soil carbon

By Sally Brown, University of Washington

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

  1. Natural Climate Solutions

  2. Deep soil inventories reveal that impacts of cover crops and compost on soil carbon sequestration differ in surface and subsurface soils

  3. Long-term effects of biosolids on soil quality and fertility

  4. Iron-mediated stabilization of soil carbon amplifies the benefits of ecological restoration in degraded lands

  5. Priming mechanisms with additions of pyrogenic organic matter to soil

Biosolids (and composts) are the good news this library.  And the library even includes articles on how to (potentially) make them even better!  We start with a big impact article on climate change.  Climate change is the bad news.  The first paper focuses on how natural climate solutions (NCS)can be a much more significant part of the answer to climate change than previously thought.  The authors go through a range of nature-based solutions with a focus on conservation, restoration and improved land management and conclude they would sequester 23.8 petagrams of CO2 per year.  These NCS can do this while still allowing for sufficient food and fiber production that we can still wear clothes and eat.  What is a petagram you may ask?  It is 1 000 000 000 tons.  In other words, an unimaginably large amount.  While they focus on forestry and tree planting, they also discuss the appropriate management of agricultural lands and the use of biochar.   They say biochar because, although gifted and great scientists, these guys don’t know the wonders of biosolids and composts.   The authors of the next two papers do.

The second paper in the library is currently causing quite the stir in compost circles.  The authors (from UC Davis) measured soil carbon concentrations to depth (2 m) in long-term field plots.  The plots included a corn rotation with conventional fertilizer, conventional + winter cover crops and organic.  The organic treatment included compost application and a winter cover crop.  Cover crops have previously been shown to increase soil carbon concentrations in the topsoil horizons.  They did that here too.  It was just when the authors dug deep that the differences started to show up.  For whatever reason, and the reason behind this is not clear or logical, the cover crop alone treated soils lost carbon in the lower depths.  It was only when compost was included that carbon concentrations in the lower depths stayed consistent - leading to a net carbon accumulation.  The take-home figure from the paper is shown below.  Big conclusion - compost is the hero.


The third paper is the same type of study as the second but with biosolids.  It comes from the land of Brexit which luckily has no bearing on long-term biosolids plots.  One of the authors on this one is Steve McGrath- long time metals guy, biosolids opponent.  The times they are a-changin'.  This study was set up in 1994 with normal and high metal biosolids added annually for 20 years.  These guys may not have dug as deep but they measured wide.  Biosolids increased soil carbon, water holding capacity with non- significant increases in water infiltration rate and soil aggregate stability.   Adding biosolids also increased soil fertility in comparison to synthetic fertilizers with increases in total nitrogen, sulfur, and extractable phosphorus.  Did I mention earthworms?  Double the number and double the weight as in the control soils.  Wheat grain N, P, and S were also increased in the biosolids amended soils.  Not only more carbon but much better soils with more nutritious wheat.  A pretty good deal, I’d say.  

So composts and biosolids are excellent tools.  Can we make them even better?  That is the subject of the final two papers in the library.  The 4th paper comes from Brazil where biosolids were used to restore a strip mine.  The biosolids did a fine job- this is no surprise.  The restored sites also showed a highly significant increase in organic matter.  The big deal here is that the authors found a very significant fraction of the stored carbon was associated with iron oxides.  Organic- mineral complexes helped to protect the carbon from microbes, preventing them from eating it.  Perhaps adding iron to biosolids may increase the carbon sequestration potential of these materials?   I see a field trial in the future…. 

The final paper comes from Cornell University.  Here the authors added char to soils and measured microbial respiration.  They found that the char, like the iron in Paper #4 formed bonds with the soil carbon, protecting it from hungry microbes looking for lunch.  The complexed carbon was harder to eat and so more stable in the soil system.  Adding biochar to composts and biosolids might be another way to make a good thing even better.