Effect of crop-specific field management and N fertilization on N2O emissions from a fine-loamy soil

Agricultural soils are a major source of atmospheric N₂O. This study was conducted to determine the effect of different crop-specific field management and N fertilization rates on N₂O emissions from a fine-loamy Dystric Eutrochrept. Fluxes of N₂O were measured for two years at least once a week on plots cropped with potatoes (Solanum tuberosum) fertilized with 50 or 150 kg N ha⁻¹ a⁻¹, winterwheat (Triticum aestivum) fertilized with 90 or 180 kg N ha⁻¹ a⁻¹, corn (Zea mays) fertilized with 65 or 130 kg N ha⁻¹ a⁻¹, and on an unfertilized, set-aside soil planted with grass (mainly Lolium perenne and Festuca rubra). The mean N₂O emission rate from the differently managed plots was closely correlated to the mean soil nitrate content in the Ap horizon for the cropping period (April to October, r ² = 0.74), the winter period (November to March, r ² = 0.93, one outlier excluded), and the whole year (r ² = 0.81). N₂O emissions outside the cropping period accounted for up to 58% of the annual emissions and were strongly affected by frost-thaw cycles. There was only a slight relationship between the amount of fertilizer N applied and the annual N₂O emission (r ² = 0.20). The mean annual N₂O-N emission from the unfertilized set-aside soil was 0.29 kg ha⁻¹. The annual N₂O-N emission from the fertilized crops for the low and the recommended rates of N fertilization were 1.34 and 2.41 kg ha⁻¹ for corn, 2.70 and 3.64 kg ha⁻¹ for wheat, and 5.74 and 6.93 kg ha⁻¹ for potatoes. The high N₂O emissions from potato plots were due to (i) high N₂O losses from the interrow area during the cropping season and (ii) high soil nitrate contents after the potato harvest. The reduction of N fertilization (fertilizer was applied in spring and early summer) resulted in decreased N₂O emissions during the cropping period. However, the emissions during the winter were not affected by the rate of N fertilization. The results show that the crop-specific field management had a great influence on the annual N₂O emissions. It also affected the emissions per unit N fertilizer applied. The main reasons for this crop effect were crop-specific differences in soil nitrate and soil moisture content. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of soil solution on the dynamics of N2O emissions: a review

In this review, which consists of two parts, major interactions between nitrous oxide (N₂>O) and soil solution are described. In the first part, as an introduction, concentrations of dissolved N₂>O in different aqueous systems are summarized. An inventory of data on maximal N₂>O concentrations in soil solution (up to 9984 g N₂>O-N l^–1>) and in soil air (up to 8300 ppm) from literature is presented. The peak N₂>O concentrations represent a N₂>O supersaturation in the soil solution up to 30000 times with respect to ambient air and a soil air N₂>O concentration about 25000 times higher than in the atmosphere. The main physico–chemical parameters (solubility, diffusion) controlling N₂>O distribution between soil solution and soil air are outlined. The influences of cultivation practice, nitrogen turnover, water content and temperature on N₂>O a ccumulation in soil solution and soil air are reviewed. In the second part some models of N₂>O dynamics in soils are discussed with emphasis on N₂>O transport processes. A simple qualitative scheme is developed to categorize the effects of the soil solution on N₂>O dynamics in soils. In this scheme the temporary, intensive N₂>O oversaturation of the soil solution is interpreted as a result of gas diffusion inhibition by water (barrier function of soil solution) resulting in an accumulation of N₂>O. In addition, N₂>O supersaturation is an indication that transitory much N₂>O can be stored in the soil solution (storage function of soil solution). Where the soil solution flows up-, down- or sidewards it can act as a relevant transport medium for dissolved N₂>O (transport function of soil solution). This scheme is applied to examples from the literature.     

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.     

Impact of different forms of N fertilizer on N2O emissions from intensive grassland

Nitrous oxide (N₂O) emissions were measured over two years from an intensively managed grassland site in the UK. Emissions from ammonium nitrate (AN) and urea (UR) were compared to those from urea modified by various inhibitors (a nitrification inhibitor, UR(N), a urease inhibitor, UR(U), and both inhibitors together, SU), as well as a controlled release urea (CR). N₂O fluxes varied through time and between treatments. The differences between the treatments were not consistent throughout the year. After the spring and early summer fertilizer applications, fluxes from AN plots were greater than fluxes from UR plots, e.g. the cumulative fluxes for one month after N application in June 1999 were 5.2 ± 1.1 kg N₂O-N ha^–1 from the AN plots, compared to 1.4 ± 1.0 kg N₂O-N ha^–1 from the UR plots. However, after the late summer application, there was no difference between the two treatments, e.g. cumulative fluxes for the month following N application in August 2000 were 3.3 ± 0.7 kg N₂O-N ha^–1 from the AN plots and 2.9 ± 1.1 kg N₂O-N ha^–1 from the UR plots. After all N applications, fluxes from the UR(N) plots were much less than those from either the AN or the UR plots, e.g. 0.2 ± 0.1 kg N₂O-N ha^–1 in June 1999 and 1.1 ± 0.3 kg N₂O-N ha^–1 in August 2000. Combining the results of this experiment with earlier work showed that there was a greater N₂O emission response to rainfall around the time of fertilizer application in the AN plots than in the UR plots. It was concluded that there is scope for reducing N₂O emissions from N-fertilized grassland by applying UR instead of AN to wet soils in cool conditions, e.g. when grass growth begins in spring. Applying UR with a nitrification inhibitor could cut emissions further.     

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%).     

Urease and nitrification inhibitors to reduce emissions of CH4 and N2O in rice production

Strategies used to reduce emissions of N₂O and CH₄ in rice production normally include irrigation management and fertilization. To date, little information has been published on the measures that can simultaneously reduce both emissions. Effects of application of a urease inhibitor, hydroquinone (HQ), and a nitrification inhibitor, dicyandiamide (DCD) together with urea (U) on N₂O and CH₄ emission from rice growing were studied in pot experiments. These fertilization treatments were carried out in the presence and absence of wheat straw, applied to the soil surface. Without wheat straw addition, in all treatments with inhibitor(s) the emission of N₂O and CH₄ was significantly reduced, as compared with the treatment whereby only urea was applied (control). Especially for the U+HQ+DCD treatment, the total emission of N₂O and CH₄ was about 1/3 and 1/2 of that in the control, respectively. In the presence of wheat straw, the total N₂O emission from the U+HQ+DCD treatment was about 1/2 of that from the control. The total CH₄ emission was less influenced. Wheat straw addition, however, induced a substantial increase in emissions of N₂O and CH₄. Hence, simultaneous application of organic materials with a high C/N ratio and N-fertilizer (e.g. urea) is not a suitable method to reduce the N₂O and CH₄ emission. Application of HQ+DCD together with urea seemed to improve the rice growth and to reduce both emissions. The NO₃ ^–-N content of the rice plants and denitrification of (NO₃ ^–+NO₂ ^–)-N might contribute to the N₂O emission from flooded rice fields.     

Subsoils: chemo-and biological denitrification, N2O and N2 emissions

Agricultural practices, soil characteristics and meteorological conditions are responsible for eventual nitrate accumulation in the subsoil. There is a lot of evidence that denitrification occurs in the subsoil and rates up to 60–70 kg ha^-1 yr^-1 might be possible. It has also been shown that in the presence of Fe²⁺ (formed through weathering of minerals) and an alkaline pH, nitrate can be chemically reduced. Another possible pathway of disappearance is through the formation of nitrite, which is unstable in acid conditions. With regard to the emission of N₂O and N₂, it can be stated that all conditions whereby the denitrification process becomes marginal are favourable for N₂O formation rather than for N₂. Because of its high solubility, however, an important amount of N₂O might be transported with drainage water.     

Recovery of groundwater N2O at the soil surface and its contribution to total N2O emissions

Production and accumulation of the major greenhouse gas nitrous oxide (N₂O) in surface groundwater might contribute to N₂O emissions to the atmosphere. We report on a ¹⁵N tracer study conducted in the Fuhrberger Feld aquifer in northern Germany. A K¹⁵NO₃ tracer solution (60 atom%) was applied to the surface groundwater on an 8 m² measuring plot using 45 injection points in order to stimulate production of ¹⁵N₂O by denitrification and to detect its contribution to emissions at the soil surface. Samples from the surface groundwater, from the unsaturated zone and at the soil surface were collected in regular intervals over a 72-days period. Total N₂O fluxes at the soil surface were low and in a range between ?7.6 and 29.1 μg N₂O-N m^?2 h^?1. ¹⁵N enrichment of N₂O decreased considerably upwards in the profile. In the surface groundwater, we found a ¹⁵N enrichment of N₂O between 13 and 42 atom%. In contrast, ¹⁵N enrichment of N₂O in flux chambers at the soil surface was very low, but a detectable ¹⁵N enrichment was found at all sampling events. Fluxes of groundwater-derived ¹⁵N-N₂O were very low and ranged between 0.0002 and 0.0018 kg N₂O-N ha^?1 year^?1, indicating that indirect N₂O emissions from the surface groundwater of the Fuhrberger Feld aquifer occurring via upward diffusion are hardly significant. Due to these observations we concluded that N₂O dynamics at the soil–atmosphere interface is predominantly governed by topsoil parameters. However, highest ¹⁵N enrichments of N₂O throughout the profile were obtained in the course of a rapid drawdown of the groundwater table. We assume that such fluctuations may enhance diffusive N₂O fluxes from the surface groundwater to the atmosphere for a short time.     

Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions

Annual cycles of NO, NO₂ and N₂O emission rates from soil were determined with high temporal resolution at a spruce (control and limed plot) and beech forest site (Höglwald) in Southern Germany (Bavaria) by use of fully automated measuring systems. The fully automated measuring system used for the determination of NO and NO₂ flux rates is described in detail. In addition, NO, NO₂ and N₂O emission rates from soils of different pine forest ecosystems of Northeastern Germany (Brandenburg) were determined during 2 measuring campaigns in 1995. Mean monthly NO and N₂O emission rates (July 1994–June 1995) of the untreated spruce plot at the Höglwald site were in the range of 20–130 µg NO-N m^-2 h^-1 and 3.5–16.4 µg N₂O-N m^-2 h^-1, respectively. Generally, NO emission exceeded N₂O emission. Liming of a spruce plot resulted in a reduction of NO emission rates (monthly means: 15–140 µg NO-N m^-2 h^-1) by 25-30% as compared to the control spruce plot. On the other hand, liming of a spruce plot significantly enhanced over the entire observation period N₂O emission rates (monthly means: 6.2–22.1 µg N₂O-N m^-2 h^-1). Contrary to the spruce stand, mean monthly N₂O emission rates from soil of the beech plot (range: 7.9–102 µg N₂O-N m^-2 h^-1) were generally significantly higher than NO emission rates (range: 6.1–47.0 µg NO-N m^-2 h^-1). Results obtained from measuring campaigns in three different pine forest ecosystems revealed mean N₂O emission rates between 6.0 and 53.0 µg N₂O-N m^-2 h^-1 and mean NO emission rates between 2.6 and 31.1 µg NO-N m^-2 h^-1. The NO and N₂O flux rates reported here for the different measuring sites are high compared to other reported fluxes from temperate forests. Ratios of NO/N₂O emission rates were >> 1 for the spruce control and limed plot of the Höglwald site and << 1 for the beech plot. The pine forest ecosystems showed ratios of NO/N₂O emission rates of 0.9 ± 0.4. These results indicate a strong differentiating impact of tree species on the ratio of NO to N₂O emitted from soil.     

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).     

Field-scale management and environmental drivers of N2O emissions from pasture-based dairy systems

Nutrient Cycling in Agroecosystems - The rational nitrogen (N) application rate for crops depends not only on yield but also on the environmental cost of N loss to the...     

Impact of mineral N fertilizer application rates on N2O emissions from arable soils under winter wheat

Nitrogen fertilizers are a major source of nitrous oxide (N₂O) emissions from arable soils. The relationship between nitrogen application rates and N₂O emissions was evaluated during the growth period of winter wheat (~140 days) at six field sites in north-western Germany. Nitrogen was applied as calcium–ammonium–nitrate, with application rates ranging between 0 and 400 kg N ha^?1. One trial was conducted in 2010, three trials in 2011 and two trials in 2012. Additionally, post-harvest N₂O emissions were evaluated at two field sites during autumn and winter (2012–2013). The emission factors (during the growth period) varied between 0.10 and 0.37 %. Annual N₂O emissions ranged between 0.46 and 0.53 % and were consistently lower across all sites and years than to the IPCC Tier 1 default value (1.0 %). Across all sites and years, the relationship between N₂O and N application rate was best described by linear regression even if nitrogen amounts applied were higher than the nitrogen uptake of the crop. Additionally, annual N₂O emissions per unit of harvested wheat grain were calculated for two field sites to assess the environmental impact of wheat grain production. Yield-scaled N₂O emissions followed a hyperbolic function with a minimum of 177 and 191 g N₂O–N t grain yield^?1 at application rates of 127 and 150 kg N ha^?1, followed by an increase at higher N application rates. This relationship indicates that wheat crop fertilization does not necessarily harm the environment through N₂O emissions compared to zero fertilization. Thus, improving nitrogen use efficiency may be the best management practice for mitigating yield-scaled N₂O emissions.     

Methane (CH4) and nitrous oxide (N2O) emissions from the system of rice intensification (SRI) under a rain-fed lowland rice ecosystem in Cambodia

The present field study investigated the effects of the system of rice intensification (SRI) on greenhouse gas emissions and rice yield, in the first field trial of its kind in Cambodia. The study was a 2 × 4 factorial design, including SRI and conventional management practices (CMP) with the following treatments: control, composted farmyard manure (FYM), mineral fertiliser (MF) and FYM + MF. The results indicated large seasonal variations of CH₄ patterns during the growing season with a peak emission of about 1,300 mg CH₄ m^?2 day^?1 under both production systems 2 weeks after rice transplanting. There was large temporal variability of CH₄ fluxes from morning to midday. Emission of N₂O was below the detection limit in both systems. Under each production system, the highest seasonal emission of CH₄ was under the FYM + MF treatment, namely 282 kg ha^?1 under CMP and 213 kg ha^?1 under SRI. Total CH₄ emission under SRI practices was reduced by 22 % in the FYM treatment, 17 % in the MF treatment and 24 % in the FYM + MF treatment compared to CMP. There was no effect of water management on CH₄ emission in the non-fertilized treatment. Grain yields were not significantly affected by the production system. Thus the yield-scaled global warming potential (GWP) was lower under SRI than CMP, namely 21 % in FYM and FYM + MF treatments, and 8 % in MF treatment. The application of mineral fertilisers moderately increased CH₄ emission but significantly increased rice yields, resulting in a significantly lower yield-scaled-GWP compared to farmyard manure.     

Evaluation of the N2O emissions from N in plant residues as affected by environmental and management factors

A review of the N₂O-N emission from crop residues was conducted based on new data published during the last decade. The result indicated that factors as type of crop, biochemical quality of residues, agricultural management, climate and season of the year, soil properties and soil moisture play a significant role in the rate of N₂O-N emissions. An emission factor (EF) equal to 1.055% of N applied in plant residues – derived from a simple linear regression of emitted N₂O-N (kg ha^?1) on N applied in crop residues (kg ha^?1) – represent an estimate that explains about 60% of emission variations. However, the EF of N applied in plant residues is not a constant but a variable coefficient that depends on environmental and management variables. The following two linear models – that estimate emitted N₂O-N (kg ha^?1) as a function of the variables N (kg ha^?1) applied in plant residues (NPR), rain (mm), temperature (°C) and temperature²(°C²) – were fitted to the dataset with 45 observations obtained from the reviewed literature. $$\hskip1.5pc\hbox{N}_{2}\hbox{O}\hbox{-}\hbox{N}=-4.154+0.00955\hbox{ NPR}+1.7278\hbox{ ApM}+0.003996\hbox{ Rain }+0.6242\hbox{ Tem }-0.0230\hbox{ Tem}^{2}$$ and $$\hbox{N}_{2}\hbox{O}\hbox{-}\hbox{N}= 0.6535 + [-0.0404 + 0.0078\hbox{ ApM }+ 0.000044\hbox{ Rain }+ 0.00567\hbox{ Tem }-0.0001975\hbox{ Tem}^{2}]\hbox{ NPR }$$ Both models provided almost equally good statistical fit to the data, with R ²=0.832 and R ²=0.829, respectively, and most regression coefficients being significant at $P < 0.01$ . Because of its internal structure, the second model is more appealing as it represents N₂O-N emission as a transformation that is affected by management and environmental variables. The following expression – that correspond to the quantities in the square bracket at the right hand side of the second model – is the coefficient for the variable N applied in crop residues, and represent the emission factor as a function of application method of plant residues, rain, temperature and temperature². $$\hskip3.5pc\hbox{EF }=-0.0404+0.0078\hbox{ ApM }+0.000044\hbox{Rain }+0.00567\hbox{ Tem }- 0.0001975\hbox{ Tem}^{2}$$ Standardization of research methodologies and data gathering and reporting, including kind of crop, N content of applied residues, agricultural management, length of the measuring period, climate, soils properties, soil temperature and water content, would facilitate further advances in studies oriented to increase the precision of N₂O-N emission estimates.     

Estimates of the interannual variations of N2O emissions from agricultural soils in Canada

The DNDC model was used to estimate direct N₂O emissions from agricultural soils in Canada from 1970 to 1999. Simulations were carried out for three soil textures in seven soil groups, with two to four crop rotations within each soil group. Over the 30-year period, the average annual N₂O emission from agricultural soils in Canada was found to be 39.9 Gg N₂O–N, with a range from 20.0 to 77.0 Gg N₂O–N, and a general trend towards increasing N₂O emissions over time. The larger emissions are attributed to an increase in N-fertilizer application and perhaps to a trend in higher daily minimum temperatures. Annual estimates of N₂O emissions were variable, depending on timing of rainfall events and timing and duration of spring thaw events. We estimate, using DNDC, that emissions of N₂O in eastern Canada (Atlantic Provinces, Quebec, Ontario) were approximately 36% of the total emissions in Canada, though the area cropped represents 19% of the total. Over the 30-year period, the eastern Gleysolic soils had the largest average annual emissions of 2.47 kg N₂O–N ha^–1 y^–1 and soils of the dryer western Brown Chernozem had the smallest average emission of 0.54 kg N₂O–N ha^–1 y^–1. On average, for the seven soil groups, N₂O emissions during spring thaw were approximately 30% of total annual emissions. The average N₂O emissions estimates from 1990 to 1999 compared well with estimates for 1996 using the IPCC methodology, but unlike the IPCC methodology our modeling approach provides annual variations in N₂O emissions based on climatic differences.     

N2O and CH4 emissions from fertilized agricultural soils in southwest France

Emissions of nitrogen compounds from heavily fertilized and irrigated maize fields have been studied in the Southwest of France, over an annual cultivation cycle. Strong nitrous oxide emissions from denitrification were observed after application of nitrogen fertilizer. Flux intensity appears to be stimulated by rain or irrigation. Emission algorithms, taking into account both nitrogen input and soil water content were established on the basis of the experimental data set. They allowed us to estimate annual nitrogen loss in the form of nitrous oxide modulated by rainfall. Production of methane is observed at the level of the water table under anoxic conditions. Nevertheless, the net flux between soil and atmosphere is negative for most of the time. When methane is produced, fluxes were very low due to methane oxidation in the soil surface layer.     

The impact of grassland ploughing on CO2 and N2O emissions in the Netherlands

The contribution of ploughing permanent grassland and leys to emissions of N₂O and CO₂ is not yet well known. In this paper, the contribution of ploughing permanent grassland and leys, including grassland renovation, to CO₂ and N₂O emissions and mitigation options are explored. Land use changes in the Netherlands during 1970–2020 are used as a case study. Three grassland management operations are defined: (i) conversion of permanent grassland to arable land and leys; (ii) rotations of leys with arable crops or bulbs; and (iii) grassland renovation. The Introductory Carbon Balance Model (ICBM) is modified to calculate C and N accumulation and release. Model calibration is based on ICBM parameters, soil organic N data and C to N ratios. IPCC emission factors are used to estimate N₂O-emissions. The model is validated with data from the Rothamsted Park Grass experiments. Conversion of permanent grassland to arable land, a ley arable rotation of 3 years ley and 3 years arable crops, and a ley bulb rotation of 6 years ley and one year bulbs, result in calculated N₂O and CO₂ emissions totalling 250, 150 and 30 ton CO₂-equivalents ha^–1, respectively. Most of this comes from CO₂. Emissions are very high directly after ploughing and decrease slowly over a period of more than 50 years. N₂O emissions in 3/3 ley arable rotation and 6/1 ley bulb rotation are 2.1 and 11.0 ton CO₂-equivalents ha^–1 year^–1, respectively. From each grassland renovation, N₂O emissions amount to 1.8 to 5.5 ton CO₂-equivalents ha^–1. The calculated total annual emissions caused by ploughing in the Netherlands range from 0.5 to 0.65 Mton CO₂-equivalents year^–1. Grassland renovation in spring offers realistic opportunities to lower the N₂O emissions. Developing appropriate combinations of ley, arable crops and bulbs, will reduce the need for conversion of permanent pasture. It will also decrease the rotational losses, due to a decreased proportion of leys in rotations. Also spatial policies are effective in reducing emissions of CO₂ and N₂O. Grassland ploughing contributes significantly to N₂O and CO₂ emissions. The conclusion can be drawn that total N₂O emissions are underestimated, because emissions from grassland ploughing are not taken into account. Specific emission factors and the development of mitigation options are required to account for the emissions and to realise a reduction of emissions due to the changes in grassland ploughing.     

Use of the 15?N gas flux method to measure the source and level of N2O and N2 emissions from grazed grassland

Understanding the contribution of nitrification and denitrification to production of nitrous oxide (N₂O), a potent greenhouse gas, is important in devising effective mitigation strategies to reduce emissions. In this study the ¹⁵N gas flux method was used to investigate N₂O and N₂ emissions following an application of ¹⁵N labelled ammonium nitrate (0.71?mol?N?m^?2) to intensive grassland swards (grazed at 2.74 or 2.05 livestock units ha^?1 year^?1) at a site in Southern Ireland. The ¹⁵N labelled fertiliser (NO₃ moiety ¹⁵N labelled at 60 at. %) was applied to designated soil areas in the field, enclosed by static chambers, in June 2009, September 2009 and March 2010. Fluxes of N₂O and N₂ were determined over 12?days on each occasion. N₂O and N₂ emissions were significantly (P?<?0.001) lower in March 2010 than in June or September 2009. There was little difference between the two swards grazed at different stocking rates on N₂O or N₂ emissions. Mean cumulative N₂O emissions over 103?h were 212.9, 279.5 and 62.06?mg?m^?2 for June 2009, September 2009 and March 2010, respectively. Mean cumulative N₂ emissions for the three time periods were 818.8, 893.8 and 87?mg?m^?2, respectively. The N₂O mole fraction averaged 0.21 and 0.23 in June 2009 and September 2009, respectively, but increased to 0.41 in March 2010 which may have been due to changes in denitrifier community composition or due to N₂O reductase being sensitive to low soil temperatures. The results point to denitrification of nitrate as the major source of N₂O at this site which may have implications for choice of fertiliser in moist temperate climates.     

NO and N2O emissions from savanna soils following the first simulated rains of the season

Data on the emissions of oxides of nitrogen from the soil during the early part of the wet season are reported for nutrient-rich and nutrient-poor sandy soils at Nylsvley, South Africa. The emissions of NO_x and N₂O following the first wetting event of the season are elevated relative to subsequent events. The observed high emission rates (76 ng N-NO m^-2 s^-1) are partially attributed to the sandiness of the soil, which permits NO to diffuse out of the soil rapidly. The pulse of high emissions following wetting is maintained for approximately 72 hours, thereafter continuing at around 20 ng NO m^-2 s^-1 while the soil remains moist. The initial pulse is suggested to be due to the accumulation of a substrate pool during the dry period, coupled with an inability of plants and microbes to use it effectively during the first few days after wetting. There were no significant differences in the peak or subsequent emission rates for either NO or N₂O between two sites of differing nitrogen mineralisation potentials. N₂O emissions averaged 8% of NO_x emissions. The enhanced emissions of NO_x which follow the first wetting after a prolonged dry period do not make a very large contribution to the annual gaseous N emission budget, but could be a significant contributor to the high tropospheric ozone levels observed over southern Africa in springtime.     

Soil N2O and CO2 emissions from cotton in Australia under varying irrigation management

Irrigation is known to stimulate soil microbial carbon and nitrogen turnover and potentially the emissions of nitrous oxide (N₂O) and carbon dioxide (CO₂). We conducted a study to evaluate the effect of three different irrigation intensities on soil N₂O and CO₂ fluxes and to determine if irrigation management can be used to mitigate N₂O emissions from irrigated cotton on black vertisols in South-Eastern Queensland, Australia. Fluxes were measured over the entire 2009/2010 cotton growing season with a fully automated chamber system that measured emissions on a sub-daily basis. Irrigation intensity had a significant effect on CO₂ emission. More frequent irrigation stimulated soil respiration and seasonal CO₂ fluxes ranged from 2.7 to 4.1 Mg-C ha^?1 for the treatments with the lowest and highest irrigation frequency, respectively. N₂O emission happened episodic with highest emissions when heavy rainfall or irrigation coincided with elevated soil mineral N levels and seasonal emissions ranged from 0.80 to 1.07 kg N₂O-N ha^?1 for the different treatments. Emission factors (EF = proportion of N fertilizer emitted as N₂O) over the cotton cropping season, uncorrected for background emissions, ranged from 0.40 to 0.53 % of total N applied for the different treatments. There was no significant effect of the different irrigation treatments on soil N₂O fluxes because highest emission happened in all treatments following heavy rainfall caused by a series of summer thunderstorms which overrode the effect of the irrigation treatment. However, higher irrigation intensity increased the cotton yield and therefore reduced the N₂O intensity (N₂O emission per lint yield) of this cropping system. Our data suggest that there is only limited scope to reduce absolute N₂O emissions by different irrigation intensities in irrigated cotton systems with summer dominated rainfall. However, the significant impact of the irrigation treatments on the N₂O intensity clearly shows that irrigation can easily be used to optimize the N₂O intensity of such a system.