Ammonia emissions from slurry applications to land
Over the last 10-15 years there has been increasing concern within Europe as to the effects of ammonia emission and subsequent deposition to sensitive ecosystems, causing eutrophication and soil acidification. Transboundary transport of emissions has led to legislation at EC level with member states being given emission ceiling targets. Research has therefore aimed at quantifying national emissions, modelling emission processes and developing mitigation strategies. Agriculture accounts for >80% of total UK ammonia emission, therefore an accurate and robust model is required to estimate emissions from this sector. National inventory methodology has improved as the database of emission measurements and survey data has grown and as models have evolved from discrete empirical calculations for individual sources to linked nitrogen flow models incorporating more process-based algorithms. Ammonia emissions from agriculture derive mainly from livestock manures (primarily from the urea content of urine) and land application of manures represents a major emission source. Research in this area has therefore aimed to improve our ability to predict losses, taking into account the major influencing factors, in order to improve inventory estimates, improve manure management decision support models for farmers and advisers and to highlight potential mitigation strategies. This requires the ability to make precise, accurate measurements and measurement technology has been developed for a range of scales. A key factor influencing ammonia emissions following applications of livestock slurries to soil is the rate and extent to which slurry infiltrates into the soil, where it will be largely protected from volatilisation. This has not previously been fully incorporated into process-based models and research presented here has provided a mechanism describing the infitration process in which the slurry dry matter concentration and the nature of that dry matter are among the important influencing factors. Measures aimed at reducing emissions from land spreading are generally regarded as the most cost-effective means of reducing emissions from agriculture. A number of slurry application techniques aimed at reducing emissions have been developed and assessed against the conventional method of surface broadcasting. These new techniques rely on either reducing the exposed slurry surface area from which emission occurs, reducing the air flow and temperature at the emitting surface (thereby increasing the resistance to ammonia transport from the emitting surface to the free atmosphere) or increasing the contact between slurry and soil. A more holistic approach to reducing emissions is via dietary manipulation, with the aim of reducing both the amount and form of nitrogen excreted by livestock. This can result in lower ammonia emissions at all stages of manure management i.e. livestock housing, manure storage and application to land.