediterranean conditions (NE Spain). Once evaluated, the STICS soil-crop model was used to simulate the 18-yr soil N2O emissions of each tillage system under increasing N rates (0, 30, 60, 90 and 120 kg N ha−1) in order to identify optimum management to reduce YSNE, being initialized with observed data. Cropping season precipitation was highly variable during the experiment, being a key regulating mechanism for crop yields and simulated soil N2O emissions. Crop yield under NT with N outperformed CT in 11 years. STICS performed reasonably well when simulating cumulative N2O emissions and ancillary variables with model efficiencies greater than 0.5. The 18-yr average simulated cumulative N2O emissions were 0.50, 0.82 and 1.09 kg N2O-N ha−1 yr−1 for CT-0, CT-60 and CT-120, respectively, and they were 0.53, 0.92 and 1.19 kg N2O-N ha−1 yr−1 for their counterparts under NT. These averages mask a large variability between years, according to precipitation. The 18-yr mean yield-scaled N2O emissions were 2.8–3.3 times lower under NT, compared to the corresponding CT treatments. Under CT, N application would increase YSNE in most years while YSNE would be more resilient to the application of increasing N rates under NT. Our work demonstrates that in rainfed Mediterranean systems NT is a win-win strategy for the equilibrium between agricultural productivity and low soil N2O emissions.

Nitrous oxide emissions in Chinese vegetable systems: A meta-analysis

China accounts for more than half of the world's vegetable production, and identifying the contribution of vegetable production to nitrous oxide (N2O) emissions in China is therefore important. We performed a meta-analysis that included 153 field measurements of N2O emissions from 21 field studies in China. Our goal was to quantify N2O emissions and fertilizer nitrogen (N) based-emission factors (EFs) in Chinese vegetable systems and to clarify the effects of rates and types of N fertilizer in both open-field and greenhouse systems. The results indicated that the intensive vegetable systems in China had an average N2O emission of 3.91 kg N2O-N ha−1 and an EF of 0.69%. Although the EF was lower than the IPCC default value of 1.0%, the average N2O emission was generally greater than in other cropping systems due to greater input of N fertilizers. The EFs were similar in greenhouse vs. open-field systems but N2O emissions were about 1.4 times greater in greenhouses. The EFs were not affected by N rate, but N2O emissions for both open-field and greenhouse systems increased with N rate. The total and fertilizer-induced N2O emissions, as well as EFs, were unaffected by the type of fertilizers in greenhouse system under same N rates. In addition to providing basic information about N2O emissions from Chinese vegetable systems, the results suggest that N2O emissions could be reduced without reducing yields by treating vegetable systems in China with a combination of synthetic N fertilizer and manure at optimized economic rates.

Streaming Urea Ammonium Nitrate with or without Enhanced Efficiency Products Impacted Corn Yields, Ammonia, and Nitrous Oxide Emissions

Core Ideas Streaming urea ammonium nitrate resulted in 11% lower corn yields compared to injected urea ammonium nitrate.Ammonia volatilization (NH3) begins immediately after application for streaming urea ammonium nitrate.In 2015–2016, NH3 loss was 3.6 fold greater for streaming urea ammonium nitrate compared to injected urea ammonium nitrate.Urease and nitrification inhibitors did not increase corn yields with streaming urea ammonium nitrate.Streaming urea ammonium nitrate with a urease inhibitor increased N2O emissions by 18.7%. Surface streaming urea ammonium nitrate (UAN) into corn (Zea mays L.) at side‐dress (v4–v6) or later in the season (v12–v14) is an emerging N fertilizer application method as it is rapid, reduces soil disturbance, and allows for flexible application times. In 2013 and 2014, side‐dress N application (130 kg N ha−1) using three streaming UAN sources was evaluated for their ability to maintain grain yield, reduce ammonia (NH3) volatilization, and mitigate nitrous oxide (N2O) emissions. The products included streaming of urea ammonium nitrate (StrUAN), urea ammonium nitrate with a urease inhibitor (StrUAN‐UI), urea ammonium nitrate with urease inhibitor plus nitrification inhibitor (StrUAN‐UI+NI), and a control (no N). The efficacy of streaming relative to traditional shallow‐injected urea ammonium nitrate (InjUAN) was also assessed. Delayed NH3 sampling related to wind‐tunnel installation in 2013 and 2014 led to additional NH3 measurements in 2015 and 2016. Average yields from StrUAN were 11% lower relative to InjUAN. The use of inhibitors did not improve yields relative to StrUAN. Ammonia volatilization was not significantly different between StrUAN and InjUAN losing 14 and 20% of applied N (2‐yr average), respectively. However in 2015 and 2016, NH3 volatilization from StrUAN was 3.6‐fold greater than InjUAN when measurements were started immediately after application. Hence lower yields in 2013 and 2014 from StrUAN likely reflect N loss to rapid volatilization during or shortly after application. The StrUAN‐UI treatment increased 2‐yr average N2O emissions by 17.3 to 18.7% relative to StrUAN or StrUAN‐UI+NI. For humid‐temperate clay loam soil, UAN streaming with/without inhibitors was not effective for maintaining yields or reducing NH3 volatilization.

Land-use type affects N2O production pathways in subtropical acidic soils

The change in land-use from woodland to crop production leads to increased nitrous oxide (N2O) emissions. An understanding of the main N2O sources in soils under a particular land can be a useful tool in developing mitigation strategies. To better understand the effect of land-use on N2O emissions, soils were collected from 5 different land-uses in southeast China: shrub land (SB), eucalyptus plantation (ET), sweet potato farmland (SP), citrus orchard (CO) and vegetable growing farmland (VE). A stable isotope experiment was conducted incubating soils from the different land use types at 60% water holding capacity (WHC), using 15NH4NO3 and NH415NO3 to determine the dominant N2O production pathway for the different land-uses. The average N2O emission rates for VE, CO and SP were 5.30, 4.23 and 3.36 μg N kg−1 dry soil d−1, greater than for SB and ET at 0.98 and 1.10 μg N kg−1 dry soil d−1, respectively. N2O production was dominated by heterotrophic nitrification for SB and ET, accounting for 51 and 50% of N2O emissions, respectively. However, heterotrophic nitrification was negligible (<8%) in SP, CO and VE, where autotrophic nitrification was a primary driver of N2O production, accounting for 44, 45 and 66% for SP, CO and VE, respectively. Denitrification was also an important pathway of N2O production across all land-uses, accounting for 35, 35, 49, 52 and 32% for SB, ET, SP, CO and VE respectively. Average N2O emission rates via autotrophic nitrification, denitrification and heterotrophic nitrification increased significantly with gross nitrification rates, NO3− contents and C:N ratios respectively, indicating that these were important factors in the N2O production pathways for these soils. These results contribute to our understanding and ability to predict N2O emissions from different land-uses in subtropical acidic soils and in developing potential mitigation strategies.

Linking nitrous oxide emissions to population dynamics of nitrifying and denitrifying prokaryotes in four full-scale wastewater treatment plants

Ammonia-oxidizing bacteria (AOB), ammonia oxidizing archaea (AOA) and N2O-reducing denitrifiers were measured by quantitative real-time PCR (qPCR) in activated sludge samples from four full-scale wastewater treatment plants (WWTPs) in South Spain, and their abundances were linked to the generation of N2O in the samples using multivariate analysis (Non-metric multidimensional scaling, MDS, and BIO-ENV). The average abundances of AOA remained in similar orders of magnitude in all WWTPs (106 copies amoA/L activated sludge mixed liquor), while significant differences were detected for AOB (105-109copies amoA/L) and N2O-reducers (107-1010copies nosZ/L). Average N2O emissions measured in activated sludge samples ranged from 0.10 ± 0.05 to 6.49 ± 8.89 mg N2O-N/h/L activated sludge, and were strongly correlated with increased abundances of AOB and lower counts of N2O-reducers. A significant contribution of AOA to N2O generation was unlikely, since their abundance correlated negatively to N2O emissions. AOB abundance was favoured by higher NO3− and NO2−concentrations in the activated sludge.

Effects of controlled-release fertilizer on nitrous oxide and nitric oxide emissions during wheat-growing season: field and pot experiments

Outdoor pot and field experiments were conducted to study the effect of controlled-release fertilizer (CRF) on nitrous oxide (N2O) and nitric oxide (NO) emissions and wheat yields during the 2011–2013. Three treatments, namely conventional N fertilizer (CF; urea and compound fertilizer, 300 kg N ha−1), CRF (210 kg N ha−1, which is 70% of the N applied in the CF treatment), and check (CK, 0 N), were applied in three replicates. The soil temperature, soil moisture, N uptake by wheat, wheat yield, and variations in NO3–N, NH4–N, N2O, and NO emissions were measured. Results showed that the average total N2O and NO emissions in the CRF treatment in the pot and field experiments decreased by 26.5 and 19.4%, respectively. The average N2O and NO emission factors decreased by 32.1 and 24.8%, respectively, compared with the CF treatment. However, the CRF wheat yields were insignificantly lower than those from the CF treatment. Results indicated that CRF could improve the efficiency of fertilizer N use, reduce labor cost, and increase economic benefit without sacrificing the yield.

Five-year study of the effects of simulated nitrogen deposition levels and forms on soil nitrous oxide emissions from a temperate forest in northern China.

Few studies have quantified the effects of different levels and forms of nitrogen (N) deposition on soil nitrous oxide (N2O) emissions from temperate forest soils. A 5-year field experiment was conducted to investigate the effects of multiple forms and levels of N additions on soil N2O emissions, by using the static closed chamber method at Xi Mountain Experimental Forest Station in northern China. The experiment included a control (no N added), and additions of NH4NO3, NaNO3, and (NH4)2SO4 that each had two levels: 50 kg N ha−1 yr−1 and 150 kg N ha−1 yr−1. All plots were treated to simulate increased N deposition on a monthly schedule during the annual growing season (March to October) and soil N2O emissions were measured monthly from March 2011 to February 2016. Simultaneously, the temperature, moisture, and inorganic N contents of soil were also measured to explore how the main factors may have affected soil N2O emission. The results showed that the types and levels of N addition significantly increased soil inorganic N contents, and the accumulation of soil NO3–-N was significantly higher than that of soil NH4+–N due to N addition. The three N forms significantly increased the average N2O emissions (P < 0.05) in the order of NH4NO3 > (NH4)2SO4 > NaNO3 by 355.95%, 266.35%, and 187.71%, respectively, compared with control. The promotion of N2O emission via the NH4+–N addition was significantly more than that via the NO3––N addition, while N addition at a high level exerted a stronger effect than at the low-level. N addition exerted significantly stronger effects on cumulative N2O emissions in the initial years, especially the third year when the increased cumulative N2O emission reached their maximum. In the later years, the increases persisted but were weakened. Increasing inorganic N concentration could change soil from being N-limited to N-rich, and then N-saturated, and so the promotion on soil available N effect increased and then decreased. Moreover, the soil NH4+–N, NO3–-N, temperature, and water-filled pore space were all positively correlated with soil N2O emissions. These findings suggest that atmospheric N deposition can significantly promote soil N2O emission, and that exogenous NH4+–N and NO3–-N inputs into temperate forests can have synergic effects on soil N2O emission. In future research, both aspects should be better distinguished in the N cycle and balance of terrestrial ecosystems by using 15N tracer methods.

Influence of carbon to nitrogen ratio on nitrous oxide emission in an Integrated Fixed Film Activated Sludge Membrane BioReactor plant

In this study a University of Cape Town (UCT) Integrated Fixed Film Activated Sludge (IFAS) Membrane BioReactor (MBR) wastewater treatment plant was monitored in terms of nitrous oxide (N2O) emissions. The short term effect on the N2O emission due to the influent carbon-to-nitrogen (C/N) ratio variation (C/N ratios of 2, 5 and 10 gCOD/gN) was evaluated. Since in a previous study, the effect of the C/N ratio was studied in the same system without biofilm (UCT-MBR configuration) the main aim here was to investigate the role of biofilms on N2O emissions. Under all the investigated C/N ratios, the N2O fluxes and the average emission factors were lower than that of previous studies with no biofilm presence. The total average N2O emission was 0.5% of the influent nitrogen with biofilm (IFAS system) and 3.5% without biofilm. This result emphasizes the potential role of the biofilms in attenuating the N2O emissions especially in the case of stress conditions (i.e., low C/N influent ratios). An increase of N2O flux from the anoxic reactor (till 28 mgN2O m−2h−1) occurred at the lowest influent C/N tested (2 gCOD/gN - phase III). At C/N equal to 2 gCOD/gN the anoxic reactor was the main source of N2O, contributing 45% of all produced N2O. This result was attributed to an incomplete denitrification caused by a lack of organic carbon and a slight increase of dissolved oxygen concentration.

Greenhouse Gas Emissions Under Different Drainage and Flooding Regimes of Cultivated Peatlands

AbstractGlobally, approximately 10–20% of peatlands have been drained for agricultural purposes. A strategy to protect peatlands and mitigate carbon dioxide (CO2) emissions, while continuing agricultural production, is the use of intermittent flooding and drainage. A potential drawback of this strategy could be increases in methane (CH4) and nitrous oxide (N2O) emissions. The objective of this study was to compare greenhouse gas (GHG) emissions from peatlands under various flooding–drainage cycles. A laboratory study was performed using intact soil cores subjected to different durations of flooding and drainage for 6 months. Average daily emissions of CO2 and N2O were significantly higher (P &lt; 0.001) under drained (667 ± 37 mg CO2–C m−2 d−1 and 135 ± 19 μg N2O–N m−2 d−1) than flooded conditions (86 ± 6 mg CO2–C m−2 d−1 and 48 ± 2 μg N2O–N m−2 d−1). Methane emissions were not influenced by drained/flooded conditions, with an average rate of 116 ± 11 μg CH4–C m−2 d−1. Peaks of CH4 and N2O emissions were observed after flooding events and lasted less than 24 h. The peak emissions were approximately 8 and 19 times higher than the mean CH4 and N2O emissions, respectively. Carbon dioxide was the dominant component of GHGs, irrespective of hydrologic regime, accounting for more than 92% of overall global warming potential. Global warming potential was inversely proportional to the flooding period, indicating that prolonging the flooding period of peatlands would help mitigate soil oxidation and GHG emissions and enhance sustainability of these agricultural peatlands.

Long-term field measurements of annual methane and nitrous oxide emissions from a Chinese subtropical wheat-rice rotation system

Long-term, year-round monitoring is essential to obtain representative estimates of methane (CH4) and nitrous oxide (N2O) emissions from agricultural systems, yet multiyear consecutive measurements of annual CH4 and N2O emissions from subtropical rice-wheat rotation systems are rare. A six-year field experiment was conducted to simultaneously monitor CH4 and N2O emissions from a subtropical rice-wheat rotation system under three N fertilizer application rates (0 [the control], 150 and 250 kg N ha−1 season−1) in southwestern China. CH4 and N2O emissions showed great seasonal and inter-annual variations along with those of temporal weather patterns because emissions were significantly correlated with soil temperature and soil moisture (floodwater depth in rice season) throughout the experimental period. Seasonal cumulative CH4 and N2O emissions were negatively correlated in both the rice and wheat seasons across different experimental treatments and years (P < 0.05). Nitrogen fertilizer application significantly inhibited CH4 emissions but enhanced N2O emissions compared to the control. The average direct N2O emission factors (EFd) for the rice seasons (0.67–0.76%) were lower than those for the wheat seasons (1.05–1.37%). Annual yield-scaled global warming potential (GWP) of CH4 and N2O emissions for the control (2572 kg CO2-eq Mg−1 grain) was over two-fold greater than for the N150 (994 kg CO2-eq Mg−1 grain) and N250 (944 kg CO2-eq Mg−1 grain) treatments (P < 0.05). However, the lowest yield-scaled GWP obtained in the study for the rice and wheat seasons were greater than the national and global averages. Thus, in addition to optimizing N fertilizer application rates, additional management practices are required to further reduce yield-scaled GWP, thereby achieving the dual goals of sustaining and/or increasing crop yields while mitigating CH4 and N2O emissions from the subtropical rice-wheat rotation systems.

Achieving Lower Nitrogen Balance and Higher Nitrogen Recovery Efficiency Reduces Nitrous Oxide Emissions in North America's Maize Cropping Systems.

Few studies have assessed the common, yet unproven, hypothesis that an increase of plant nitrogen (N) uptake and/or recovery efficiency (NRE) will reduce nitrous oxide (N2O) emission during crop production. Understanding the relationships between N2O emissions and crop N uptake and use efficiency parameters can help inform crop N management recommendations for both efficiency and environmental goals. Analyses were conducted to determine which of several commonly used crop N uptake-derived parameters related most strongly to growing season N2O emissions under varying N management practices in North American maize systems. Nitrogen uptake-derived variables included total aboveground N uptake (TNU), grain N uptake (GNU), N recovery efficiency (NRE), net N balance (NNB) in relation to GNU [NNB(GNU)] and TNU [NNB(TNU)], and surplus N (SN). The relationship between N2O and N application rate was sigmoidal with relatively small emissions for N rates <130 kg ha−1, and a sharp increase for N rates from 130 to 220 kg ha−1; on average, N2O increased linearly by about 5 g N per kg of N applied for rates up to 220 kg ha−1. Fairly strong and significant negative relationships existed between N2O and NRE when management focused on N application rate (r2 = 0.52) or rate and timing combinations (r2 = 0.65). For every percentage point increase, N2O decreased by 13 g N ha−1 in response to N rates, and by 20 g N ha−1 for NRE changes in response to rate-by-timing treatments. However, more consistent positive relationships (R2 = 0.73–0.77) existed between N2O and NNB(TNU), NNB(GNU), and SN, regardless of rate and timing of N application; on average N2O emission increased by about 5, 7, and 8 g N, respectively, per kg increase of NNB(GNU), NNB(TNU), and SN. Neither N source nor placement influenced the relationship between N2O and NRE. Overall, our analysis indicated that a careful selection of appropriate N rate applied at the right time can both increase NRE and reduce N2O. However, N2O reduction benefits of optimum N rate-by-timing practices were achieved most consistently with management systems that reduced NNB through an increase of grain N removal or total plant N uptake relative to the total fertilizer N applied to maize. Future research assessing crop or N management effects on N2O should include N uptake parameter measurements to better understand N2O emission relationships to plant NRE and N uptake.

Nitrapyrin addition mitigates nitrous oxide emissions and raises nitrogen use efficiency in plastic-film-mulched drip-fertigated cotton field.

Nitrification inhibitors (NIs) have been used extensively to reduce nitrogen losses and increase crop nitrogen nutrition. However, information is still scant regarding the influence of NIs on nitrogen transformation, nitrous oxide (N2O) emission and nitrogen utilization in plastic-film-mulched calcareous soil under high frequency drip-fertigated condition. Therefore, a field trial was conducted to evaluate the effect of nitrapyrin (2-chloro-6-(trichloromethyl)-pyridine) on soil mineral nitrogen (N) transformation, N2O emission and nitrogen use efficiency (NUE) in a drip-fertigated cotton-growing calcareous field. Three treatments were established: control (no N fertilizer), urea (225 kg N ha-1) and urea+nitrapyrin (225 kg N ha-1+2.25 kg nitrapyrin ha-1). Compared with urea alone, urea plus nitrapyrin decreased the average N2O emission fluxes by 6.6–21.8% in June, July and August significantly in a drip-fertigation cycle. Urea application increased the seasonal cumulative N2O emission by 2.4 kg N ha-1 compared with control, and nitrapyrin addition significantly mitigated the seasonal N2O emission by 14.3% compared with urea only. During the main growing season, the average soil ammonium nitrogen (NH4+-N) concentration was 28.0% greater and soil nitrate nitrogen (NO3--N) concentration was 13.8% less in the urea+nitrapyrin treatment than in the urea treatment. Soil NO3--N and water-filled pore space (WFPS) were more closely correlated than soil NH4+-N with soil N2O fluxes under drip-fertigated condition (P<0.001). Compared with urea alone, urea plus nitrapyrin reduced the seasonal N2O emission factor (EF) by 32.4% while increasing nitrogen use efficiency by 10.7%. The results demonstrated that nitrapyrin addition significantly inhibited soil nitrification and maintained more NH4+-N in soil, mitigated N2O losses and improved nitrogen use efficiency in plastic-film-mulched calcareous soil under high frequency drip-fertigated condition.