Soil water content and freezing temperature affect freeze–thaw related N2O production in organic soil

An organic agricultural soil was exposed to freeze–thaw cycles (FTC) using either intact soil cores or cores packed with homogenized soil. The cores were first exposed to two mild FTCs (–1.5°C/+4°C) with soil water content being 56–85% of the water-filled pore space (WFPS). Both intact and packed soil cores showed high N₂O emissions when the soil was thawing and had high WFPS. The second freeze–thaw cycle induced lower N₂O emission than the first. After the mild FTCs, a deep frost (–15°C) was applied. This greatly increased the N₂O emissions when the soil was thawing. Freezing–thawing had a smaller effect on CO₂ than on N₂O release. The results show that both soil moisture and the severity of frost modify the N₂O burst after thawing, and N₂O release (denitrification) was favoured more by FTC than heterotrophic microbial activity (CO₂ production) in general. The possible reason for this difference is discussed.     

Nitrous oxide emissions from paddy soil in three rice-based cropping systems in China

Rice-flooding fallow, rice-wheat, and double rice-wheat systems were adopted in pot experiment in an annual rotation to investigate the effects of cropping system on N₂O emission from rice-based cropping systems. The annual N₂O emission from the rice-wheat and the double rice-wheat cropping systems were 4.3 kg N ha^–1 and 3.9 kg N ha^–1, respectively, higher than that from rice-flooding fallow cropping system, 1.4 kg N ha^–1. The average N₂O flux was 115 and 118 g N m^–2 h^–1 for rice season in rice-wheat system and early rice season in double rice-wheat system, respectively, 68.6 and 35.3 g N m^–2 h^–1 for the late rice season in double rice-wheat system and rice season in rice-flooding fallow, respectively, and only 3.1–5.3 g N m^–2 h^–1 for winter wheat or flooding fallow season. Temporal variations of N₂O emission during rice growing seasons differed and high N₂O emission occurred when soil conditions changed from upland crop to flooded rice.     

Emissions of N2O from Scottish agricultural soils, as a function of fertilizer N

Potato fields and cut (ungrazed) grassland in SE Scotland gave greater annual N₂O emissions per ha (1.0–3.2 kg N₂O–N ha^-1) than spring barley or winter wheat fields (0.3–0.8 kg N₂O–N ha^-1), but in terms of emission per unit of N applied the order was potatoes > barley > grass > wheat. On the arable land, especially the potato fields, a large part of the emissions occurred after harvest.When the grassland data were combined with those for 2 years' earlier work at the same site, the mean emission over 3 years, for fertilization with ammonium nitrate, was 2.24 kg N₂O–N ha^-1 (0.62% of the N applied). Also, a very strong relationship between N₂O emission and soil nitrate content was found for the grassland, provided the water-filled pore space was > 70%. Significant relationships were also found between the emissions from potato fields and the soil mineral N content, with the added feature that the emission per unit of soil mineral N was an order of magnitude larger after harvest than before, possibly due to the effect of labile organic residues on denitrification.Generally the emissions measured were lower, as a function of the N applied, than those used as the basis for the current value adopted by IPCC, possibly because spring/early summer temperatures in SE Scotland are lower than those where the other data were obtained. The role of other factors contributing to emissions, e.g. winter freeze–thaw events and green manure inputs, are discussed, together with the possible implications of future increases in nitrogen fertilizer use in the tropics.     

Two-year field study on the emission of N2O from coarse and middle-textured Belgian soils with different land use

In the following study N₂O emissions from 3 different grasslands and from 3 different arable lands, representing major agriculture areas with different soil textures and normal agricultural practices in Belgium, have been monitored for 1 to 2 years. One undisturbed soil under deciduous forest was also included in the study. Nitrous oxide emission was measured directly in the field from vented closed chambers through photo-acoustic infrared detection. Annual N₂O emissions from the arable lands ranged from 0.3 to 1.5 kg N ha⁻¹ y⁻¹ and represent 0.3 to 1.0% of the fertilizer N applied. Annual N₂O emissions from the intensively managed grasslands and an arable land sown with grass were significantly larger than those from the cropped arable lands. Emissions ranged from 14 to 32 kg N ha⁻¹ y⁻¹, representing fertilizer N losses between 3 and 11%. At the forest soil a net N₂O uptake of 1.3 kg N₂O-N ha⁻¹ was recorded over a 2-year period. It seems that the N₂O-N loss per unit of fertilizer N applied is larger for intensively managed and heavily fertilized (up to 500 kg N ha⁻¹) grasslands than for arable lands and is substantially larger than the 1.25% figure used for the global emission inventory. Comparison of the annual emission fluxes from the different soils also indicated that land use rather than soil properties influenced the N₂O emission. Our results also show once again the importance of year-round measurements for a correct estimate of N₂O losses from agricultural soils: 7 to 76% of the total annual N₂O was emitted during the winter period (October–February). Disregarding the emission during the off-season period can lead to serious underestimation of the actual annual N₂O flux. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of nitrous oxide emissions from two rice-based cropping systems in China

A field experiment was conducted to investigate the effects of winter management and N fertilization on N₂O emission from a double rice-based cropping system. A rice field was either cropped with milk vetch (plot V) or left fallow (plot F) during the winter between rice crops. The milk vetch was incorporated in situ when the plot was prepared for rice transplanting. Then the plots V and F were divided into two sub-plots, which were then fertilized with 276 kg urea-N ha^–1 (referred to as plot VN and plot FN) or not fertilized (referred to as plot VU and plot FU). N₂O emission was measured periodically during the winter season and double rice growing seasons. The average N₂O flux was 11.0 and 18.1 g N m^–2 h^–1 for plot V and plot F, respectively, during winter season. During the early rice growing period, N₂O emission from plot VN averaged 167 g N m^–2 h^–1, which was eight- to fifteen-fold higher than that from the other three treatments (17.8, 21.0 and 10.8 g N m^–2 h^–1 for plots VU, FN, and FU, respectively). During the late rice growing period, the mean N₂O flux was 14.5, 11.1, 12.1 and 9.9 g N m^–2 h^–1 for plots VN, VU, FN and FU, respectively. The annual N₂O emission rates from green manure-double rice and fallow-double rice cropping systems were 3.6 kg N ha^–1 and 1.3 kg N ha^–1, respectively, with synthetic N fertilizer, and were 0.99 kg N ha^–1 and 1.12 kg N ha^–1, respectively, without synthetic N fertilizer. Generally, both green manure N and synthetic fertilizer N contribute to N₂O emission during double rice season.     

Nitrous oxide formation potential of various humid tropic soils of Malaysia: A laboratory study

Nitrous oxide (N₂O) is formed mainly during nitrification and denitrification. Inherent soil properties strongly influence the magnitude of N₂O formation and vary with soil types. A laboratory study was carried out using eight humid tropic soils of Malaysia to monitor NH₄ ⁺ and NO₃ ^– dynamics and N₂O production. The soils were treated with NH₄NO₃ (100 mg N kg^–1 soil) and incubated for 40 days at 60% water-filled pore space. The NH₄ ⁺ accumulation was predominant in the acid soils studied and NO₃ ^– accumulation/disappearance was either small or stable. However, the Munchong soil depicted the highest peak (238 g N₂O-N kg^–1 soil d^–1) at the beginning of the incubation, probably through a physical release. While the Tavy soil showed some NO₃ ^– accumulation at the end of the study with a maximum N₂O flux of 206 g N₂O-N kg^–1 soil d^–1, both belong to Oxisols. The other six soils, viz. Rengam, Selangor, Briah, Bungor, Serdang and Malacca series, formed smaller but maximum peaks in an decreasing order of 116 to 36 g N₂O-N kg^–1 soil d^–1. Liming the Oxisols and Ultisols raised the soil pH, resulting in NO₃ ^– accumulation and N₂O production to some extent. As such the highest N₂O flux of 130.2 and 77.4 g N₂O-N kg^–1 soil d^–1 was detected from the Bungor and Malacca soils, respectively. The Selangor soil, belonging to Inceptisol, did not respond to lime treatment. The respective total N₂O formations were 3.63, 1.92 and 1.69 mg N₂O-N kg^–1 soil from the Bungor, Malacca and Selangor soils, showing an increase by 49 and 99% over the former two non-limed soils. Under non-limed conditions, the indigenous soil properties, viz. Ca⁺⁺ content, %clay, %sand and pH of the soils collectively could have influenced the total N₂O formation.     

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.     

Nitrous oxide emissions for 6 years from a gray lowland soil cultivated with onions in Hokkaido, Japan

We studied nitrous oxide (N₂O) emissions every growing season (April to October) for 6 years (19952000), in a Gray Lowland soil cultivated with onions in central Hokkaido, Japan. Emission of N₂O from the onion field ranged from 0.00 to 1.86 mgN m^–2 h^–1. The seasonal pattern of N₂O emission was the same for 6 years. The largest N₂O emissions appeared near harvesting in August to October, and not, as might be expected, just after fertilization in May. The seasonal patterns of soil nitrate (NO₃ ^–) and, ammonium (NH₄ ⁺) levels and the ratio of N₂O to NO emission indicated that the main process of N₂O production after fertilization was nitrification, and the main process of N₂O production around harvest time was denitrification. N₂O emission was strongly influenced by the drying–wetting process of the soil, as well as by the high soil water content. The annual N₂O emission during the growing season ranged from 3.5 to 15.6 kgN ha^–1. The annual nitrogen loss by N₂O emission as a percentage of fertilizer-N ranged from 1.1 to 6.4%. About 70% of the annual N₂O emission occurred near harvesting in August to October, and less than 20% occurred just after fertilization in May to July. High N₂O fluxes around the harvesting stage and a high proportion of N₂O emission to total fertilizer-N appeared to be probably a characteristic of the study area located in central Hokkaido, Japan.     

Effect of climate change on greenhouse gas emissions from arable crop rotations

The DAISY soil–plant–atmosphere model was used to simulate crop production and soil carbon (C) and nitrogen (N) turnover for three arable crop rotations on a loamy sand in Denmark under varying temperature, rainfall, atmospheric CO₂ concentration and N fertilization. The crop rotations varied in proportion of spring sown crops and use of N catch crops (ryegrass). The effects on CO₂ emissions were estimated from simulated changes in soil C. The effects on N₂O emissions were estimated using the IPCC methodology from simulated amounts of N in crop residues and N leaching. Simulations were carried out using the original and a revised parameterization of the soil C turnover. The use of the revised model parameterization increased the soil C and N turnover in the topsoil under baseline conditions, resulting in an increase in crop N uptake of 11 kg N ha^–1 y^–1 in a crop rotation with winter cereals and a reduction of 16 kg N ha^–1 y^–1 in a crop rotation with spring cereals and catch crops. The effect of increased temperature, rainfall and CO₂ concentration on N flows was of the same magnitude for both model parameterizations. Higher temperature and rainfall increased N leaching in all crop rotations, whereas effects on N in crop residues depended on use of catch crops. The total greenhouse gas (GHG) emission increased with increasing temperature. The increase in total GHG emission was 66–234 kg CO₂-eq ha^–1 y^–1 for a temperature increase of 4°C. Higher rainfall increased total GHG emissions most in the winter cereal dominated rotation. An increase in rainfall of 20% increased total GHG emissions by 11–53 kg CO₂-eq ha^–1 y^–1, and a 50% increase in atmospheric CO₂ concentration decreased emissions by 180–269 kg CO₂-eq ha^–1 y^–1. The total GHG emissions increased considerably with increasing N fertilizer rate for a crop rotation with winter cereals, but remained unchanged for a crop rotation with spring cereals and catch crops. The simulated increase in GHG emissions with global warming can be effectively mitigated by including more spring cereals and catch crops in the rotation.     

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.     

Effect of soil water contents in the non-rice growth season on CH4 emission during the following rice-growing period

A pot experiment was carried out to investigate the effect of soil water content in the non-rice growth season (winter season) on CH₄ emission during the following rice-growing period. The results showed that CH₄ fluxes increased significantly with the increase of soil water content in the winter season, except air-dry water condition. The mean CH₄ fluxes of treatments with soil water contents in the winter of 3.89–5.37% (air-dry), 25–35%, 50–60%, 75–85% and 107% (flooded) of field water capacity (FWC) were 13.04, 4.04, 8.61, 13.26 and 20.47 mg m^–2 h^–1, respectively. Antecedent soil water contents also markedly affected temporal variation patterns of CH₄ fluxes and soil redox potential (E_h) during the rice-growing period. The higher soil water contents in the winter season were, the quicker soil E_h decreased, and the earlier CH₄ emission occurred after rice transplanting, except air-dry water condition. Though the seasonal mean CH₄ flux was significantly correlated with the seasonal mean soil E_h, the seasonal variation of CH₄ fluxes was not always significantly correlated with soil E_h. For the treatment flooded in the fallow season, there was no significant correlation between CH₄ flux and soil E_h, but there was significant correlation between CH₄ flux and soil temperature during rice growth season. In contrast, for the other four treatments, it was soil E_h, not soil temperature that significantly affected the temporal variation of CH₄ emissions. Soil water contents in the fallow season significantly influenced concentrations of soil labile organic carbon (including undecomposed plant debris), active Fe and Mn immediately before rice transplanting. The mean CH₄ fluxes during rice-growing period were significantly correlated with soil labile organic carbon contents (positively) and contents of soil active Fe and Mn (negatively).     

The fate of nitrogen from incorporated cover crop and green manure residues

Nitrogen retention and release following the incorporation of cover crops and green manures were examined in field trials in NE Scotland. These treatments reduced the amounts of nitrate-N by between 10–20 kg ha^-1 thereby lowering the potential for leaching and gaseous N losses. However, uptake of N by overwintering crops was low, reflecting the short day-lengths and low soil temperatures associated with this part of Britain. Vegetation that had regenerated naturally was as effective as sown cover crops at taking up N over winter and in returning N to the soil for the following crop. Incorporation of residues generally resulted in lower mineralisation rates and reduced N₂O emissions than the cultivation of bare ground, indicating a temporary immobilisation of soil N following incorporation. Emissions from incorporated cover crops ranged from 23–44 g N₂O-N ha^-1 over 19 days, compared with 61 g N₂O-N ha^-1 emitted from bare ground. Emissions from incorporated green manures ranged from 409–580 g N₂O-N ha^-1 over 53 days with 462 g N₂O-N ha^-1 emitted from bare ground. Significant positive correlations between N₂O and soil NO₃ ^- after incorporation (r=0.8–0.9; P<0.001 and r=0.1–0.4; P<0.05 for cover crops and green manures, respectively) suggest that this N₂O was mainly produced during nitrification. There was no significant effect of either cover cropping or green manuring on the N content or yield of the subsequent oats crop, suggesting that N was not sufficiently limiting in this soil for any benefits to become apparent immediately. However, benefits of increased sustainability as a result of increased organic matter concentrations may be seen in long-term organic rotations, and such systems warrant investigation.     

Similarity and differences in the oxidative dehydrogenation of C2–C4 alkanes over nano-sized VOx species using N2O and O2

The effect of O₂ and N₂O on alkane reactivity and olefin selectivity in the oxidative dehydrogenation of ethane, propane, n-butane, and iso-butane over highly dispersed VO_x species (0.79 V/nm²) supported on MCM-41 has been systematically investigated. For all the reactions studied, olefin selectivity was significantly improved upon replacing O₂ with N₂O. This is due to suppressing CO_x formation in the presence of N₂O. The most significant improving effect of N₂O was observed for iso-butane dehydrogenation: S(iso-butene) was ca. 67% at X(iso-butane) of 25%.Possible origins of the superior performance of N₂O were derived from transient experiments using ¹⁸O₂ traces. ¹⁸O¹⁶O species were detected in ¹⁸O₂ and ¹⁸O₂–C₃H₈ transient experiments indicating reversible oxygen chemisorption. In the presence of alkanes, the isotopic heteroexchange of O₂ strongly increased. Based on the distribution of labeled oxygen in CO_x and in O₂ as well as on the increased CO_x formation in sequential O₂–C₃H₈ experiments, it is suggested that non-lattice oxygen species (possibly of a bi-atomic nature) originating from O₂ are non-selective ones and responsible for CO_x formation. These species are not formed from N₂O.     

Nitrous oxide emissions from agricultural fields during winter and spring thaw as affected by management practices

Highest rates of N₂O emissions from fertilized as well as natural ecosystems have often been measured at spring thaw. But, it is not clear if management practices have an effect on winter and spring thaw emissions, or if measurements conducted over several years would reveal different emission patterns depending on winter conditions. In this study, we present N₂O fluxes obtained using the flux-gradient approach over four winter and spring thaw periods, spanning from 1993 to 1996, at two locations in Ontario, Canada. Several agricultural fields (bare soil, barley, soybean, canola, grass, corn) subjected to various management practices (manure and nitrogen fertilizer addition, alfalfa ploughing, fallowing) were monitored. Nitrous oxide emissions from these fields from January to April over four years ranged between 0 and 4.8 kg N ha^-1. These thaw emissions are substantial and should be considered in the nitrous oxide budgets in regions where thaw periods occur. Our study indicates that agricultural management can play a role in mitigating these emissions. Our data show that fallowing, manure application and alfalfa incorporation in the fall lead to high spring emissions, while the presence of plants (as in the case of alfalfa or grass) can result in negligible emissions during thaw. This presents an opportunity for mitigation of N₂O emissions through the use of over-wintering cover crops.     

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.     

Using a boundary line approach to analyze N2O flux data from agricultural soils

Predicting the N₂O flux from soils is difficult because of the complex interplay of the various processes involved. In this study a boundary line approach was used to apply results from mechanistic experiments to N₂O flux data resulting from measurements on field scale in southern Germany. Boundary lines were fitted to the rim of the data points in scattergrams depicting readily obtainable soil variables against the measured N₂O flux. The boundary line approach is based on the hypothesis that this line depicts the functional dependency between the two variables. For determining these boundary lines a novel method was applied. The function best representing the relationship between the N₂O flux and soil temperature had a maximum above 23 °C and the one between the N₂O flux and the water filled pore space (WFPS, to represent water content) had a maximum at 72% WFPS. In the range of 0–20 mg N kg^-1 the relationship between N₂O flux and nitrate in the soil was best described by a linear function, whereas in the range of 0–35 mg N kg^-1 a Michaelis–Menten function was more appropriate. The boundary lines specified in this study are in agreement with existing theoretical concepts as well as experimental results obtained under controlled and field conditions as reported in the literature. Therefore, the boundary line approach can be used to improve empirical models for predicting the N₂O flux in the field.     

Oxidative coupling of methane over Ce4+-doped Ba3 WO6 catalysts: investigation on oxygen species responsible for catalytic performance

Ce⁴⁺ doped Ba₃ WO₆ complex oxides were used as catalysts for methane oxidative coupling (MOC), and characterized by XPS and O₂-TPD-MS techniques. The results indicate that the ratio of electrophilic oxygen species O^– and O ₂ ^– to lattice oxygen on the surface is crucial for C₂ selectivity. By adjusting the relative amount of cations in Ba-W-Ce complex oxides with perovskite superstructure interstitial oxygen species can be created which benefits C₂ selectivity by raising the relative amount of (O^– + O ₂ ^– ) on the surface.     

Detectability of15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of¹⁵N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.) -winter rye (Secale cereale L.) crop rotation. Nitrogen as¹⁵N-depleted (NH₄)₂SO₄ was applied at five rates (0, 84, 168, 252, and 336 kg N ha^–1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO ₃ ^- -N fraction following corn harvest (140-d after application). Low levels of exchangeable NH ₄ ⁺ -N (<2.5 mg kg^–1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha^–1 N rate. Atom %¹⁵N concentrations in the nonexchangeable NH ₄ ⁺ -N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO ₃ ^- -N levels below the 10 kg ha^–1 level needed for accurate isotope-ratio analysis. Atom %¹⁵N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha^–1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom %¹⁵N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of¹⁵N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.Research partially supported by grants from the National Fertilizer and Environmental Research Center/TVA and the Virginia Division of Soil and Water Conservation.     

Nitrous oxide flux from a solid dairy manure pile measured using a micrometeorological mass balance method

A micrometeorological mass balance technique was used to quantify the N₂O flux from a solid dairy manure pile under field conditions. Flux was determined using time-averaged measurements of wind speed, and nitrous oxide concentration using a tunable diode laser trace gas analyzer. A total of 66 hourly flux averages were collected and values were never lower than 200 ng N₂O-N m^–2 s^–1. The mean hourly N₂O flux was 4865 ng N₂O-N m^–2 s^–1 (0.42 g N m^–2 day^–1), which is of the same order of magnitude, albeit higher, as previously observed for a similar solid pig manure storage.     

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.