Rachel Thorman

An assessment of nitrification inhibitors to reduce nitrous oxide emissions from UK agriculture

A trial was conducted consisting of 14 experiments across sites in England of contrasting soil type and annual rainfall to assess the effectiveness of nitrification inhibitors (predominantly dicyandiamide (DCD) but limited assessment also of 3, 4-dimethylpyrazole phosphate (DMPP) and a commercial product containing two pyrazole derivatives) in reducing direct nitrous oxide (N2O) emissions from fertilizer nitrogen (N), cattle urine and cattle slurry applications to land. Measurements were also made of the impact on ammonia (NH3) volatilization, nitrate (NO3−) leaching, crop yield and crop N offtake. DCD proved to be very effective in reducing direct N2O emissions following fertilizer and cattle urine applications, with mean reduction efficiencies of 39, 69 and 70% for ammonium nitrate, urea and cattle urine, respectively. When included with cattle slurry a mean, non-significant reduction of 56% was observed. There were no N2O emission reductions observed from the limited assessments of the other nitrification inhibitors. Generally, there were no impacts of the nitrification inhibitors on NH3 volatilization, NO3− leaching, crop yield or crop N offtake. Use of DCD could give up to 20% reduction in N2O emissions from UK agriculture, but cost-effective delivery mechanisms are required to encourage adoption by the sector. Direct N2O emissions from the studied sources were substantially lower than IPCC default values and development of UK country-specific emission factors for use in inventory compilation is warranted.

Nitrous oxide mitigation in UK agriculture

Nitrous oxide (N2O) makes the single largest contribution to greenhouse gas (GHG) emissions from UK and European Union agriculture. Ambitious government targets for GHG mitigation are leading to the implementation of changes in agricultural management in order to reduce these emissions (mitigation measures). We review the evidence for the contribution of those measures with the greatest mitigation potential which provide an estimated 4.3 t CO2e ha−1 y−1 GHG reduction in the UK. The mitigation options considered were: using biological fixation to provide nitrogen (N) inputs (clover, Trifolium), reducing N fertilizer, improving land drainage, avoiding N excess, fully accounting for manure/slurry N, species introduction (including legumes), improved timing of mineral fertilizer N application, nitrification inhibitors, improved timing of slurry and manure application, and adopting systems less reliant on inputs. These measures depend mostly on increasing the efficiency of N fertilizer use and improving soil conditions; however, they provide the added benefit of increasing the economic efficiency of farming systems, and can often be viewed as “win-win” solutions.

Algorithm for estimating the crop height effect on ammonia emission from slurry applied to cereal fields and grassland

Ammonia (NH3) emission following the application of livestock slurry to agricultural land is a significant source of atmospheric NH3, and not only poses a risk to the environment through eutrophication and acidification of sensitive ecosystems, but may also result in a loss of plant-available nitrogen (N). The band-spread slurry application technologies of trailing hose and trailing shoe have been shown to reduce NH3 emissions and consequently to increase plant uptake of slurry-applied N. There is a need to improve the precision in calculating the reduction in NH3 emission nationally and at farm level, and this may be achieved through the development of algorithms that more accurately estimate NH3 emission and also support the assessment of fertilizer efficiency of slurry that is band-applied to crops. Therefore, this study reviewed studies of NH3 emission from slurry band applied with a trailing hose or shoe with the objective of developing an algorithm for calculating the reduction efficiency of the band application technique in relation to crop height. The developed algorithm predicted that for slurry applications to cereal crops, the reduction efficiency would increase by slightly less than 1% for every 1 cm increase in crop height. For slurry application to grassland, the reduction efficiency was predicted to increase by approximately 5% for every 1 cm increase in sward height. The developed algorithm was used in combination with the ALFAM model, which predicts NH3 emission from slurry applications to bare soil, to provide monthly coefficients for estimating NH3 emission from slurry applied to bare soil, cereal crops and grassland for Denmark and Southern England. Inclusion of the crop height algorithm in the emission factor derivation predicted, for example, a decrease in the emission factor for cattle slurry application by trailing hose to winter wheat from c. 25% to c. 15% of applied ammoniacal N for applications in January and May, respectively. While the algorithm developed in this study would benefit from wider validation, should more published data become available, the present paper demonstrates its potential value as an educational tool for farmers and advisers in developing more sustainable manure management strategies and for inclusion in emission factor calculations for national inventories to both improve the temporal distribution of emissions from slurry application and to better reflect improved management practices of farmers.

Modelling nitrous oxide emissions from grazed grassland systems

Grazed grassland systems are an important component of the global carbon cycle and also influence global climate change through their emissions of nitrous oxide and methane. However, there are huge uncertainties and challenges in the development and parameterisation of process-based models for grazed grassland systems because of the wide diversity of vegetation and impacts of grazing animals. A process-based biogeochemistry model, DeNitrification-DeComposition (DNDC), has been modified to describe N(2)O emissions for the UK from regional conditions. This paper reports a new development of UK-DNDC in which the animal grazing practices were modified to track their contributions to the soil nitrogen (N) biogeochemistry. The new version of UK-DNDC was tested against datasets of N(2)O fluxes measured at three contrasting field sites. The results showed that the responses of the model to changes in grazing parameters were generally in agreement with observations, showing that N(2)O emissions increased as the grazing intensity increased.

The contribution of cattle urine and dung to nitrous oxide emissions: Quantification of country specific emission factors and implications for national inventories

Urine patches and dung pats from grazing livestock create hotspots for production and emission of the greenhouse gas, nitrous oxide (N2O), and represent a large proportion of total N2O emissions in many national agricultural greenhouse gas inventories. As such, there is much interest in developing country specific N2O emission factors (EFs) for excretal nitrogen (EF3, pasture, range and paddock) deposited during gazing. The aims of this study were to generate separate N2O emissions data for cattle derived urine and dung, to provide an evidence base for the generation of a country specific EF for the UK from this nitrogen source. The experiments were also designed to determine the effects of site and timing of application on emissions, and the efficacy of the nitrification inhibitor, dicyandiamide (DCD) on N2O losses. This co-ordinated set of 15 plot-scale, year-long field experiments using static chambers was conducted at five grassland sites, typical of the soil and climatic zones of grazed grassland in the UK. We show that the average urine and dung N2O EFs were 0.69% and 0.19%, respectively, resulting in a combined excretal N2O EF (EF3), of 0.49%, which is <25% of the IPCC default EF3 for excretal returns from grazing cattle. Regression analysis suggests that urine N2O EFs were controlled more by composition than was the case for dung, whilst dung N2O EFs were more related to soil and environmental factors. The urine N2O EF was significantly greater from the site in SW England, and significantly greater from the early grazing season urine application than later applications. Dycandiamide reduced the N2O EF from urine patches by an average of 46%. The significantly lower excretal EF3 than the IPCC default has implications for the UKs national inventory and for subsequent carbon footprinting of UK ruminant livestock products.

Modeling nitrous oxide emissions from three United Kingdom farms following application of farmyard manure and green compost

Organic fertilizers, such as manure and compost, are promising additions for synthetic fertilizers in order to increase soil fertility and crop yields. However, the organic fertilizers applied to soils may increase nitrous oxide (N2O, a greenhouse gas) emissions due to their lower C/N ratios, and therefore potentially contribute to global warming. Very few studies have used process-based models to assess the environmental advantages and drawbacks of compost soil amendments compared to other field treatments. In this study, the UK-DNDC model was modified for simulation of nitrous oxide (N2O) fluxes emitted from the soils treated with green compost and farmyard manure at three UK farms (WE, PW and NW): one winter wheat and two grasslands. The results show that the annual overall N2O emissions were 1.45 kg N ha-1 y-1 for WE treated with farmyard manure, 0.71 for WE with green compost, 1.09910 for PW treated with farmyard manure, 0.94 for PW treated with green compost, 1.19 for NW treated with farmyard manure, and 1.18 for NW treated with green compost. A two dimensional linear model was developed to correlate nitrogen loading and soil pH for calculations of emissions and emission factors (EFs). The linear model could fit the emissions obtained from the UK-DNDC model well. The squares of correlation coefficients of the emissions between two models are 0.993 and 0.985 for farmyard manure and green compost, respectively. Analysis of correlation coefficients between N2O emissions and air temperature, precipitation as well as the time period between fertilizer application and sample measurement (PFS) for the three sites treated with farmyard manure and compost indicated that N2O emissions were mainly related to PFS. The modified DNDC model provides an approach to estimating N2O emissions from compost amended soils.