Direct N2O Emissions Research Articles

Quantifying On‐Farm Nitrous Oxide Emission Reductions in Food Supply Chains

AbstractReducing nitrous oxide (N2O) emissions from agriculture is critical to limiting future global warming. In response, a growing number of food retailers and manufacturers have committed to reducing N2O emissions from their vast networks of farmer suppliers by providing technical assistance and financial incentives. A key challenge for such companies is demonstrating that their efforts are leading to meaningful progress toward their climate mitigation commitments. We show that a simplified version of soil surface nitrogen (N) balance—or partial N balance—the difference between N inputs to and outputs from a farm field (fertilizer N minus crop N), is a robust indicator of direct N2O emissions from fields with maize and other major rainfed temperate‐region crops. Furthermore, we present a generalized environmental model that will allow food‐supply‐chain companies to translate aggregated and anonymized changes in average N balance across their supplying farms into aggregated changes in N2O emissions. This research is an important first step, based on currently available science, in helping companies demonstrate the impact of their sustainability efforts.

Tea-planted soils as global hotspots for N2O emissions from croplands

Tea-planted soils generally receiving high nitrogen (N) fertilizer doses are more vulnerable to acidification, and turn into significant sources of the potent greenhouse gas nitrous oxide (N2O). However, little is known about the magnitude of soil N2O emissions from global tea plantations. Based on a global meta-analysis of field experimental data collected from major tea growing countries, we quantify annual N2O emissions, calculate direct emission factors (EFd) and identify key environmental controls of emissions from tea plantations. However, most data are from China and Japan, which is to be expected given that tea plantations in these countries represent >60% of the global area and the vital environmental research community in both countries. Results suggest that annual N2O emissions from soils of global tea plantations are on average 17.1 kg N ha−1 (or 8008 kg CO2-eq ha−1), being substantially greater than those reported for cereal croplands (662–3757 kg CO2-eq ha−1). The global mean EFd for N applications to tea plantations equals 2.31% (with a 95% confidence interval of 1.91%–2.71%), being two times higher than the Intergovernmental Panel on Climate Change default value of 1%. Across tea plantations worldwide, total N2O emissions are estimated to be 57–84 Gg N yr−1, or 1.5%–12.7% of total direct cropland N2O emissions. Given that tea plantations account for only 0.3% of total cropland area, our finding highlights that tea-planted soils are global hotspots for N2O emissions and that these systems might be prime targets for climate change mitigation in the agricultural sector. Considering that tea is a high price commodity for which consumers may be willing to apply pressure for more climate-smart production, possible mitigation efforts include use of controlled-release fertilizers or nitrification inhibitors, and application of biochar and/or lime for increasing soil pH; i.e. measures that increase N use efficiency while reducing the climate footprint of tea production.

Nitrogen dynamics in a winter-wheat field in Ehime, southwestern Japan

ABSTRACT Nitrogen (N) is the most important and limiting factor in crop production. Thus, a clear understanding of its cycle is important to improve its efficiency and reducing environmental pollution. A 2-year experiment was conducted to investigate the N flows on a winter-wheat field during 2017 to 2019 in Ehime Prefecture, Japan. We also evaluated N budgets based on N input and N output among different N flows in a N dynamic model. Full fertilizer (coated urea at 120 N kg ha−1) application was used. Two additional treatments: no and low fertilizer N were set up in a randomized block design with four replications to estimate N sources of plant uptake and emitted N2O. Direct and indirect N2O emission were measured and estimated using a closed chamber technique and emission factor, respectively. N leaching and soil mineralized N were estimated using microlysimeters. Bulk N deposition was measured using the rain trap. Total plant N uptake (208 kg N ha−1) and aboveground yield (190 kg N ha−1) were major N outflows from the field. N in plants originated from fertilizer and soil N were 58.1% and 41.9%, respectively. Leached N (30.1 kg N ha−1) from the surface 30-cm depth of soil was also a major outflow. Mineralized soil N was estimated as 104 kg N ha−1 in the surface soil and contributed to 91% of leached N. Higher mean nitrogen use efficiency (101%) of fertilizer N was possibly because of utilization of coated fertilizer and climatic conditions in the field. Indirect and direct N2O emissions were quite low. A negative N budget (−86.8 kg N ha−1) suggested that supplementation of N by organic matter, such as compost and biochar, which can suppress N leaching, would be required to maintain soil N status and environmentally friendly farming.

Grazing under Irrigation Affects N2O-Emissions Substantially in South Africa

Fertilized agricultural soils serve as a primary source of anthropogenic N2O emissions. In South Africa, there is a paucity of data on N2O emissions from fertilized, irrigated dairy-pasture systems and emission factors (EF) associated with the amount of N applied. A first study aiming to quantify direct N2O emissions and associated EFs of intensive pasture-based dairy systems in sub-Sahara Africa was conducted in South Africa. Field trials were conducted to evaluate fertilizer rates (0, 220, 440, 660, and 880 kg N ha−1 year−1) on N2O emissions from irrigated kikuyu–perennial ryegrass (Pennisetum clandestinum–Lolium perenne) pastures. The static chamber method was used to collect weekly N2O samples for one year. The highest daily N2O fluxes occurred in spring (0.99 kg ha−1 day−1) and summer (1.52 kg ha−1 day−1). Accumulated N2O emissions ranged between 2.45 and 15.5 kg N2O-N ha−1 year−1 and EFs for mineral fertilizers applied had an average of 0.9%. Nitrogen in yielded herbage varied between 582 and 900 kg N ha−1. There was no positive effect on growth of pasture herbage from adding N at high rates. The relationship between N balance and annual N2O emissions was exponential, which indicated that excessive fertilization of N will add directly to N2O emissions from the pastures. Results from this study could update South Africa’s greenhouse gas inventory more accurately to facilitate Tier 3 estimates.

Changes in direct CO2 and N2O emissions from a loam Haplic Luvisol under conventional moldboard and reduced tillage during growing season and post-harvest period of red clover

Abstract The objectives of the study were to: (1) assess the strength of associations of direct CO2 and N2O emissions with the seasonal variations in the relevant soil properties under both tillage systems; 2) evaluate how CT and RT affect magnitudes of seasonal CO2 and N2O fluxes from soil. Field studies were carried out on plots for conventional tillage (up to 0.22–0.25 m) and reduced tillage (up to 0.10–0.12 m) during the growing season and post-harvest period of red clover. The results showed that daily CO2 emissions significantly correlated only with soil temperature during the growing season under conventional and reduced tillage. Soil temperature demonstrated its highest influence on daily N2O emissions only at the beginning of the growing season in both tillage systems. There were no significant inter-system differences in daily CO2 and N2O emissions from soil during the entire period of observations. Over the duration of post-harvest period, water-filled pore space was a better predictor of daily CO2 emissions from soils under CT and RT. The conventional and reduced tillage did not cause significant differences in cumulative N2O and CO2 fluxes from soil.

N2O emission factors of full-scale animal manure windrow composting in cold and warm seasons

Emission of nitrous oxide (N2O) during animal manure composting is of great concern, and its emission factor (EF) is important for greenhouse gas emission inventory, while the EF is still uncertain due to limited on-site full-scale observations worldwide. In this study, N2O emissions were monitored during different seasons in a full-scale swine manure windrow composting with pile volume of about 76.5 m3. The results showed that the maximum N2O flux during the cold season (CS) was 23 times higher than during the warm season (WS), significant differences in the contribution to direct N2O emissions were observed in three composting stages, and shaded-side N2O emission was higher than sunny-side emission. The direct N2O emission factors of animal manure composting were 0.0046, 0.0002 kg N2O-N/kgTN (dry weight) in the CS and WS, respectively. Scenario analysis results showed that windrow composting is a suitable manure management that emits less N2O than solid storage.

Nitrous oxide emissions during microalgae-based wastewater treatment: current state of the art and implication for greenhouse gases budgeting.

Microalgae can synthesise the ozone depleting pollutant and greenhouse gas nitrous oxide (N2O). Consequently, significant N2O emissions have been recorded during real wastewater treatment in high rate algal ponds (HRAPs). While data scarcity and variability prevent meaningful assessment, the magnitude reported (0.13-0.57% of the influent nitrogen load) is within the range reported by the Intergovernmental Panel on Climate Change (IPCC) for direct N2O emissions during centralised aerobic wastewater treatment (0.016-4.5% of the influent nitrogen load). Critically, the ability of microalgae to synthesise N2O challenges the IPCC's broad view that bacterial denitrification and nitrification are the only major cause of N2O emissions from wastewater plants and aquatic environments receiving nitrogen from wastewater effluents. Significant N2O emissions have indeed been repeatedly detected from eutrophic water bodies and wastewater discharge contributes to eutrophication via the release of nitrogen and phosphorus. Considering the complex interplays between nitrogen and phosphorus supply, microalgal growth, and microalgal N2O synthesis, further research must urgently seek to better quantify N2O emissions from microalgae-based wastewater systems and eutrophic ecosystems receiving wastewater. This future research will ultimately improve the prediction of N2O emissions from wastewater treatment in national inventories and may therefore affect the prioritisation of mitigation strategies.

Biochar amendment mitigates greenhouse gases emission and global warming potential in dairy manure based silage corn in boreal climate

About 11% of the global anthropogenic greenhouse gases (GHGs) emissions result from agricultural practices. Dairy manure (DM) application to soil is regarded as a best management practice due to C sequestration and improvement of soil physiochemical properties. However, GHGs emissions from the soil following the DM application could offset its advantages. Biochar (BC) is known to affect N transformation and GHGs emissions from soil. There had been considerably less focus on the BC amendment and its effects on GHGs emissions following DM application under field conditions. The objectives of this study were; i) to determine the temporal patterns and cumulative GHGs fluxes following DM and inorganic nitrogen (IN) application and, ii) to investigate BC amendment impact on DMY, GWP, direct N2O emission factor (EFd) and the response of CH4 emissions (RC) in DM based silage corn. To achieve these objectives a two-year field experiment was conducted with these treatments: 1) DM with high N conc. (DM1: 0.37% N); 2) DM with low N conc. (DM2: 0.13% N); 3) IN; 4) DM1+BC; 5) DM2+BC; 6) IN + BC; and 7) Control (N0); and were laid out in randomized complete block design with four replications. BC amendment to DM1, DM2 and IN significantly reduced cumulative CO2 emission by 16, 25.5 and 26.5%, CH4 emission by 184, 200 and 293% and N2O emission by 95, 86 and 93% respectively. It also reduced area-scaled and yield-scaled GWP, EFd, RC and enhanced DMY. Thus, BC application showed great potential to offset the negative effects of DM application i.e GHGs emissions from the silage corn cropping system. Further research is needed to evaluate soil organic carbon and nitrogen dynamics (substrates for GHG emissions) after DM and BC application on various soil types and cropping systems under field conditions.

Life cycle assessment of mulch use on Okanagan apple orchards: Part 1 - Attributional

Food production contributes substantially to anthropogenic environmental impacts, including 19–29% of greenhouse gas (GHG) emissions. Use of wood and bark chip mulch as a soil cover has previously been found to reduce direct N2O emissions and increase soil organic carbon on apple orchards. The current study expanded the scope of this prior investigation to include the “upstream” processes that support production and transportation of the mulch used on orchards using ISO-compliant life cycle assessment. An “attributional” life cycle assessment was conducted to determine the net life cycle environmental impacts of the production of apples on an Okanagan orchard, focusing on the relative contribution of bark and wood chip mulch used as a soil amendment, both compared to other life cycle stages and processes, and to a system producing apples without the use of mulch. This research focused on only the orchard-level GHG reductions benefit associated with mulch use. A variety of environmental impact categories were examined including human toxicity, freshwater aquatic ecotoxicity, depletion of abiotic resources (elements, ultimate reserves), photochemical oxidation, ozone layer depletion, terrestrial ecotoxicity, acidification potential, climate change, eutrophication, land use – land competition, and energy use (including non-renewable: fossil, nuclear, primary forest; and renewable: biomass, geothermal, solar, water and wind). When the entire life cycle was considered, apples produced using mulch had higher life cycle GHG emissions than those without mulch, as well as higher impacts in all of the other impact categories considered. Due to these higher emissions with mulch, the results do not support the recommendation of mulch application on orchards as a GHG mitigation strategy, nor with respect to the other impact categories considered.

Is farmers’ agricultural production a carbon sink or source? – Variable system boundary and household survey data

The widespread and scattered smallholders in China have a profound impact on agricultural production in response to climate change. Using the data of peasant household obtained from survey and the factor coefficients collected in Shaanxi Province of China, the carbon footprint (CF) of farmer’s agricultural production (FAP) was estimated by a multi system boundary scenarios approach to judge the contribution of FAP for climate change reasonably. The results showed that the agricultural activities’ carbon footprint (ACF), farm’s carbon footprint (FCF) and product’s carbon footprint (PCF) of wheat, maize, rice and apple crops in FAP exhibited significant differences in four scenarios, and the positive climate externalities of FAP rely on its own duality of carbon effect. Distribution structure of ACF in FAP subsystem under the four scenarios indicated that allocation of emission responsibility has a significant impact on the carbon effect of FAP. The categories of key source in FAP were greenhouse gas emissions from heterotrophic soil respiration in agro-ecosystem (AE), chemical fertilizer in raw material production system (RMPS), diesel or gasoline in RMPS and agricultural production system and direct N2O emissions in AE, and the key sink categories were carbon sequestration from crop biomass and cropland management in AE. Therefore, the ecological benefits should be embedded in the economic benefit-oriented agricultural production policy objectives, in order to reduce the dependence on high energy-consuming industrial products in FAP, optimize the agricultural industrial structure and guide farmers from cleaner production behavior. Besides, the uncertainties of CF accounting method drop significantly which we improved by integrating carbon-nitrogen cycle into life cycle assessment theory.

Erratum to: Greenhouse gas mitigation through dairy manure acidification

Erratum to: Greenhouse gas mitigation through dairy manure acidification

Calculation of the carbon footprint for family farms using the Farm Accountancy Data Network: A case from Lithuania

This study appraises the greenhouse gas (GHG) emissions for Lithuanian family farms based on the Intergovernmental Panel on Climate Change (IPCC) guidelines using Lithuanian emission factors from Lithuania’s National Inventory report (LNIR). Family farm activity data are derived from Lithuanian sample of the Farm Accountancy Data Network (FADN) for 2016. The GHG emission profile for Lithuanian family farms includes the estimates for methane (CH4) from enteric fermentation of domestic livestock population, CH4 from manure management, direct and indirect nitrous oxide (N2O) emissions from manure management, direct and indirect N2O emissions from managed soils, and carbon dioxide (CO2) from combustion of fuel. In Lithuanian family farms, on average, the key source categories of on-farms emissions were CH4 from enteric fermentation and N2O direct emissions from agricultural soils, as they together constituted 69.3% of the total emissions. The environmental pressures related to family farming were measured in terms of the Carbon Footprint (CF), which refers to the total amount of GHG produced by farming activities, and Carbon Intensity (CI) using a range of metrics including CI per land area, total output and Livestock Unit (LU). In 2016, CF stood at the average value of 57.8 t CO2eq farm−1, CI per UAA – 1.5 t CO2eq ha−1, CI per total output – 2.7 kg CO2eq EUR−1 and per LU – 6.0 t CO2eq LU−1.

Soil N2O emissions, N leaching and marine eutrophication in life cycle assessment – A comparison of modelling approaches

Nitrogen fertilisation is an essential part of modern agriculture, providing food for a growing human population, but also causing environmental impacts when reactive nitrogen (N) is released to the environment. The amount and impact of these emissions are difficult to quantify in life cycle assessment (LCA), due to their site-dependent nature. This study compared seven models for direct soil nitrous oxide (N2O) emissions, seven models for N leaching and five characterisation models for marine eutrophication impact assessment, selected to represent medium-effort options for accounting for spatial variation in emissions and impact assessment. In a case study, the models were applied to wheat cultivation at two Swedish sites to estimate climate and marine eutrophication impact. Direct N2O emissions estimated by the models varied by up to five-fold at one of the sites and contributed 21–56% of the total climate impact. Site-dependent models gave both lower and higher N2O emissions estimates than the site-generic Tier 1 model from the Intergovernmental Panel on Climate Change (IPCC). Estimated N leaching also varied by up to fivefold at one of the sites and contributed 47–93% of the total eutrophication potential, depending on model choice. All site-dependent models estimated lower N leaching than the site-generic IPCC Tier 1 model. Marine eutrophication impact estimates varied by almost an order of magnitude depending on characterisation model choice. The large variation between models found in this study highlights the importance of model choice for N emissions and marine eutrophication impact assessment in LCA of crop cultivation. Due to the divergence of model outcomes and different limitations of some of the models, no general recommendations on choosing soil N2O emissions model, N leaching model or characterisation model for marine eutrophication could be given.

Nitrous oxide emissions from oilseed rape cultivation were unaffected by flash pyrolysis biochar of different type, rate and field ageing

Nitrous oxide (N2O) emission from winter oilseed rape (WOSR) cultivation may compromise the sustainability of oilseed rape biodiesel. Typically, greenhouse gas budgets of WOSR cultivation assume an N2O emission factor (EF) of 1% of the N added in fertilizer and crop residues. Management options to reduce direct soil emissions of N2O include the application of biochar, but efficacy and mechanisms of N2O suppression are elusive. We measured N2O emissions in a WOSR field trial on a sandy loam soil in Denmark over 402 days in 2017–2018, comparing biochar applications from two feedstocks (wheat straw and pig manure fibers), two application rates (1.5 and 15 Mg ha−1) and field ageing of up to three years. Further, a controlled incubation experiment was performed to examine the effect of biochar dose and ageing on N2O production and consumption by denitrification. Biochar treatments had no significant effects on cumulative N2O emissions (1.71–2.78 kg N ha−1 yr−1). Likewise, no significant effects were found on crop yield, yield-scaled N2O emission, soil mineral N content, gravimetric soil moisture or pH. The fertilizer induced EF was 0.51% which is well below the IPCC Tier 1 EF of 1%. High doses of fresh, but not field-aged biochar suppressed N2O production under anoxic conditions ex situ, suggesting that biochar with sufficient liming capacity could mitigate N2O emissions from denitrification also under field conditions. Yet, rates of up to 15 Mg ha−1 flash pyrolysis biochar in the current in situ study, which comprised a pronounced summer drought, showed no significant N2O mitigation. This highlights the need for selecting dedicated biochars and doses and test them in multi-year studies to conclude on their N2O mitigating effect. Yet, in relation to sustainability of WOSR cultivation for biodiesel, the current study suggests that C sequestration by biochar is not compromised by increased N2O emissions.

Potential use of a new nitrogen trading tool to assess nitrogen management practices to protect groundwater quality

The Nitrogen Loss and Environmental Assessment Package (NLEAP) with Geographic Information Systems (GIS), version 5.0, with a Nitrogen Trading Tool (NTT) is a model that can be used to integrate site-specific information about soils, weather, and management to conduct quick assessments of nitrogen (N) management practices to increase N use efficiencies while minimizing N losses. This tool is a valuable resource that can be used to conduct quick assessments of risky combinations of management practices and landscapes across several fields to identify hot spot areas that are susceptible to nitrogen losses. The NLEAP GIS 5.0 NTT can quickly assess the temporal and spatial N losses of a given scenario simulated across all the fields and subtract them from the baseline scenario. This allows the user to compare scenarios and use the tool as a nitrogen trading tool to assess the potential reductions in nitrate leaching or emissions of nitrous oxide that may be achieved with a given management practice. The tool can be used to quickly assess the potential to use management practices to reduce the potential losses of reactive nitrogen to groundwater and the atmosphere, as well as the potential to trade these reductions in reactive nitrogen in water or air quality markets. It can quickly calculate the potential savings (reductions in) direct and indirect N2O emissions (and the carbon sequestration equivalent) achieved due to improvements in N management practices. The carbon sequestration equivalent could also be traded in air quality markets. There is potential to use the tool to assess which areas are more sensitive to nitrate leaching, which can impact groundwater quality, and to assess what would be the best management practices to implement in these areas for protection of groundwater quality.

Production of spinach in intensive Mediterranean horticultural systems can be sustained by organic-based fertilizers without yield penalties and with low environmental impacts

Agricultural production of leafy vegetables in Mediterranean countries aims to achieve high yields without elevated nitrate contents in the edible parts. This implies an adjusted nutrient management, especially of nitrogen (N), in irrigated horticultural systems under semiarid conditions. These horticultural systems are highly relevant in SE Spain from an economic perspective. However, the management of N fertilizer, generally applied in large amounts (150–250 kg N ha−1 in a split application), could trigger losses of reactive N to the environment. The use of novel fertilizers may fulfill the nitrogen requirements of the crop, but should also help to decrease the environmental impacts of production, thus achieving carbon-neutral horticultural systems through (e.g.) enhancement of carbon (C) stocks and greenhouse gases (GHG) emission abatement. In this experiment, eight different fertilizing scenarios at a normalized N application rate of 150 kg N ha−1 were assessed in terms of crop yields, nutrients uptake, C stocking capacity, and yield-scaled GHG emissions. Inorganic NPK fertilizers, digestates, biosolids, composts, and vermicomposts were included among this set of fertilizers. Our results show that organic-based stabilized materials, especially composts, lowered the NO3− concentration in spinach leaves, in comparison to organic raw materials and synthetic fertilizers. They also produced yields similar to those of slow-release synthetic fertilizers, but with a significant increase in soil organic C 61 days after application. In general, N2O emissions were positively affected by the treatments. Nevertheless, direct N2O emissions were generally low (the highest emission factor, 0.13, being for the biosolid treatment) due to the combined mitigating effect of both the edapho-climatic conditions and the management practices. In general, cumulative CO2 emissions were high in all organic scenarios compared to the control treatment (299 kg C-CO2 ha−1), the highest values being observed in the treatment with biosolid (589 kg C-CO2 ha−1), probably due to differences in the labile organic C contents. In conclusion, some of the organic-based treatments showed multiple positive effects: on crop quality (i.e. leaf N content), crop yields, and GHG mitigation potential. Based on our results, the use of these materials represents an optimized N fertilizer strategy to help achieve a circular economy, by closing nutrient loops and decreasing the environmental impacts of horticultural production systems in semiarid regions of southern Europe.

Greenhouse gas emissions from synthetic nitrogen manufacture and fertilization for main upland crops in China

BackgroundA significant source of greenhouse gas (GHG) emissions comes from the manufacture of synthetic nitrogen (N) fertilizers consumed in crop production processes. And the application of synthetic N fertilizers is recognized as the most important factor contributing to direct N2O emissions from agricultural soils. Based on statistical data and relevant literature, the GHG emissions associated with synthetic N manufacture and fertilization for wheat and maize in different provinces and agricultural regions of China were quantitatively evaluated in the present study.ResultsDuring the 2015–2017 period, the average application rates of synthetic N for wheat and maize in upland fields of China were 222 and 197 kg ha−1, respectively. The total consumption of synthetic N on wheat and maize was 12.63 Mt year−1. At the national scale, the GHG emissions associated with the manufacture of synthetic N fertilizers were estimated to be 41.44 and 59.71 Mt CO2-eq year−1 for wheat and maize in China, respectively. And the direct N2O emissions derived from synthetic N fertilization were estimated to be 35.82 and 69.44 Gg N2O year−1 for wheat and maize, respectively. In the main wheat-cultivating regions of China, area-scaled GHG emissions were higher for Inner Mongolia, Jiangsu and Xinjiang provinces. And for maize, Gansu, Xinjiang, Yunnan, Shannxi and Jiangsu provinces had higher area-scaled GHG emissions. Higher yield-scaled GHG emissions for wheat and maize mainly occured in Yunnan and Gansu provinces.ConclusionsThe manufacture and application of synthetic N fertilizers for wheat and maize in Chinese croplands is an important source of agricultural GHG emissions. The current study could provide a scientific basis for establishing an inventory of upland GHG emissions in China and developing appropriate mitigation strategies.

Feasibility of stabilised nitrogen fertilisers decreasing greenhouse gas emissions under optimal management in sprinkler irrigated conditions

Stabilised nitrogen (N) fertilisers with nitrification and urease inhibitors have been proposed to abate greenhouse gas (GHG) emissions in agrosystems. Nevertheless, differences in their application and in the management of water and nitrogen rates make it difficult to evaluate their actual utility. The aim of this study was to analyse the possibility for GHG emissions reduction in a 3-year rotation (maize-maize-wheat) by substituting the traditional split-urea application to maize by a single side-dress application of stabilised urea fertiliser. The experiment was performed in 24 drainage lysimeters in two contrasting soil types (Shallow and Deep) under efficient irrigation practices and adjusted N rates under Mediterranean conditions. Nitrous oxide (N2O) and methane (CH4) were measured using static closed unvented chambers, and the soil mineral N was monitored through periodic soil samplings. CH4 emissions were generally negligible with occasional tendency the soil acting as a sink more than as a net source. Direct N2O emissions during the whole rotation showed lower values when a nitrification inhibitor (3,4-dimethylpyrazole phosphate) was added than with conventional urea (Deep soil: 73% lower, p < 0.05; Shallow soil: 60% lower, ns). Urease inhibitors (N-(n-butyl) thiophosphoric triamide and monocarbamide dihydrogen sulphate) could not abate direct N2O emissions, and their effect depended on the soil type. However, all stabilised fertilisers mitigated N2O emissions in Deep soil when scaled by grain yield (average 54%). Indirect N2O emissions associated with nitrate leaching were not affected by the treatments but contributed more to total N2O emissions in Shallow soil (12%) than in Deep soil (6%). These results suggest that adequate use of nitrification inhibitors could have environmental benefits without lessening agronomic production.

Impact of water table levels and winter cover crops on greenhouse gas emissions from cultivated peat soils

Drainage and cultivation have turned peatlands from carbon (C) sinks into hotspots for greenhouse gas (GHG) emissions. Raising the water table and planting of winter cover crops are potential strategies to help reduce peat oxidation and re-initiate net C accumulation during the non-cropping period. However, the effects of these practices as well as their interactions on GHG emissions remain unclear. Here, we carried out an outdoor mesocosm experiment to elucidate the effect of water table levels (−30 cm and −50 cm) and winter cover crop cultivation (vetch, rye, no plant) on carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes during the winter period (November-April). Soil-atmosphere GHG exchange, GHG concentrations within the peat profile and soil water solute concentrations were monitored. Our results showed that high water table significantly reduced ecosystem respiration, while it had no net effect on N2O and CH4 fluxes. Uptake of available N by the cover crop significantly reduced nitrate in soil solution, thereby lowering the potential for leaching and both direct and indirect N2O emissions. No interactive effects between water table levels and cover crops were detected for any of the measured GHG fluxes. Seasonal variations of GHG fluxes were positively correlated with soil air concentrations at −15 cm and −40 cm depths, which were further regulated by dissolved organic C, nitrate concentration, and anaerobic conditions in the soil. This study suggests that there is great potential to raise water table levels and introduce green cover crops to reduce GHG emissions. Further studies are needed to achieve a complete evaluation of these strategies outside of the growing season, which may provide a significant mitigation benefit in C-rich cultivated peatlands.

Nitrous oxide (N2O) emissions during real domestic wastewater treatment in an outdoor pilot-scale high rate algae pond

In order to determine if nitrous oxide (N2O) emissions could affect the sustainability of microalgae-based pond systems, N2O emissions were recorded from an outdoor 900 L pilot high rate algal pond (HRAP) fed primary wastewater over 1 year. The HRAP was mixed using a paddle wheel and operated at a hydraulic retention time (HRT) of 7.5–10 days. Direct N2O emissions ranged from 4.1 to 6400 μg N-N2O·m−2·d−1 (median of 560 μg N-N2O·m−2·d−1, n = 28) at 10 days HRT, and 70–18300 μg N-N2O·m−2·d−1 (median of 4200 μg N-N2O·m−2·d−1, n = 22) at 7.5 days HRT. Using 25–75% of the data, we estimated that HRAPs designed for nitrogen removal operated at 7.5 days HRT (i.e. 9.5 m2·capita-1 required) would generate 12–53 g N2O·capita·yr-1 which is 4–17 fold higher than the default value of 3.2 g N2O·capita·yr-1 given by the Intergovernmental Panel on Climate Change for centralized wastewater treatment plants with controlled nitrification and denitrification steps. When indirect N2O emissions (via nitrogen discharge and ammonia volatilization) are included, a HRAP operated at 7.5 days HRT could generate total emissions equivalent to 21–138 g N2O·capita−1·yr−1. When expressed as a % of the nitrogen input load into the system, the HRAP direct emissions (i.e. 0.13–0.57%) and total (i.e. 0.23–1.5%) where within the range of 0–14.6% reported in the literature for centralized wastewater treatment.