By carrying out a life cycle analysis (LCA), the cohort from Denmark’s Aarhus University and Ireland’s University College Dublin assessed GHG emissions data to determine the carbon footprint of slurry fertilizers throughout their life cycle: from production to application.
The research also offers insight into which slurry management technologies have the lowest impact on climate and can positively contribute to reducing greenhouse gas (GHG) emissions from agricultural inputs.
The LCA assessed three stages: feedstock production (including the impact of transport, energy use and emissions); feedstock processing (such as anaerobic digestion and separation), and the use of fertilizer for crop production, where the effects of associated emissions such as nitrous oxide, methane and ammonia were measured.
The researchers also looked at the impact of organic nitrogen fertilizers produced from slurry processing, e.g. anaerobic co-digestion and separation.
According to the findings, treated slurry – whether partially or highly-processed – had a lower global warming potential (GWP) compared to untreated slurry; with anaerobic digestion and separation techniques being the least climate-intensive for producing organic fertilizer.
The global warming potential of different organic fertilizers
Digestate fertilizer had a 24% GWP of that of untreated slurry inclusive of the avoided emissions from biogas use; this was reduced to -6% when the production of 1 ton of spring barley grain yield was factored in.
For liquid fertilizer, the values were 21% and 6%, respectively.
Ammonium sulfate fertilizers had the largest GWP at 49%, increasing to 177% post-field application.
(See 'sources' below for more information)
The authors also estimated that replacing untreated slurry with digestate, or the liquid fertilizer of digestate, leads to net GWP reductions of nearly 80%, and of up to 106% if the emissions of 1 ton of grain yield is factored in.
The researchers also found that methane emissions were responsible for much of untreated slurry’s climate impact; while for liquid and ammonium sulfate fertilizers, the biggest offenders were emissions from field application.
Meanwhile, untreated slurry had the greatest carbon sequestration potential compared to treated fertilizers.
How to reduce emissions from fertilizers
The study also discusses how emissions from using different types of fertilizer could be minimized through smart applications.
For example, using less liquid fertilizer in wet fields could help mitigate the risk of nitrous oxide emissions; while anaerobic digestion could reduce methane emissions from slurry during storage.
In addition, liquid fertilizers may help reduce soil crusting, which encourages nitrous oxide production.
Timing is also key – for example, spreading manure-based fertilizer within the growing season can help reduce soil nitrous oxide emissions, the authors stated.
“Achieving additional [carbon dioxide] reduction from organic fertilizers requires a focus on mitigating GHG emissions during storage of slurry-based fertilizers (mainly methane), and after their field application (mainly nitrous oxide). This implies the need to select and evaluate GHG mitigation measures to maximize GHG reduction while avoiding crop yield loss. Mitigation options mentioned include acidification during storage, cooling, using a porous cover to support [methane] oxidation, and adopting appropriate field application methods and timing,” the authors explained.
How much carbon is produced by organic fertilizers is also down to the type of feedstock and their post-treatment stages, they added.
In conclusion, the authors commented: “Our study shows that the carbon footprint of the production and storage of alternative organic fertilizers per 100kg TN ranged from 21% to 49% of the baseline scenario using [untreated cattle slurry]. Upon application to the field, the carbon footprint per ton [dry matter] spring barley grain yield ranged from −6% to 177% of the baseline scenario.
“The primary contributors to the carbon footprint of these fertilizers were emissions during storage and after field application, while biogas production from anaerobic digestion significantly reduced the GWP.
“This highlights the substantial potential for mitigating climate impact by replacing untreated slurry with alternative fertilizers derived from anaerobic digestion, primarily through reduced emissions during storage and biogas production.”
Sources:
Climate impact of alternative organic fertilizers using life cycle assessment
Xiaoyi Meng et al
Published: 15 November 2024, IOP Publishing Ltd, Environmental Research Letters, Volume 19, Number 12
DOI: 10.1088/1748-9326/ad8589