By Sally Brown, University of Washington

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

  1. A Soil-Science revolution upends plans to fight climate change
  2. Dynamic stability of soil carbon: reassessing the “permanence” of soil carbon sequestration.
  3. Winter cover crops increase readily decomposable soil carbon, but compost drives total soil carbon during eight years of intensive, organic vegetable production in California
  4. Soil carbon response to long-term biosolids application
  5. Closing the Loop

June and July we focused on parts per billion (PFAS).  For August, I scaled it up to tons (soil health and carbon).  Now for September we are going giga tons!  The library this month is in many ways an extension of the August library, how organics can impact soil carbon sequestration and soil health.  Here the inspiration came from an article in Quanta Magazine- not so common in this library.  I found out about this article via an email from Maile.  A panicked email from Maile at that.  The article (first in the series) is a classic example of the sky is falling genre.  Here the author notes that our understanding of soil carbon and its associated longevity has changed.  We used to put soil carbon into two basic categories:

  • The new stuff
  • The old stuff (consisting of)
    • Fulvic and humic compounds

These fulvic and humic compounds where gigantic and would cause severe indigestion for any microbe foolish enough to try to eat them.  Here is a picture from Wikipedia


These compounds were going to be the silver bullet for climate change.  Well, it turns out that they don’t really exist.  Hence the article and hence the hysteria.  We have a new and developing understanding of how organic matter functions in soils.  Out with the new and old, and in with things like MAOM (mineral associated organic matter) and POM (particulate organic matter). 

Maile sent an email out to some of the trusted minds in the W 4170 group looking for a reaction to the article and got the following responses: 



Wise words!

From here we go to Article #2 that provides a detailed and nuanced description of how soil scientists currently understand organic matter cycling and persistence.  One of the authors is Kate Scow from UC Davis.  She is a microbiologist and has published some critical papers on this topic.  The main points of this article are that just because a single piece of carbon may not hang around for centuries, soil carbon as a whole is a critical tool for carbon sequestration. If one considers equilibrium carbon concentrations rather than that specific molecule you get a much more realistic and nuanced understanding of how organic matter in soils function.  This, they argue, should be integrated into policy.  

Here is a figure from the paper showing a simplified version of how we now understand carbon to behave:


The authors also emphasize the potential of soil C at lower depths.  Here there are fewer microbes around looking to chow down and so the C may last longer.

The paper sites a wide number of studies.  For paper #3, I picked a recent one.  Here the authors are looking at an organic vegetable cropping system in California.  These systems are intensively tilled and managed, producing two crops a year.  That tillage reduces carbon and over time can degrade the soil. 


Here the authors did a full carbon balance looking at the role of cover crops (every year or once every 4 years) as well as use of a yard waste compost.  The conclusions were that the cover crops and the compost (understand here that they were using a low N compost) had different roles in maintaining the soils health.  The cover crops provided more food for the microbes and more nutrients for the microbes and the plants.  The compost provided for more soil carbon to be accumulated.  Here is a figure from the paper showing the range of carbon inputs in the different systems.  Note that the yields for both the vegetables and the cover crops were highest where cover crops were grown every year.  Soil C storage was about the same as long as the compost was added.


Paper #4 takes us to agronomic systems in CA where biosolids have been applied over time.  Here the authors are Rebecca Ryals and her graduate student (who will be speaking at Biofest).  FYI- Biofest is on line and not expensive this year.  Might be worth it for those who typically can’t attend due to distance and cost.  Anyway, here the authors look at changes in soil C and N over time at three sites.  The authors sampled down to 100 cm and measured total C and N as well as microbially active C and N.  The sites had different cumulative loading rates as well as different rates of C and N storage from the biosolids. 


The take home here is that the biosolids are helping the soils but how much help and how they help is something that we need to do more research to fully understand. 

As articles 2-4 show, soils are complex systems and while we can say with confidence (as did the guys from W 4170) that organics help, we are continuing to learn how to best strengthen them.  They can have a pivotal role in sequestering carbon and making sure we can eat.  Shame on article #1 for crying wolf and not doing their homework.

To reinforce this, and to show that this is something that soil stewards have known for a long time we go to Article #5.  This is actually a chapter from David Montgomery’s book Growing a Revolution.  This is a great book and an easy read.  Much of it focuses on farmers who are integrating restorative practices into their farms and the benefits that they are seeing.  This chapter talks about adding organics back to the land.  Montgomery points out that the very same guys who did so much to advance chemical fertilization actually understood that adding organics back to soil was critical for soil functioning, what we now call soil health.  The chapter starts with the history and travels to Tagro where Montgomery sees the modern- day version of adding organics back to soil at work.  It is a great read and fun to see some familiar names.  You can also watch him give a talk about this book (not much on Tagro makes it to the talk) on YouTube:

While I didn’t reply to Maile’s email, hopefully this library will add enough information to both back up what Greg and Jim’s replies added and reinforce that every ton of organics we add to soil is a ton closer to healthy, resilient soil.  Soils that also store carbon.