K. A. Smith

Nitrous oxide emissions from fertilised grassland: A 2-year study of the effects of N fertiliser form and environmental conditions

Abstract The aim was to investigate the effects of different N fertilisers on nitrous oxide (N2O) flux from agricultural grassland, with a view to suggesting fertiliser practices least likely to cause substantial N2O emissions, and to assess the influence of soil and environmental factors on the emissions. Replicate plots on a clay loam grassland were fertilised with ammonium sulphate (AS), urea (U), calcium nitrate (CN), ammonium nitrate (AN), or cattle slurry supplemented with AN on three occasions in each of 2 years. Frequent measurements were made of N2O flux and soil and environmental variables. The loss of N2O-N as a percentage of N fertiliser applied was highest from the supplemented slurry (SS) treatment and U, and lowest from AS. The temporal pattern of losses was different for the different fertilisers and between years. Losses from U were lower than those from AN and CN in the spring, but higher in the summer. The high summer fluxes were associated with high water-filled pore space (WFPS) values. Fluxes also rose steeply with temperature where WFPS or mineral N values were not limiting. Total annual loss was higher in the 2nd year, probably because of the rainfall pattern: the percentage losses were 2.2, 1.4, 1.2, 1.1 and 0.4 from SS, U, AN, CN and AS, respectively. Application of U in the spring and AN twice in the summer in the 2nd year gave an average emission factor of 0.8% – lower than from application of either individual fertiliser. We suggest that similar varied fertilisation practices, modified according to soil and crop type and climatic conditions, might be employed to minimise N2O emissions from agricultural land.

Nitrification and denitrification as sources of nitric oxide and nitrous oxide in a sandy loam soil

Emissions of nitric oxide (NO) and nitrous oxide (N2O) from a freely drained sandy loam, fertilized with (NH4)2SO4 or KNO3 (100 kg N ha−1) with or without the addition of the nitrification inhibitor dicyandiamide (DCD), were measured. The addition of N fertilizers increased emissions of NO and N2O. For plots fertilized with (NH4)2SO4, NO emissions increased from 2.4 to 46.9 ng NO-N m−2 s−1 (2.1–40.5 g NO-N ha−1 day−1), in the first 7 days after fertilizer application. Nitrous oxide emission rates were considerably lower, ranging from 0.95 to 7.4 ng N2O-Nm−2s−1 (0.82–6.4 g N2O-Nha−1 day−1). Nitrification rather than denitrification was the source of the NO emitted from the soil; additions of DCD inhibited the emissions by at least 92%. Nitrous oxide, on the other hand, was a product of both nitrification and denitrification. When soils were dry, N2O was produced predominantly by nitrification and DCD reduced emissions by at least 40%. In contrast, in wet conditions denitrification was the main source of N2O and emissions were not inhibited by DCD. Nitric oxide emissions correlated significantly with soil temperature (30 mm depth), the air temperature inside the chamber, soil available NH4+, and were significantly reduced by watering the soil. Apparent activation energies, calculated from the temperature response in the NO emission rates, ranged from 30 to 71 kJmol−1. It was concluded from the close links between air temperature in the chamber and the NO emission rates that the NO was produced very close to the soil surface. During nitrification the rate of depletion of NH4+-N emitted as NO-N was 5.5 × 10−5s−1. It was estimated that for cultivated fields 0.15–0.75% of the applied NH4+ fertilizer is released as NO.

Comparison of N2O emissions from soils at three temperate agricultural sites: simulations of year-round measurements by four models

Nitrous oxide (N2O) flux simulations by four models were compared with year-round field measurements from five temperate agricultural sites in three countries. The field sites included an unfertilized, semi-arid rangeland with low N2O fluxes in eastern Colorado, USA; two fertilizer treatments (urea and nitrate) on a fertilized grass ley cut for silage in Scotland; and two fertilized, cultivated crop fields in Germany where N2O loss during the winter was quite high. The models used were daily trace gas versions of the CENTURY model, DNDC, ExpertN, and the NASA-Ames version of the CASA model. These models included similar components (soil physics, decomposition, plant growth, and nitrogen transformations), but in some cases used very different algorithms for these processes. All models generated similar results for the general cycling of nitrogen through the agro-ecosystems, but simulated nitrogen trace gas fluxes were quite different. In most cases the simulated N2O fluxes were within a factor of about 2 of the observed annual fluxes, but even when models produced similar N2O fluxes they often produced very different estimates of gaseous N loss as nitric oxide (NO), dinitrogen (N2), and ammonia (NH3). Accurate simulation of soil moisture appears to be a key requirement for reliable simulation of N2O emissions. All models simulated the general pattern of low background fluxes with high fluxes following fertilization at the Scottish sites, but they could not (or were not designed to) accurately capture the observed effects of different fertilizer types on N2O flux. None of the models were able to reliably generate large pulses of N2O during brief winter thaws that were observed at the two German sites. All models except DNDC simulated very low N2O fluxes for the dry site in Colorado. The US Trace Gas Network (TRAGNET) has provided a mechanism for this model and site intercomparison. Additional intercomparisons are needed with these and other models and additional data sets; these should include both tropical agro-ecosystems and new agricultural management techniques designed for sustainability.

Nitric oxide emissions from agricultural soils in temperate and tropical climates: sources, controls and mitigation options

Global annual NO emissions from soil are of the order of 10 Tg NO-N. This is about half the amount fossil fuel combustion processes contribute to the annual global NOx budget. Reducing the emissions of soil derived NOx requires an understanding of the source of the flux and the processes that determine its magnitude. A thorough investigation of possible mitigation strategies and the consequences of their implementation is also necessary. The ratio of NO and N2O emissions from soils can be used as an indicator of the dominant NO production pathway operating. Fertilizer application (rate, type and time of application), soil temperature, soil water content and soil management practices all affect the emission rate and are reviewed. Mitigation options include reduction in N fertilizer use through an increase in fertilizer use efficiency, preferential use of NH4NO3 instead of urea, improved timing of fertilizer application, the use of nitrification and urease inhibitors, improving the fertilizer uptake efficiency of crops in tropical agriculture and changes in land management. Several of the viable mitigation strategies, mainly those increasing fertilizer use efficiency, have the capacity to reduce global annual NO emissions by 4% (0.4 Tg NO-N y-1). For other strategies including use of inhibitors, changing cultivation or land use, the possible reductions are too uncertain to justify quantification on the basis of present knowledge.

Enhancing the carbon sink in European agricultural soils: including trace gas fluxes in estimates of carbon mitigation potential.

The possibility that the carbon sink in agricultural soils can be enhanced has taken on great political significance since the Kyoto Protocol was finalised in December 1997. The Kyoto Protocol allows carbon emissions to be offset by demonstrable removal of carbon from the atmosphere. Thus, forestry activities (Article 3.3) and changes in the use of agricultural soils (Article 3.4) that are shown to reduce atmospheric CO2levels may be included in the Kyoto emission reduction targets. The European Union is committed to a reduction in CO2 emissions to 92% of baseline (1990) levels during the first commitment period (2008–2012). We have shown recently that there are a number of agricultural land-management changes that show some potential to increase the carbon sink in agricultural soils and others that allow alternative forms of carbon mitigation (i.e. through fossil fuel substitution), but the options differ greatly in their potential for carbon mitigation. The changes examined were, (a) switching all animal manure use to arable land, (b) applying all sewage sludge to arable land, (c) incorporating all surplus cereal straw, (d) conversion to no-till agriculture, (e) use of surplus arable land to de-intensify 1/3 of current intensive crop production (through use of 1/3 grass/arable rotations), (f) use of surplus arable land to allow natural woodland regeneration, and (g) use of surplus arable land for bioenergy crop production. In this paper, we attempt for the first time to assess other (non-CO2) effects of these land-management changes on (a) the emission of the other important agricultural greenhouse gases, methane and nitrous oxide, and (b) other aspects of the ecology of the agroecosystems. We find that the relative importance of trace gas fluxes varies enormously among the scenarios. In some such as the sewage sludge, woodland regeneration and bioenergy production scenarios, the inclusion of trace gases makes only a small (<10%) difference to the CO2-C mitigation potential. In other cases, for example the no-till, animal manure and agricultural de-intensification scenarios, trace gases have a large impact, sometimes halving or more than doubling the CO2-C mitigation potential. The scenarios showing the greatest increase when including trace gases are those in which manure management changes significantly. In the one scenario (no-till) where the carbon mitigation potential was reduced greatly, a small increase in methane oxidation was outweighed by a sharp increase in N2O emissions. When these land-management options are combined to examine the whole agricultural land area of Europe, most of the changes in mitigation potential are small, but depending upon assumptions for the animal manure scenario, the total mitigation potential either increases by about 20% or decreases by about 10%, shifting the mitigation potential of the scenario from just above the EUs 8% Kyoto emission reduction target (98.9 Tg C y−1) to just below it. Our results suggest that (a) trace gas fluxes may change the mitigation potential of a land management option significantly and should always be considered alongside CO2-C mitigation potentials and (b) agricultural management options show considerable potential for carbon mitigation even after accounting for trace gas fluxes.

General CH4 oxidation model and comparisons of CH4 Oxidation in natural and managed systems

Fluxes of methane from field observations of native and cropped grassland soils in Colorado and Nebraska were used to model CH 4 oxidation as a function of soil water content, temperature, porosity, and field capacity (FC). A beta function is used to characterize the effect of soil water on the physical limitation of gas diffusivity when water is high and biological limitation when water is low. Optimum soil volumetric water content (W opt ) increases with PC. The site specific maximum CH 4 oxidation rate (CH 4max ) varies directly with soil gas diffusivity (D opt ) as a function of soil bulk density and FC. Although soil water content and physical properties are the primary controls on CH 4 uptake, the potential for soil temperature to affect CH 4 uptake rates increases as soils become less limited by gas diffusivity, Daily CH 4 oxidation rate is calculated as the product of CH 4max , the normalized (0-100%) beta function to account for water effects, a temperature multiplier, and an adjustment factor to account for the effects of agriculture on methane flux. The model developed with grassland soils also worked well in coniferous and tropical forest soils. However, soil gas diffusivity as a function of field capacity, and bulk density did not reliably predict maximum CH 4 oxidation rates in deciduous forest soils, so a submodel for these systems was developed assuming that CH 4max is a function of mineral soil bulk density. The overall model performed well with the data used for model development (r 2 = 0.76) and with independent data from grasslands, cultivated lands, and coniferous, deciduous, and tropical forests (r 2 = 0.73, mean error < 6%).

Comparison of CH4 oxidation rates in woodland, arable and set aside soils

CH4 fluxes and various soil properties were measured over three successive years at a field site on a loamy sand soil in eastern Scotland, to determine which factors influence CH4 oxidation rate. This site included three adjacent areas with contrasting land use: woodland, arable land and set aside land. The CH4 oxidation rates in the arable soil were less than half the corresponding rates in the woodland soil. The CH4 oxidation rates in the set aside soil were even lower, indicating that there is no immediate recovery when cultivation and fertilisation are abandoned. In the woodland and set aside soils, a seasonal variation in CH4 oxidation rate was found, but in the arable soil there was no such trend. The CH4 oxidation rate was negatively correlated with soil moisture content (P < 0.001) in the woodland soil and positively correlated with soil temperature (P < 0.001) in the set aside soil. In the arable soil, CH4 oxidation rate was related to moisture content only in dry summer conditions, when the relationship was positive (P < 0.001). These relationships suggest that CH4 oxidation was controlled partly by diffusion and partly by biological activity. A negative correlation was found between soil ammonium concentration and CH4 oxidation rate in the woodland soil (P < 0.001), indicating that ammonium inhibited CH4 oxidation in that environment.

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.

Fluxes of nitric and nitrous oxides from agricultural soils in a cool temperate climate

Fluxes of NO and N2O from sandy loam soils cropped with winter wheat and a clay loam soil under ryegrass, with and without the addition of NH4NO3 fertilizer, were measured using static and dynamic chamber methods. Nitric oxide fluxes ranged from −0.3 (deposition) to 6.9 (emission) ng NO-N m−2 s−1. The corresponding N2O flux ranged from 0 to 91 (emission) ng N2O-N m−2 s−1. The NO flux was temperature dependent. Activation energies ranged from 40 to 81 kJ mol−1. Nitric oxide and N2O fluxes increased linearly with soil available nitrogen (NH4 + NO3). Emissions of NO and N2O were not detectable from unfertilized ryegrass plots. Instead, nitric oxide was absorbed by the soil and vegetation at a maximum rate of 0.31 ng NO-N m−2 s−1. The aeration state of the soil controlled the relative rates of NO and N2O emission. Nitric oxide was the major gas emitted from well aerated soils, conditions that favour nitrification. The NO/N2O emission ratio was >100 for the coarse-textured sandy loam soil and the clay loam soil only during low rainfall periods. Nitrous oxide was the major gas emitted from less aerated soils, conditions that allowed denitrification to occur. The NO/N2O emission ratio was <0.001 for the clay loam soil when rainfall was high and soils were wet. Extrapolation to the U.K. situation showed that agricultural land may account for 2–6% of the total annual NOx emission and for 16–64% of the total annual N2O emission in the U.K.

Measurement of nitrous oxide emissions from fertilized grassland using closed chambers

The aims of this study were to use closed chambers to improve estimates of N 2 O-N losses from intensively managed grassland on poorly drained soils and to provide measurements for comparison with fluxes determined simultaneously using micrometeorological methods. A 10-ha field on clay soil in central Scotland received 185 kg NH 4 NO 3 -N ha −1 on April 3, 1992. Twenty-four closed chambers were installed, six in a 2-3-ha area grazed by cattle the previous summer, the remainder in an ungrazed area. Fluxes were measured regularly for 3 weeks. Nitrous oxide accumulation in the chambers was determined by gas chromatography. No flux was detected before fertilization. After fertilization, fluxes from the ungrazed and grazed areas were 153±9 and 551±101 g N 2 O-N ha −1 d −1 , respectively (means and standard errors of all measurements)