The United Kingdom has set legally binding targets to reduce ammonia (NH3) emissions by 16% in 2030, based on a 2005 baseline. The latest inventory found that agriculture is responsible for approximately 90% of total UK NH3 emissions, with the manure management continuum contributing 60% of agriculture emissions. Such a loss of NH3 must be targeted if the 2030 target is to be achieved, but also to improve farm sustainability as NH3 emission represents a significant loss of nitrogen (N) from manures. Current Best Available Techniques (BAT) of reducing agricultural NH3 emissions include slotted animal housing floors, slurry store covers, and slurry injection, all of which can be prohibitively expensive. Slurry acidification, the process of adding concentrated acid to slurry, altering the slurry pH to adjust the NH3:NH4+ ratio strongly in favour of NH4+, is commonly used in Denmark and has been found to successfully reduce NH3 emissions at all stages of the manure management continuum. However, little research has been carried out on the impact of acidified slurry on soil and crop quality from a UK perspective. This Defra funded research focused on one of five national trial sites, aimed to assess the potential of including slurry acidification in the farmer toolkit to help achieve the UK NH3 reduction targets.
This study is comprised of multiple experimental chapters furthering the understanding of the impacts of slurry acidification on soil and crop quality, as well as providing policy-targeted research investigating the most efficient means to undertake acidification. The study begins with an overall introduction outlining the work undertaken (Chapter 1), as well as a literature review which explores the current literature within the research topic and establishes knowledge gaps within the current research (Chapter 2). Chapter 3 details a multi-year field experiment assessing the impact of acidified cattle slurry compared to a conventional cattle slurry, both applied by surface broadcast and band spread. This chapter also included nitrogen response plots (0 – 150 kg N ha-1), to assess fertiliser replacement values of acidified slurry. The experiment was carried out over 2 years and represented 4 single applications, spring and summer of each year, as well as plots which received repeated application of up to four applications (ranging from 67 – 125 kg N ha-1). The latter was used to assess any long term and residual impacts of applying acidified slurry. Soil chemistry - including soil pH, electrical conductivity, inorganic N and P - soil biology – including micro-, meso-, and macro-fauna, soil respiration, and greenhouse gas emissions – and soil physical characteristics were all analysed. Overall, Chapter 3 found that there were no long-term impacts of acidified slurry on either soil or crop quantity over a 2 year period. Soil pH was found to reduce by up to 0.75 units when compared to the control, directly under the band in band spread plots, but was buffered to control levels by the end of each growing period. Plots receiving acidified slurry were found to have greater soil concentrations of NH4+ without increasing NO3-. Consequently, this resulted in numerically greater N fertiliser replacement values (NFRV), and N use efficiency (NUE) by up to an additional 50%.
Alongside the field experiments, multiple bench scale experiments were carried out (Chapter 4). This included two different experiments using a Desktop Ammonia Volatilisation System (DAVoS) in combination with intact soil cores. These experiments assessed the impact of using different slurry pH levels on NH3 emissions, greenhouse gas emissions, and soil inorganic N. The typical pH target for Danish in-house acidification, pH 5.5, was found to be a compromise between NH3 emission reduction and the pH of slurry applied. At pH 5.5, NH3-N loss was found to be 17% of NH4-N applied, significantly lower than pH 6.5 (33%) and conventional slurry (39%), while greenhouse gases did not significantly increase. A further experiment using the DAVoS compared different application techniques in combination with acidification. Surface broadcast conventional slurry represented current UK agricultural standard practice, while shallow injection represented the BAT option for farmers. When applied in combination with acidification the results presented indicate that over a 14-day period, cumulative NH3 emissions from acidified surface broadcast cattle slurry were not significantly different from conventional injection. Both methods were found to have a NH3 loss of 3% of total N applied without significantly increasing short-term N2O emissions. As found in the field experiments, the DAVoS experiments showed an increase in soil solution NH4+ in acidified cores. The increase in soil NH4+ was also found in Chapter 5. Here, repacked soil cores had acidified slurry applied and were analysed at a micro-scale using a microtome over a 42-day period. NH4+ was found to be contained within the top 25 mm of cores for the entire period, while a clear inhibition of nitrification occurred with conventional slurry having greater peak NO3- concentrations (2861 mg NO3-N kg-1) compared to acidified slurry (2173 mg NO3-N kg-1).
The final experimental (Chapter 6) took on a policy-driven approach where slurry buffering was assessed in direct response to recommended acid quantities (2.2 kg t-1 slurry) added to slurry in Danish in-field acidification. This study found that the Danish recommend quantity would on average reduce a 20 UK sample of cattle slurries by pH 0.7, and a more appropriate quantity of acid for UK slurries would be 8 kg t-1 slurry. However, this represents an average value, with the key conclusion from Chapter 6 being that the unique nature of pH buffering in each slurry will require a different dose of acid.
Overall, slurry acidification was found to have limited medium- to long-term impacts on soil and crop quality in the UK. The experiments included in the study highlighted the potential of the technology to reduce NH3 emissions whilst increasing soil NH4+ without increasing NO3- concentrations and N2O emissions. Such evidence is supported by increased NFRV and NUE in plots receiving acidified slurry at each harvest. This research supports the inclusion of slurry acidification in a farmer’s toolkit as an option to reduce NH3 emissions.