Iain P. McTaggart

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.

Nitrous oxide emissions from grassland and spring barley, following N fertiliser application with and without nitrification inhibitors

Abstract The aims of this study were to assess the effectiveness of the nitrification inhibitors dicyandiamide (DCD) and nitrapyrin on reducing emissions of nitrous oxide (N2O) following application of NH4+ or NH4+-forming fertilisers to grassland and spring barley. DCD was applied to grassland with N fertiliser applications in April and August in 1992 and 1993, inhibiting N2O emissions by varying amounts depending on the fertiliser form and the time of application. Over periods of up to 2 months following each application of DCD, emissions of N2O were reduced by 58–78% when applied with urea (U) and 41–65% when applied with ammonium sulphate (AS). Annual emissions (April to March) of N2O were reduced by up to 58% and 56% in 1992–1993 and 1993–1994, respectively. Applying DCD to ammonium nitrate (AN) fertilised grassland did not reduce emissions after the April 1993 fertilisation, but emissions following the August application were reduced. Nitrapyrin was only applied once, with the April fertiliser applications in 1992, reducing N2O emissions over the following 12 months by up to 40% when applied with U. When N fertiliser was applied in June without DCD, the DCD applied in April was still partly effective; N2O emissions were reduced 50%, 60% and 80% as effectively as the emissions following the April applications, for AS in 1993, U in 1992 and 1993, respectively. In 1992 the persistence of an inhibitory effect was greater for nitrapyrin than for DCD, increasing after the June fertiliser application as overall emissions from U increased. There was no apparent reduction in effectiveness following repeated applications of DCD over the 2 years. N2O emissions from spring barley, measured only in 1993, were lower than from grassland. DCD reduced emissions from applied U by 40% but there was no reduction with AN. The results demonstrate considerable scope for reducing emissions by applying nitrification inhibitors with NH4+ or NH4+-forming fertilisers; this is especially so for crops such as intensively managed grass where there are several applications of fertiliser nitrogen per season, as the effect of inhibitors applied in April persists until after a second fertiliser application in June.

N2O, NO, and NH3 Emissions from Soil after the Application of Organic Fertilizers, Urea and Water

Agricultural soil is a major source of nitrous oxide (N2O), nitric oxide (NO) and ammonia (NH3). Little information is available on emissions of these gases from soils amended with organic fertilizers at different soil water contents. N2O, NO and NH3 emissions were measured in large-scale incubations of a fresh sandy loam soil and amended with four organic fertilizers, [poultry litter (PL), composted plant residues (CP), sewage sludge pellets (SP) and cattle farm yard manure (CM)], urea fertilizer (UA) or a zero-N control (ZR) for 38 days. Fertilizers were added to soil at 40, 60 or 80% water-filled pore space (WFPS). The results showed that urea and organic fertilizer were important sources of N2O and NO. Total N2O and NO emissions from UA ranged from 0.04 to 0.62%, and 0.23 to 1.55% of applied N, respectively. Total N2O and NO emissions from organic fertilizer treatments ranged from 0.01 to 1.65%, and <0.01 to 0.55% of applied N, respectively. The lower N2O and NO emissions from CP and CM suggested that applying N is these forms could be a useful mitigation option. Comparison of the NO-N/N2O-N ratio suggested that nitrification was more dominant in UA whereas denitrification was more dominant in the organic fertilizer treatments. Most N was lost from PL and UA as NH3, and this was not influenced significantly by WFPS. Emissions of NH3 from UA and PL ranged from 62.4 to 69.6%, and 3.17 to 6.11% of applied N, respectively.

Influence of soil physical properties, fertiliser type and moisture tension on N2O and NO emissions from nearly saturated Japanese upland soils

In Japan, upland soils are an important source of nitrous oxide (N2O) and nitric oxide (NO) gas emissions. This paper reports on an investigation of the effect of soil moisture near saturation on N2O and NO emission rates from four upland soils in Japan of contrasting texture. The aim was to relate these effects to soil physical properties. Intact cores of each soil type were incubated in the laboratory at different moisture tensions after fertilisation with NH4-N, NO3-N or zero N. Emissions of N2O and NO were measured regularly over a 16–20 day period. At the end of the incubation, soil cores were analysed for physical properties. Moisture and N fertiliser significantly affected rates of emissions of both N2O and NO with large differences between the soil types. Nitrous oxide emissions were greatest in the finer-textured soils, whereas NO emissions were greater in the coarser-textured soils. Emissions of N2O increased at higher moisture contents in all soils, but the magnitude of increase was much greater in finer-textured soils. Nitric oxide emissions were only significant in soils fertilised with NH4-N and were negatively correlated with soil moisture. Analysis of soil properties showed that there was a strong relationship between the magnitude of emissions and soil physical properties. The importance of soil wetness to gas emissions was mainly through its influence on soil air-filled porosity, which itself was related to gas diffusivity. From the results of this research, we can now estimate likely effects of soil texture on emissions through the influence of soil type on soil aeration and soil drainage. This is of particular value in modelling N2O and NO emissions from soil moisture status and land use inputs.

The effect of fertilizer placement on nitrogen uptake and yield of wheat and maize in Chinese loess soils

Field trials were carried out to study the fate of15N-labelled urea applied to summer maize and winter wheat in loess soils in Shaanxi Province, north-west China. In the maize experiment, nitrogen was applied at rates of 0 or 210 kg N ha−1, either as a surface application, mixed uniformly with the top 0.15 m of soil, or placed in holes 0.1 m deep adjacent to each plant and then covered with soil. In the wheat experiment, nitrogen was applied at rates of 0, 75 or 150 kg N ha−1, either to the surface, or incorporated by mixing with the top 0.15 m, or placed in a band at 0.15 m depth. Measurements were made of crop N uptake, residual fertilizer N and soil mineral N. The total above-ground dry matter yield of maize varied between 7.6 and 11.9 t ha−1. The crop recovery of fertilizer N following point placement was 25% of that applied, which was higher than that from the surface application (18%) or incorporation by mixing (18%). The total grain yield of wheat varied between 4.3 and 4.7 t ha−1. In the surface applications, the recovery of fertilizer-derived nitrogen (25%) was considerably lower than that from the mixing treatments and banded placements (33 and 36%). The fertilizer N application rate had a significant effect on grain and total dry matter yield, as well as on total N uptake and grain N contents. The main mechanism for loss of N appeared to be by ammonia volatilization, rather than leaching. High mineral N concentrations remained in the soil at harvest, following both crops, demonstrating a potential for significant reductions in N application rates without associated loss in yield.

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 μg 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.

Nitrous Oxide Flux from Fertilised Grassland: Strategies for Reducing Emissions

Nitrous oxide emissions from soils, through the processes of nitrification and denitrification, are increased by N fertiliser application. The effect of choosing a form of N fertiliser appropriate to the environmental conditions, and the effect of nitrification inhibitors, on N2O emissions, were studied on a grassland site over two growing seasons. Total emissions were greatest from urea fertiliser, with the greatest losses coming from urea applications in the warmer, drier months of June and August, suggesting that the N2O was formed mainly via nitrification. The greatest emissions of N2O from ammonium nitrate fertiliser occurred after applications in April in relatively wet conditions, when the emissions from urea and ammonium sulphate were low, indicating that denitrification was probably responsible. The application of the nitrification inhibitor dicyandiamide (DCD) reduced emissions from the ammonium-based fertilisers by up to 64%. A single application of nitrapyrin reduced emissions by up to 52%. When urea fertiliser was applied in the spring, followed by ammonium nitrate later in the growing season, the emissions of N2O were lower than when only urea or only ammonium nitrate were used throughout the season; the reduction was similar to that obtained using the nitrification inhibitors.

Potential nitrogen availability and fertiliser recommendations

In a series of trials, uptake of N derived from soil by cereals was compared with potentially available N in the soil, as estimated by boiling with 2 M KC1 for four hours and measuring the ammonium-N released. In previous work, between 1988 and 1990, nitrogen fertilisers were applied to spring barley at recommended rates, and microplots using 15N were used to discriminate between soil-derived and fertiliser-derived nitrogen. In 1991 and 1992 the trials were extended to include winter cereals (barley and wheat) with different rates of fertiliser. Where fertiliser additions were reduced below the normal recommended additions, no reductions in yield or N content of the cereal grain were observed. The uptake of soil-derived nitrogen by the grain in spring barley varied between 34 and 108 kg ha−1. It was correlated significantly with potentially available N over the four trial years (r=0.56;p<0.01). The uptake of soil-derived N by winter cereals was found not to correlate with potentially available N. When sites with a high soil organic matter content were excluded, the correlation between soil N uptake and soil organic-carbon was significant between 1988 and 1990 (r=0.96; p<0.001); however, this relationship was not apparent in 1991 and 1992 at sites with a wider range of organic-carbon contents and management histories.