By Sally Brown, University of Washington

Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members. 

  1. Carbon sequestration value of biosolids applied to soil: A global meta-analysis
  2. Carbon balance for biosolids use in commercial Douglas Fir plantations in the Pacific Northwest
  3. Revegetation of degraded ecosystems into grasslands using biosolids as an organic amendment: A meta-analysis
  4. Long-Term Biosolids Applications to Overgrazed Rangelands Improve Soil Health
  5. Long-term biosolids land application influences soil health

Let’s step back a few (000 000 000 000) zeros and instead of thinking in parts per billion/ trillion that require specialized equipment and several extraction days to stuff you can pretty much see with your naked eye.  Let’s measure in tons (1 kg x 1000) instead of ppb (1 kg x 10-000000000).  It is this perspective that lets you see the value of the material.  Yet again, let me remind you all that we work with some excellent sh**t!.  The library this month will focus on how biosolids improve soils and so improve the world.  And I say this not just as a soil scientist.

We start with a meta -analysis that looked at a wide range of studies to come up with an estimate of how much carbon you add (persistent over time) to the soil following biosolids applications.  Here is the take home

3.3% increase in soil carbon per hectare when you add 100 Mg of biosolids.

That is a lot.  If you figure that soil weighs 1000 tons per acre and has an initial C concentration of 2%.  You add biosolids and get up to 5%.  That is an extra 30 tons of C or 110 tons of CO2 per acre. 


The paper has qualifications- different soil types, temperature and moisture regimes will get different results but the main point is – that is a very high rate of C storage.  Note- that NW Biosolids dollars funded at least one of the sited studies.

Paper #2 is local, fresh off the press and confirms the findings of Paper #1.  Here we looked at soil carbon sequestration in forest plantations where King County has been applying LOOP for decades.  We looked at two soils- miles apart from each other, both planted in Douglas Fir.  In the soil with higher carbon to start with, we saw no storage from biosolids.  In the lower C storage- quite the opposite.  The figure says it all.


Note that the average here is pretty similar to the average seen in Paper #1.

Paper #3 is another meta- analysis.  Here the focus is on whether biosolids work well to restore native grasslands.  Increasing soil carbon is a necessary first step to this process but it gets more complex when you consider plant species richness and other factors.  They found across the 59 studies that they looked at that biosolids were highly effective at increasing plant cover and also productivity.  Species richness (the variety of types of plants) did not change.  Biosolids worked best in warmer places and seeding increased plant cover more quickly than not seeding.  They also found that biosolids were less effective if used to restore a burned area.  All responses were reported as the ratio to the control.  Key figure from the paper shown below.

august 2


The final two papers are also focused on biosolids applied to grassland.  They each focus on a distinct site. They both use a range of measures to evaluate soil health.  Both come from Jim Ippolito, a professor at Colorado State.  In Paper #4, the focus is on a degraded rangeland and either on a single high rate (30 Mg ha) or on multiple lower rate applications to the rangeland.  One and done is the best approach.  Biosolids, applied once at the higher rate were more effective than multiple applications at lower rates for the soil chemical properties (hear that, regulators?).  However, the lower rate over time was better for the biological indicators.  Overall, the authors say that applying up to 30 Mg ha, with an optimal rate of 10 Mg ha is what you need to restore a grassland. 

The last paper uses the same soil health tools to look at the impact of biosolids on dryland wheat- another type of grassland.  The study was based on a long-term trial in CO.  Jim presented the preliminary results at a Biofest a few years back.  The field was tilled, most likely minimizing improvements in physical properties and also likely reducing carbon sequestration.  Nevertheless, the biosolids were the winner over synthetic fertilizer.  This figure gets you the key points and is very easy to follow.


What all of these papers show is that despite the ppb of PFAS or whatever people are tearing their hair out about at the moment, biosolids are a highly effective tool for restoring soils- by adding carbon back into the system and as a consequence, improving the health and diversity of those systems.  That is the take home message.