Rates and controls of nitrous oxide and nitric oxide emissions following conversion of forest to pasture in Rondônia

Tropical soils are important sources of nitrous oxide (N₂O) and nitric oxide (NO) emissions from the Earths terrestrial ecosystems. Clearing of tropical rainforest for pasture has the potential to alter N₂O and NO emissions from soils by altering moisture, nitrogen supply or other factors that control N oxide production. In this review we report annual rates of N₂O and NO emissions from forest and pastures of different ages in the western Brazilian Amazon state of Rondônia and examine how forest clearing alters the major controls of N oxide production. Forests had annual N₂O emissions of 1.7 to 4.3 kg N ha^-1 y^-1 and annual NO emissions of 1.4 kg N ha^-1 y^-1. Young pastures of 1–3 years old had higher N₂O emissions than the original forest (3.1–5.1 kg N ha^-1 y^-1) but older pastures of 6 years or more had lower emissions (0.1 to 0.4 kg N ha^-1 y^-1). Both soil moisture and indices of soil N cycling were relatively poor predictors of N₂O, NO and combined N₂O + NO emissions. In forest, high N₂O emissions occurred at soil moistures above 30 water-filled pore space, while NO emissions occurred at all measured soil moistures (18–43). In pastures, low N availability led to low N₂O and NO emissions across the entire range of soil moistures. Based on these patterns and results of field fertilization experiments, we concluded that: (1) nitrification was the source of NO from forest soils, (2) denitrification was not a major source of N₂O production from forest soils or was not limited by NO- supply, (3) denitrification was a major source of N₂O production from pasture soils but only when NO₃^- was available, and (4) nitrification was not a major source of 3 NO production in pasture soils. Pulse wettings after prolonged dry periods increased N₂O and NO₃^- emissions for only short periods and not enough to appreciably affect annual emission rates. We project that Basin-wide, the effect of clearing for pasture in the future will be a small reduction in total N₂O emissions if the extensive pastures of the Amazon continue to be managed in a way similar to current practices. In the future, both N₂Oand NO fluxes could increase if uses of pastures change to include greater use of N fertilizers or N-fixing crops. Predicting the consequences of these changes for N oxide production will require an understanding of how the processes of nitrification and denitrification interact with soil type and regional moisture regimes to control N₂O and NO production from these new anthropogenic N sources.     

The influence of controlled release fertilisers and the form of applied fertiliser nitrogen on nitrous oxide emissions from an andosol

A laboratory experiment was conducted to determine whether applying controlled release nitrogen fertilisers could reduce nitrous oxide emissions from an andosol maintained at different water contents, compared with applying standard nitrogen fertiliser. The effect of the form of N applied (NH₄-N or NO₃-N) was also investigated. Soil was collected from an arable field and sub-samples were treated with controlled release or standard fertiliser, applied at a rate of 200 g N g^–1 dry soil either as NH₄ ⁺ or NO₃ ^–. The soils were maintained at 40%, 55%, 70% or 85% water filled pore space (WFPS) and incubated at 25 °C for 50 days. Gas samples were collected and analysed every 3–4 days and soil samples were analysed on five occasions during the incubation. Emissions of N₂O were much greater from ammonium sulphate than from calcium nitrate fertiliser, indicating that nitrification was the main source of the N₂O. Emissions at 85% WFPS were greater than at the lower water contents in all treatments. The use of controlled release NH₄-N fertilisers reduced and delayed the maximum peak of emissions, but at 55% and 70% WFPS this did not always result in lower total emissions. Emissions from the controlled release NO₃-N fertiliser were very low, but only significantly lower than from standard NO₃-N fertiliser at water contents below 85% WFPS. The results demonstrate that choosing the appropriate form of fertiliser in relation to expected soil moisture could significantly reduce N₂O emissions. Applying the fertiliser in a controlled-release form could further reduce emissions by reducing the length of time that fertiliser nitrogen is present in the soil and available for nitrification or denitrification.     

N2O and NO emissions from different N sources and under a range of soil water contents

Emissions of nitrous oxide (N₂O) and nitric oxide (NO) have been identified as one of the most important sources of atmospheric pollution from grasslands. Soils are major sources for the production of N₂O and NO, which are by-products or intermediate products of microbial nitrification and denitrification processes. Some studies have tried to evaluate the importance of denitrification or nitrification in the formation of N₂O or NO but there are few that have considered emissions of both gases as affected by a wide range of different factors. In this study, the importance of a number of factors (soil moisture, fertiliser type and temperature) was determined for N₂O and NO emissions. Nitrous oxide and NO evolution in time and the possibility of using the ratio NO:N₂O as an indicator for the processes involved were also explored. Dinitrogen (N₂) and ammonia (NH₃) emissions were estimated and a mass balance for N fluxes was performed. Nitrous oxide and NO were produced by nitrification and denitrification in soils fertilised with and by denitrification in soils fertilised with . Water content in the soil was the most important factor affecting N₂O and NO emissions. Our N₂O and NO data were fitted to quadratic (r=0.8) and negative exponential (r=0.7) equations, respectively. A long lag phase was observed for the N₂O emitted from soils fertilised with (denitrification), which was not observed for the soils fertilised with (nitrification) and was possibly due to a greater inhibiting effect of low temperatures on microbial activity controlling denitrification rather than on nitrification. The use of the NO:N₂O ratio as a possible indicator of denitrification or nitrification in the formation of N₂O and NO was discounted for soils fertilised with . The N mass balance indicated that about 50 kg N ha⁻¹ was immobilised by microorganisms and/or taken up by plant roots, and that most of the losses ocurred in wet soils (WFPS >60%) as N₂ and NH₃ losses (>55%).     

Contribution of nitrification and denitrification to N2O production in peat, clay and loamy sand soils under different soil moisture conditions

Agricultural soils are a significant source of nitrous oxide (N₂O). Since mitigation of greenhouse gas emissions is needed in all sectors of society, it is important to identify the processes producing N₂O and the factors affecting the production rates in agricultural soils. This study aimed to elucidate the N₂O production in peat, clay and loamy sand at four different soil moisture conditions (40, 60, 80 and 100% Water Filled Pore Space). The ace­tylene inhibition technique was used to evaluate the contribution of nitrification to N₂O production. Nitrous oxide production responded markedly to soil moisture in all three soils. The highest N₂O production, measured at the wettest soils (100% WFPS), was up to four orders of magnitude higher than that at the dry soils (40% WFPS). In dry conditions N₂O production decreased in the order of peat > clay > loamy sand, while in wet conditions the highest N₂O production was measured in loamy sand, then in peat, and the lowest in clay soils. Nitrification was the dominant N₂O producing process in all the soils at 60% WFPS. In the sandy soil 70% of the total N₂O production originated from nitrification, while in the peat soil most of the total N₂O production originated from denitrification. Data on processes producing N₂O in agricultural soils are needed to develop process-based models that could reduce the uncertainty of the emission estimates in greenhouse gas inventories.     

N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions

The number of published N₂O and NO emissions measurements is increasing steadily, providing additional information about driving factors of these emissions and allowing an improvement of statistical N-emission models. We summarized information from 1008 N₂O and 189 NO emission measurements for agricultural fields, and 207 N₂O and 210 NO measurements for soils under natural vegetation. The factors that significantly influence agricultural N₂O emissions were N application rate, crop type, fertilizer type, soil organic C content, soil pH and texture, and those for NO emissions include N application rate, soil N content and climate. Compared to an earlier analysis the 20% increase in the number of N₂O measurements for agriculture did not yield more insight or reduced uncertainty, because the representation of environmental and management conditions in agro-ecosystems did not improve, while for NO emissions the additional measurements in agricultural systems did yield a considerable improvement. N₂O emissions from soils under natural vegetation are significantly influenced by vegetation type, soil organic C content, soil pH, bulk density and drainage, while vegetation type and soil C content are major factors for NO emissions. Statistical models of these factors were used to calculate global annual emissions from fertilized cropland (3.3 Tg N₂O-N and 1.4 Tg NO-N) and grassland (0.8 Tg N₂O-N and 0.4 Tg NO-N). Global emissions were not calculated for soils under natural vegetation due to lack of data for many vegetation types.     

Measurements of N2O and NO emissions during tomato cultivation using a flow-through chamber system in a glasshouse

Nitrous oxide (N₂O) and nitric oxide (NO) fluxes resulting from long-term tomato cultivation in a glasshouse were continuously determined using the flow-through chamber method over the course of three cultivation periods. Gas concentrations were measured using an nondispersive infrared (gas filter correlation/infra-red) analyzer and a chemiluminescence-based analyzer, respectively. Following a basal application of fertilizer, daily N₂O and NO emission rates increased, with peaks lasting from 40 to 140 days. Short-term fluctuations in daily N₂O and NO emissions were affected by differences in nitrogen application, soil water, and soil temperature. Diurnal changes in N₂O and NO fluxes during the period of peak emissions depended primarily on soil temperature. Following the application of a top dressing (N as urea or calcium nitrate) in the irrigation water, the N₂O and NO fluxes increased immediately, with a very short period of peak emissions (1–5 h) after urea application. The duration of the peak period in daily accumulated N₂O and NO emissions following application of the top dressing ranged from 3 to 10 days.     

Increased emissions of nitric oxide and nitrous oxide following tillage of a perennial pasture

About 40% of the agricultural land in the European Union (EU) is grassland used for animal production. When grassland is tilled, organically bound carbon and nitrogen are released, providing substrates for nitrifying and denitrifying microorganisms. The aim of this study was to examine the immediate effects of tillage of a perennial grassland carried out on different dates, on the emissions of nitric oxide (NO) and nitrous oxide (N₂O), monitored intensively over a 5-day period, in a humid, dairy farming area of northern Spain. Soil was tilled 12 days and 2 days prior to fertiliser application. Tillage, time of tillage, and N fertiliser application affected NO and N₂O emissions. Tillage 12 days before the start of the flux measurements resulted in higher emissions than tillage one day before, the difference being related to differences in soil mineral N and water-filled pore space (WFPS). Emissions of NO peaked at a WFPS of 50–60%, while N₂O fluxes peaked at 70–90% WFPS. Loss of N was greater as N₂O than as NO. The total loss of N as N₂O plus NO ranged from 0.027 kg N ha^–1 in unfertilised plots to 0.56 kg N ha^–1 in the tilled and N fertilised plot. Thereafter emissions decreased rapidly to low values. The results of this study indicate that tillage of perennial grassland may release large amounts of NO and N₂O, the amounts also depending on moisture conditions and addition of N fertiliser. We suggest that in order to reduce such emissions, application of N fertiliser should not immediately follow tillage of perennial grassland, as there is an extra supply of N from mineralisation of organic matter at this time.     

Toward Improved Coefficients for Predicting Direct N<Subscript>2</Subscript>O Emissions from Soil in Canadian Agroecosystems

Agricultural soils emit nitrous oxide (N₂O), a potent greenhouse gas. Predicting and mitigating N₂O emissions is not easy. To derive national coefficients for N₂O emissions from soil, we collated over 400 treatment evaluations (measurements) of N₂O fluxes from farming systems in various ecoregions across Canada. A simple linear coefficient for fertilizer-induced emission of N₂O in non-manured soils (1.18% of N applied) was comparable to that used by the Intergovernmental Panel on Climate Change (IPCC) (1.25% of N applied). Emissions were correlated to soil and crop management practices (manure addition, N fertilizer addition and inclusion of legumes in the rotation) as well as to annual precipitation. The effect of tillage on emissions was inconsistent, varying among experiments and even within experiments from year to year. In humid regions (e.g., Eastern Canada) no-tillage tended to enhance N₂O emissions; in arid regions (e.g., Western Prairies) no-tillage sometimes reduced emissions. The variability of N₂O fluxes shows that we cannot yet always distinguish between potential mitigation practices with small (e.g., <10%) differences in emission. Our analysis also emphasizes the need for developing consistent experimental approaches (e.g., ‘control’ treatments) and methodologies (i.e. measurement period lengths) for estimating N₂O emissions.     

Emissions of nitrous oxide from boreal agricultural clay and loamy sand soils

Long-term studies of greenhouse gas fluxes from agricultural soils in different climate regions are needed to improve the existing calculation models used in greenhouse gas inventories. The aim of this study was to obtain more information on nitrous oxide (N₂O) emissions from agricultural mineral soils in the boreal region. N₂O emissions were studied during 2000–2002 on two soil types in Finland, a loamy sand and a clay with plots of grass, barley and fallow. N₂O fluxes were measured with static chambers throughout the year. Other parameters measured were water filled pore space (WFPS), soil mineral nitrogen concentration, soil porosity, soil temperature and depth of soil frost. The annual fluxes from the clay soil ranged from 3.7 to 7.8 kg N ha^–1 and those from sandy loam from 1.5 to 7.5 kg N ha^–1. On average 60% of the annual fluxes occurred outside the growing season, from October to April. Increasing the number of freeze-thaw events was found to increase the fluxes during winter and during the thawing period in spring. The results suggest that N₂O fluxes from these boreal mineral soils do not vary much as a function of applied fertiliser N and could probably be better estimated from soil physical properties, including soil porosity.     

Measurement and modeling of nitrous and nitric oxide emissions from a tea field in subtropical central China

Tea fields represent an important source of nitrous oxide (N₂O) and nitric oxide (NO) emissions due to high nitrogen (N) fertilizer applications and very low soil pH. To investigate the temporal characteristics of N₂O and NO emissions, daily emissions were measured over 2½ years period using static closed-chamber/gas chromatograph and chemiluminescent measurement system in a tea field of subtropical central China. Our results revealed that N₂O and NO fluxes showed similar temporal trends, which were generally driven by temporal variations in soil temperature and soil moisture content and were also affected by fertilization events. The measured average annual N₂O and NO emissions were 10.9 and 3.3 kg N ha^?1 year^?1, respectively, highlighting the high N₂O and NO emissions from tea fields. To improve our understanding of N-cycling processes in tea ecosystems, we developed a new nitrogenous gas emission module for the water and nitrogen management model (WNMM, V2) that simulated daily N₂O and NO fluxes, in which the NO was simulated as being emitted from both nitrification and nitrite chemical decomposition. The results demonstrated that the WNMM captured the general temporal dynamics of N₂O (NSE = 0.40; R² = 0.52, RMSE = 0.03 kg N ha^?1 day^?1, P < 0.001) and NO (NSE = 0.41; R² = 0.44, RMSE = 0.01 kg N ha^?1 day^?1, P < 0.001) emissions. According to the simulation, denitrification was identified as the dominant process contributing 76.5% of the total N₂O emissions, while nitrification and nitrite chemical decomposition accounted for 52.3 and 47.7% of the total NO emissions, respectively.     

Nitrogen dynamics during till and no-till pasture restoration sequences in Rondônia,Brazil

The clearing of tropical rain forest in the Amazon basin has created large areas of cattle pasture that are now declining in productivity. Practices adopted by ranchers to restore productivity to degraded pastures have the potential to alter soil N availability and gaseous N losses from soils. We examined how soil inorganic N pools, net N mineralization and net nitrification rates, nitrification potential and NO and N₂O emissions from soils of a degraded pasture responded to the following restoration treatments: (1) soil tillage followed by replanting of grass and fertilization, (2) no-till application of non-selective herbicide, planting of rice, harvest followed by no-till replanting of grass and fertilization, and (3) the same no-till sequence with soybeans instead of rice. Tillage increased soil NH₄⁺ and NO₃^? pools but NH₄⁺ and NO₃^? pools remained relatively constant in the control and no-till treatments. Cumulative rates of net N mineralization and net nitrification during the first 6 months after treatment varied widely but were hightest in the tilled treatment. Emissions of NO and N₂O fluxes increased with tillage and with N fertilization. There were no clear relationships among rates of N fertilizer application, net N mineralization, net nitrification, NO, N₂O and total N oxide emissions. Our results indicate that pasture restoration sequences involving tilling and fertilizing will increase emissions of N oxides, but the magnitude of the increase is likely to differ based on timing of fertilizer application relative to the presence of plants and the magnitude of plant N demand. Emissions of N oxides appear to be decreased by the use of restoration sequences that minimize reductions in pasture grass cover.     

Nitrous Oxide Emissions from a Fertilized and Grazed Grassland in the South East of Ireland

Agriculture currently accounts for 28% of national greenhouse gas emissions in Ireland. Nitrous oxide (N₂O) emissions from agricultural soils account for 38% of this total. A 2-year study was conducted, using the chamber technique on a fertilized and grazed grassland to quantify the effect of fertilizer application rate, soil and meteorological variables on N₂O emissions. Three N fertilizer regimes (0, 225 & 390 kg N ha⁻¹) were imposed with three replicates of each treatment. Nitrogen fertilizer was applied as urea (46% N) in spring with calcium ammonium nitrate (CAN-26% N) applied in the summer (June–September). Rotational grazing was practiced using steers. Nitrous oxide emissions arising from the unfertilized plots (0 N) were consistently low. Emissions from the N-fertilized plots (225 & 390 kg N ha⁻¹) were concentrated in relatively short periods (1–2 weeks) following fertilizer applications and grazing, with marked differences between treatments, relative patterns and magnitudes of emissions at different times of the year and between years. Variation in N₂O emissions throughout both years was pronounced with mean coefficients of variation of 116% in year 1 and 101% in year 2. The study encompassed two climatologically contrasting years. As a result the N₂O–N loss, as a percent of the N applied in the cooler and wetter 2002 (0.2–2.0%), were similar to those used for N-fertilized grasslands under the Intergovernmental Panel on Climate Change (IPCC) N₂O emission inventory calculation methodology (1.25% ± 1). In contrast, the percentage losses in the warmer and drier 2003 (3.5–7.2%) were substantially higher.     

Modelling nitrous oxide emissions from dairy-grazed pastures

Soil N₂O emissions were measured during four seasons from two highly productive grass-clover dairy pastures to assess the influences of soil moisture, temperature, availability of N (NH ₄ ⁺ and NO ₃ ^– ) and soluble C on N₂O emissions, and to use the emission data to validate and refine a simulation model (DNDC). The soils at these pasture sites (Karapoti fine sandy loam, and Tokomaru silt loam) differed in texture and drainage characteristics. Emission peaks for N₂O coincided with rainfall events and high soil moisture content. Large inherent variations in N₂O fluxes were observed throughout the year in both the ungrazed (control) and grazed pastures. Fluxes averaged 4.3 and 5.0 g N₂O/ha/day for the two ungrazed sites. The N₂O fluxes from the grazed sites were much higher than for the ungrazed sites, averaging 26.4 g N₂O/ha/day for the fine sandy loam soil, and 32.0 g N₂O/ha/day for the silt loam soil. Our results showed that excretal and fertiliser-N input, and water-filled pore space (WFPS) were the variables that most strongly regulated N₂O fluxes. The DNDC model was modified to include the effects of day length on pasture growth, and of excretal-N inputs from grazing animals; the value of the WFPS threshold was also modified. The modified model NZ-DNDC simulated effectively most of the WFPS and N₂O emission pulses and trends from both the ungrazed and grazed pastures. The modified model fairly reproduced the real variability in underlying processes regulating N₂O emissions and could be suitable for simulating N₂O emissions from a range of New Zealand grazed pastures. The NZ-DNDC estimates of total yearly emissions of N₂O from the grazed and ungrazed sites of both farms were within the uncertainty range of the measured emissions. The measured emissions changed with changes in soil moisture resulting from rainfall and were about 20% higher in the poorly drained silt loam soil than in the well-drained sandy loam soil. The model accounts for these climatic variations in rainfall, and was also able to pick up differences in emissions resulting from differences in soil texture.     

N2O and NO emissions from a field of Chinese cabbage as influenced by band application of urea or controlled-release urea fertilizers

A field experiment was conducted in an Andosol in Tsukuba, Japan to study the effect of banded fertilizer applications or reduced rate of fertilizer N (20% less) on emissions of nitrous oxide (N₂O) and nitric oxide (NO), and also crop yields of Chinese cabbage during the growing season in 2000. Six treatments were applied by randomized design with three replications, which were; no N fertilizer (CK); broadcast application of urea (BC); band application of urea (B); band application of urea at a rate 20% lower than B (BL); band application of controlled-release urea (CB) and band application of controlled-release urea at a rate 20% lower than CB (CBL). The results showed that reduced application rates, applied in bands, of both urea (BL) and controlled-release urea fertilizer (CBL) produced yields that were not significantly lower than yields from the full rate of broadcast urea (BC). The emissions of N₂O and NO from the reduced fertilizer treatments (BL, CBL) were lower than that of normal fertilizer rates (B, CB). N₂O and NO emissions from controlled-release urea applied in band mode (CB, CBL) were less than those from urea applied in band mode (B, BL). The total emissions of N₂O and NO indicated that applying fertilizers in band mode mitigated NO emission from soils, but N₂O emissions from banded urea (B) were no lower than from broadcast urea (BC).     

Effects of the depth of NO and N2O productions in soil on their emission rates to the atmosphere: analysis by a simulation model

Many factors are concerned in the changing forms of nitrogen compounds in soil, so it is not easy to make precise models to simulate the concentration profiles of soil nitric oxide (NO) and nitrous oxide (N₂O) and their emission rates under various soil conditions. We prepared a simple mathematical simulation model based on soil concentration profiles of NO and N₂O. The profiles were measured at lysimeters filled with Andosol soil and fertilized with ammonium sulfate at rate of 200 kgNha^-1, incorporating to plow layer (Hirose & Tsuruta, 1996). In this model, diffusion of gases in soil followed Fick's law and the diffusion coefficient was adopted from Sallam et al. (1984). The gas production rate was set up at constant value in the site of gas production, and the gaseous consumption followed Michaelis-Menten kinetics. By changing only the depth of NO and N₂O production in soil in this model, we obtained the following results.(1) When the depth of gas production was set at near the soil surface (NO: 0–10 cm, N₂O: 0-8 cm), the emission rates of both gases corresponded with the results of the lysimeter-measurement.(2) When the depth of gas production was shifted down 10 cm deeper (NO: 10–20 cm, N₂O: 10-18 cm), the gas emission rate of NO decreased to 1.3% of (1), while that of N₂O was almost the same as (1).(3) In the case that the total intensity of produced gases was not changed from (1) or (2), but that the extent of gas productions expanded 3 times wider (NO: 0–30 cm, N₂O: 0–24 cm) than (1) or (2), the emission rates of NO and N₂O became 26% and 95% of (1), respectively.The above results suggest the possibility of mitigating NO emission by setting the site of gaseous production in deeper soil, e.g. by means of deep application of fertilizer.     

Comparing measured and Expert-N predicted N2O emissions from conventional till and no till corn treatments

Agricultural soils are a major source of the greenhouse gas nitrous oxide (N₂O). Nitrous oxide emission models can be used to predict the effectiveness of N₂O mitigation strategies; however, these models require rigorous testing before they can be used with confidence. Expert-N, a modular process based N₂O emission model, was tested to determine its ability at predicting nitrogen (N) cycling in the soil–plant–atmosphere system under Canadian agroclimatic conditions. Ancillary data and N₂O emissions were collected/measured from a corn cultivated clay-loam soil that was under different tillage and red clover treatments. The treatments were conventional till (CT) with and without red clover (rc) underseeded in the previous year's wheat crop (CT-Crc and CT-C, respectively), and no till (NT) with and without red clover underseeded in the previous year's wheat crop (NT-Crc and NT-C, respectively). Expert-N provided good estimates of N₂O emissions, and predictions correlated well (positive) with the measured emissions (r ² 0.55–0.83). There was no statistically significant difference between measured and predicted daily emissions. The predicted emissions, integrated over the growing season (25 May–4 October, 1995), were 0.56, 0.57, 0.62, and 0.62 kg N₂O-N ha^–1 for CT-C, CT-Crc, NT-C, and NT-Crc, respectively. The measured emissions over the same period were 1.29, 1.07, 0.96, and 1.04 kg N₂O-N ha^–1 for CT-C, CT-Crc, NT-C, and NT-Crc, respectively. The modelled emissions underestimated the integrated measured emissions by 35–55%; however, the integrated measured emissions had an estimated uncertainty of ±35%. The model provided good predictions of the soil temperatures, moisture contents, and soil nitrate levels with no significant difference from the measured data. Correlations between modelled and measured values for these soil properties in the first 30 cm soil layer were positive and high with r ² 0.71–0.93.     

Effects of deep application of urea on NO and N2O emissions from an Andisol

A modeling study revealed that the depth of nitric oxide (NO) production in soil is crucial for its flux, while that of nitrous oxide (N₂O) is not. To verify this result, laboratory experiments with soil columns classified as Andisol (Hydric Hapludand) were conducted, with changing the depth of urea application, at 0–0.1 or 0.1–0.2 m. All the NO concentration profiles in soil exhibited a sharp peak at each fertilized layer within 5 days of fertilizer application. NO concentration in soil decreased abruptly as the distance from the fertilized layer increased. These findings imply that NO is produced mainly within the fertilized layer, but does not diffuse widely in the soil columns, because of rapid NO uptake within the soil. As a result, the NO flux from soil columns fertilized at 0.1–0.2 m depth over the 48-day study period was reduced to almost the same rate as that of the unfertilized one. The total NO emissions from soil columns unfertilized and fertilized at 0–0.1 and 0.1–0.2 m depth were 0.02, 1.39 (± 0.05) and 0.05 (± 0.03) kg N ha^–1, respectively, suggesting that NO emission derived from N fertilizer could be reduced to 2% by shifting the depth of fertilizer application by 0.1 m. On the other hand, soil N₂O concentration profiles exhibited a gentler peak, because of the lower uptake by soil. N₂O fluxes were affected more by the soil conditions, e.g. soil water content, than the distance between fertilized depth and soil surface. The total N₂O emissions from soil columns unfertilized and fertilized at 0–0.1 and 0.1–0.2 m were 0.02, 0.16 (± 0.03) and 0.25 (± 0.04) kg N ha^–1, respectively.     

Canadian greenhouse gas mitigation options in agriculture

In 1991, on farm management practices contributed 57.6 Tg CO₂ equivalent in greenhouse gas emissions, that is, about 10% of the anthropogenic GHG emissions in Canada. Approximately 11% of these emissions were in the form of CO₂, 36% in the form of CH₄ and 53% in the form of N₂O. The CO₂ emissions were from soils; CH₄ emissions were from enteric fermentation and manure, and N₂O emissions were primarily a function of cropping practices and manure management. With the emissions from all other agricultural practices included, such as the emissions from fossil fuels used for transportation, manufacturing, food processing etc., the agricultural sector's contributions were about 15% of Canada's emissions. In this publication, several options are examined as to their potential for reducing greenhouse gas emissions. These involve soil and crop management, soil nutrient management, improved feeding strategies, and carbon storage in industrial by-products. The Canadian Economic Emissions Model for Agriculture (CEEMA) was used to predict the greenhouse gas emissions for the year 2010, as well as the impact of mitigation options on greenhouse gas emissions from the agricultural sector. This model incorporates the Canadian Regional Agricultural sub-Model (CRAM), which predicts the activities related to agriculture in Canada up to 2010, as well as a Greenhouse Gas Emissions sub-Model (GGEM), which estimates the greenhouse gas emissions associated with the various agricultural activities. The greenhouse gas emissions from all agricultural sources were 90.5 Tg CO₂ equivalent in 1991. Estimates based on CEEMA for the year 2010 indicate emissions are expected to be 98.0 Tg CO₂ equivalent under a business as usual scenario, which assumes that the present trends in management practices will continue. The agricultural sector will then need to reduce its emissions by about 12.9 Tg CO₂ equivalent below 2010 forecasted emissions, if it is to attain its part of the Canadian government commitment made in Kyoto. Technologies focusing on increasing the soil carbon sink, reducing greenhouse gas emissions and improving the overall farming efficiency, need to be refined and developed as best management practices. The soils carbon sink can be increased through reduced tillage, reduced summer fallowing, increased use of grasslands and forage crops, etc. Key areas for the possible reduction of greenhouse gas emissions are improved soil nutrient management, improved manure storage and handling, better livestock grazing and feeding strategies, etc. The overall impact of these options is dependent on the adoption rate. Agriculture's greenhouse gas reduction commitment could probably be met if soils are recognized as a carbon sink under the Kyoto Accord and if a wide range of management practices are adopted on a large scale. None of these options can currently be recommended as measures because their socio-economic aspects have not been fully evaluated and there are still too many uncertainties in the emission estimates. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrous oxide mitigation potential of reduced tillage and N input in durum wheat in the Mediterranean

In Italy, managed soils account for about 50% of annual national emissions of nitrous oxide (N₂O), thus the effect of agricultural practices on N₂O emissions must be studied in order to develop mitigation strategies. Soil N₂O emissions were measured in two field campaigns (2013–2014 and 2014–2015) on durum wheat in a Mediterranean environment to test the mitigation potential of reduced tillage and nitrogen (N) fertilization rate. N₂O emissions were measured with a fully-transportable instrument developed during the project LIFE?+?IPNOA “Improved flux Prototypes for N₂O emission reduction from Agriculture” and equipped with an infrared laser detector. Reducing tillage from ploughing to minimum tillage had no effect on average daily N₂O flux, while decreasing the N rate from 170 to 110 kg N ha^?1 reduced the average daily N₂O flux, without negatively affecting the grain yield. Furthermore, N₂O daily flux were positively correlated with soil water filled pore space, NO₃-N, and NH₄-N concentrations, and they were largely variable between the two field campaigns as a result of different environmental and management conditions (i.e.: rainfall, different amount of crop residues incorporated in soil). Overall, the innovative fully-transportable instrument performed well in the field and allowed us to conclude that decreasing the N fertilizer rate was a valuable option to mitigate N₂O emissions without negative effects on wheat productivity.     

Quantification of N-losses as NH3, NO, and N2O and N2 from fertilized maize fields in southwestern France

Emissions of nitrogen compounds (NO, NH₃, N₂O and N₂) from heavily fertilized (280 kg(N) ha^-1) and irrigated maize fields were studied over an annual cultivation cycle in southwestern France. NO and N₂O emissions were measured by chamber techniques throughout the year. During fertilization and maize growth periods, chamber measurements were intensified and complemented by flux-gradient micrometeorological measurements of NO_x and NH₃. The two methods used, Bowen ratio and a simplified aerodynamical techniques, agree quite well and quantify NO_x and NH₃ flux variations during the period of intense emission which followed fertilizer application. Over a yearly cycle, nitrogen loss in the form of NH₃, NO and N₂O were calculated using micrometeorological flux measurements and emission algorithms calibrated with field data (chambers). The soil denitrification potential represented by the ratio N₂O/(N₂O+N₂) was measured in the laboratory to calculate potential total gaseous nitrogen loss. Taking into account all uncertainties, the total N loss into the atmosphere represents 30 to 110 kg(N) ha^-1 with about less than 1% as NH₃, 40% as NO, 14% as N₂O and 46% as N₂. This is in agreement with the agronomic nitrogen budget based on the N fertilizer input and soil furniture and, on the N-output by crops and crop residues, which displays a net imbalance of 50 to 100 kg(N) ha^-1.