By Sally Brown, University of Washington
Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members.
Global nitrous oxide emission factors from agricultural soils after addition of organic amendments : A meta-analysis
Nitrous oxide emissions respond differently to mineral and organic nitrogen sources in contrasting soil types
Effects of organic and inorganic fertilizers on greenhouse gas (GHG) emissions in tropical forestry
Soil nitrous oxide emissions from agricultural soils in Canada: Exploring relationships with soil, crop and climatic variables
Nitrous oxide emissions from clayey soils amended with paper sludges and biosolids of separated pig slurry
It has been 10 years since the Biosolids Emissions Assessment Model (BEAM) came out. No - BEAM is not some electronic vehicle. That would be the LEAF model. This is the model that SYLVIS developed for the Canadian government to quantify the carbon costs/benefits associated with different biosolids process and end-use/disposal practices - access the BEAM User Guide here.
I helped out with that, as did Ned Beecher (NEBRA) and Andrew Carpenter (Northern Tilth). We three also wrote a paper about the model that was published in Environmental Science & Technology. Happy to send that along but the focus of the library for the next few months will be on publications that have come out since then. Just like the 503 regulations, the BEAM model can stand to be looked at over time to see what type of updates and revisions might be available. The focus for this library is nitrous oxide (N20) emissions from land application.
Take home - in order for N2O to be formed you need low oxygen concentrations and a carbon source for microbes.
The IPCC default factor for N2O emissions is 1% of the total N applied. In the original BEAM model we had different factors for N2O emission based on soil texture and whether the biosolids were dried. We assumed emissions of 0.5% total N for coarse textured soils and 2.3% of total N for fine textured soils. These factors were reduced when the biosolids solids content was > 80% for fine textured soils by 50%.
So what has 10 years brought? The first article in the library is a survey paper on N2O emissions from soils that have received organic amendments. One of the authors here is Philippe Rochette. Dr. Rochette works for Ag Canada and is a recognized and reasonable expert on this topic. The authors surveyed all of the peer review papers on this topic to come up with revised values for default emission factors. They ended up finding 38 usable studies that were done in 12 countries. The overall N2O emissions factor across all of the organic amendments included was 0.57±0.3% of total N- lower than the IPCC factor of 1 for synthetic fertilizers. They also found that emissions varied based on type of amendment and type of soil that it was applied to. The three types of amendments were:
• High risk (1.21 ± 0.14%)
o Animal slurries
o Waste waters
• Medium risk (0.35 ± 0.13%)
o Solid manure
o Compost + fertilizer
o Crop residues + fertilizer
• Low risk (0.02± 0.13%)
o Crop residues
o Paper mill sludge and pellets
They found higher emissions when amendments were added with fertilizer and also a 2.8x increase in emissions when materials were applied to finer textured soils.
If you look at the study in greater depth you notice a few things. There are very few biosolids studies- the references that I saw were only for biosolids added with fertilizers. So the default value for biosolids is not really useful. It is also not clear that when the authors say ‘biosolids’ that they are referring to municipal wastewater solids. The 4th paper (author overlap) defines biosolids very broadly as including animal solids and pulp and paper residuals. Also telling was the information on pig slurry. Pig slurry with > 10% N, 5% solids, a C:N ratio <5 had higher emissions than animal slurries with solids contents > 5% or more stable products (C:N>30). The authors say ‘..’pig slurry contains high amounts of NH4 and easily decomposable organic C that can, in concert, directly stimulate soil denitrifers… thereby further stimulating N2O production through denitrification’. Cow manure- with more stable C compounds had lower emissions than pig slurry. They found that if mineral N (NO3 and NH3) were less than 0.3% dry weight emissions were lower. We can use that to make some small refinements to BEAM based on biosolids characteristics. Looking over the data set they found C:N>21 would not increase N2O emissions. More rain led to greater emissions and higher organic matter in the soil resulted in a small increase in emissions. Again - some ways to refine the BEAM model.
The second paper is also from Rochette. It presents results from a field study in Canada where different types of animal manures were applied to a sandy and a clayey soil. In Table 3 you see that the liquid swine manure (LSM) released 7.6 kg N2O per ha-1 in the silty clay soil and 0.5 kg N2O per ha-1 in the sandy loam for N2O emissions factors of 4.8% and 0.1%, respectively. Emissions were concentrated in the spring and fall when moisture content of the soils was highest.
Third paper takes us to Brazil, to a reforestation study where biosolid and biosolids compost were applied to a degraded pasture. The biosolids had been aerobically stabilized and the compost was produced with biosolids mixed with wood waste. Application rates were low. Gas samples were collected over time with no replication across plots. The soil was an Ultisol- a well weathered soil. These are typically high in clay but texture wasn’t reported. The compost had the highest emissions (2.11%) with biosolids coming in second (0.94%). So both of these are lower than what Rochette saw in the high clay soil.
Fourth paper takes us back to Rochette and back to Canada. This paper was published more recently (2018) than the first two from Rochette. Like the first paper, it also presents a compilation of results from other studies rather than original research. The authors found that high precipitation and fine soil texture were significant factors. They also found that biosolids derived N- with biosolids here defined as : ‘Sources of the biosolid organic N in this study included aerobically digested pig slurry, de-inking paper sludge, filtered pig slurry, anaero- bically digested and flocculated pig slurry, and mixed primary and secondary paper pulp sludge’ behaved differently and as a result, were analyzed separately. Here the authors did a stepwise regression, seeing what factors were responsible for N2O emissions. For synthetic fertilizers precipitation was responsible for 38% of the variation observed in the data - higher rain, more anaerobic conditions, higher rates of denitrification and more N2O. Amount of N applied took care of another 15% of the variability. More N, more N2O. Sand content decreased emissions as did higher pH (has to do with microbial denitrification pathways). These variables were able to account for 70% of the variability in the data. For organic sources of N, the process was much simpler. About 62% of the variability in the data was explained by soil texture with another 20% depending on whether it was an annual or perennial system (perennial systems have lower emissions). Here is the take home in terms of emissions factors:
Emissions in Western CA were much lower likely due to lower rainfall and higher pH soils:
From here we move to the last paper in the library, this one directly comparing different types of animal waste application to control and synthetic nitrogen in clayey soils in Canada. Raw pig manure as well as stabilized pig manure and paper sludges were land applied. The treated manures and the pulp sludges are referred to as biosolids. There is a high probability that the behavior of these materials is similar to the behavior of the stuff that we call biosolids. The emissions factor for raw pig slurry was 2.5 with a decrease to 1.6 after the material had been through an anaerobic digestor. This and the first paper in the library are the two take homes as far as I am concerned. Number 4 as well- but talk about a dense read. My brain was giving off N2O after that one.
So - some progress after 10 years. We can adjust BEAM further for soil texture, rainfall and type of crop. We can say with more certainty that drier, digested materials and composts emit lower amounts of N2O than wet, raw slurries. Step by step.