Blog Entry #1 Slurry Spreading – Ammonia Emissions and Reduction Strategies
Slurry spreading is a significant source of ammonia emissions, accounting for approximately 34% of total agricultural NH3 emissions in NI. Slurry spreading is a diffuse source of ammonia, with land spreading events generating significant localised peaks in ammonia, but with emissions (and resulting concentrations in the air) dropping off fairly rapidly in the hours and days following spreading.
Slurry spreading by splash plate has been the most typical application method in NI for many decades. However, this form of spreading incurs high ammonia losses as the slurry is dispersed widely, exposed to the air during application, and spread over a large ground area, creating a large emissions source. The loss of ammonia represents the loss of a valuable source of nutrient nitrogen from the slurry, which could otherwise have been utilised by the crop.
Low Emission Slurry Spreading (LESS) methods reduce slurry exposure to the atmosphere when spreading and consequently reduce ammonia emissions. Ammonia emissions are also influenced by the types of slurry being spread. Cattle slurries with a 6% dry matter content typically have 2.5kg total nitrogen (N) per m3 slurry, with 40-50% of this total N in the form of ammonium-N. Pig slurries with a 4% dry matter typically have 3.6 kg total N per m3 slurry, with 70% of the total N as ammonium-N. The ammonium-N component of the slurry is that which is predisposed to loss as ammonia. Consequently, ammonia emissions from pig slurry applications are higher than from cattle slurry applications, per m3 of slurry.
The table below outlines some typical slurry application methods and their ammonia emission reduction potential compared with the traditional splash plate. In recent times, there has been a move towards use of these lower emission methods with 27% of slurry in Northern Ireland now estimated to be spread using LESS equipment.
Since 2019, slurry at AFBI Hillsborough has been spread by low emissions slurry spreading techniques. The use of an umbilical system is also common on the farm.
At the AFBI Hillsborough ammonia monitoring site we have measured a number of concentration peaks associated with slurry spreading in recent months, as slurry spreading resumed following the end of the winter closed period. It is inferred that the concentration peaks would have been significantly higher had low emission spreading techniques not been used.
Here we present some examples:
Slurry Spreading Event 1
Slurry spreading using trailing shoe occurred on March 10 in fields immediately to the east of the ammonia monitoring site (Magennis field, highlighted yellow).
On March 11 at approximately 10 am, the winds changed from a south easterly to an easterly breeze, allowing us to see ammonia from the slurry applied field for a brief period at the monitoring site. Ammonia concentrations peaked at 128 ug m3 at 11.30 at the monitoring site. By 12.30 a change in the wind to a southerly direction and light rainfall mean that the peak in ammonia, as detectable at the measurement site, subsided quickly. Had slurry been spread by splashplate we would have expected a significantly higher concentration peak.
Emissions from landspread slurry are highest for approximately 36-48 hrs after application, after which emissions and local concentrations decrease rapidly. However, emissions from a slurry spreading event can continue to be above background levels for over a week following application.
Slurry Spreading Event 2
There are a number of interesting ammonia peaks during this period, including concentration peaks associated with emissions from local livestock grazing and animal housing and slurry store emissions, which we frequently see at the measurement site under prevailing southerly and south westerly winds.
The largest peak, of 55 ug m3 is associated with slurry spreading by trailing shoe on Magennis Hill (highlighted yellow). The difference in magnitude between this peak and the previously identified slurry spreading peak is determined by the distance between the spreading location and the monitoring site and the windspeed among other factors.
Summary
In summary, slurry spreading is a significant source of ammonia emissions within a livestock-dominated agricultural sector, such as that in Northern Ireland. Ammonia emissions at slurry spreading are influenced by the type of slurry being spread, the spreading method and weather conditions. LESS techniques offer significant potential for ammonia reduction as outlined. Wind speed and direction play a significant role in transporting emitted ammonia from emission sources and influence the variability in ammonia concentrations through an agricultural landscape over space and time. The AFBI Hillsborough monitoring site gives us an insight into ammonia concentrations on the Hillsborough farm and how these are related to both on-farm activities, which generate ammonia emissions, and weather conditions.
Subsequent blog posts will consider ammonia emissions from other major farm sources, as captured at Hillsborough farm, and what can be done to mitigate these.
