By Sally Brown, University of Washington

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

  1. Towards a global-scale soil climate mitigation strategy

  2. Quantifying carbon for agricultural soil management: from the current status toward a global soil information system

  3. The simple AMG model accurately simulates organic carbon storage in soils after repeated application of exogenous organic matter

  4. RothC simulation of carbon accumulation in soil after repeated application of widely different organic amendments

  5. Major limitations to achieving “4 per 1000” increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom

A shout out to Deirdre Griffin LaHue for her work in organizing an on- line week of presentations on soil health at Soilcon (  I was able to listen to pieces and look forward to listening to more.  Soil health is all the rage.  Gwyneth Paltrow may even have heard of it.   The key to soil health is organic matter.  And we all know that biosolids and composts are the most effective ways to increase soil organic matter (See June 2020, Oct 2019, February 2018, February 2009).  This gives you one of the best two for one deals around; improved soil health and a tool to fight climate change (soil carbon storage).  This month I wanted to take a look to see how our part of the solution is being recognized by people who don’t work directly with ‘cake’.

The first paper is one of many recent ones that I found that focus on the potential for soil carbon storage to be a viable option for taking CO2 out of the air.  Like most, this one focuses on the global scale.  How to fix soils not just in New Jersey but also New Caledonia.  The authors note that three new initiatives have focused on increasing soil carbon storage as a means to remove carbon and increase resilience.  One of these is the ‘4p1000 initiative’ whose name reflects the fact that an increase in 4% of the global soil carbon reserves would be equivalent to the increase in CO2 emissions annually; one could cancel the other out.  The authors note that increases in soil C storage come with a range of benefits- including increased soil health. They outline basic tools to improve soil health and C storage.  The figure from the paper is shown below. GAP stands for good agricultural practices and does not mean that you have to wear jeans from that company as you tend to your field.

c sequestration

The authors note the difficulty of mapping soils- to see where the really bad ones are.  They talk about different solutions and where each solution would likely have the most significant impact.  Char makes it into this table for highly weathered soils in tropical regions.  Here is how they talk about us and what we do:

There is a general agreement that the most effective way to accumulate SOC is to increase C inputs (Fig. 1 (eg., refs. 29,30); organic substrates fulfill this purpose only partly if at all, because meaningful increases of carbon sequestration at one farm or region must occur without simultaneous reductions in SOC at another location from where this material is transported from. Hence, organic C inputs into soil must be produced on-site, i.e., by enhancing crop production and green manure71.

What they’re saying is that Joe can’t steal the cattle manure from Susie and claim credit for it. They point out that in Germany farmers have to document where their manure comes from-domestic or imported.  The authors of this article are not even in the universe of food scrap composting or biosolids beneficial use. Those things don’t show up on the radar for most of the world.  

For a trip to the universe where food scraps and biosolids do exist we go to article #2.  This article is written by the authors of the Comet model- the model used by USDA to estimate changes in carbon storage as a result of changes in farming practices.  I looked for a quantitative analysis of the Comet model- it is hard to find.  Keith Paustian, the lead author, is also responsible for the CENTURY model, an early and influential model to predict C storage in soil.  Comet is being used here and I included this article so you would get a feel of where the locals are coming from.  The authors start with a welcome repetition of much of the first article- all about how the importance of soil carbon is being recognized right, left and sideways!  Chalk one up for the team.  They continue that there are three ways farmers could be compensated for adopting practices to increase soil C- government subsidies, offset credits, or credits from companies that purchase agricultural product. Then there is a discussion over how to measure changes in soil C with direct soil samples as the gold standard.  That isn’t practical on a non- replicated field trial level so they then come up with the next best thing:  modeled data.  There is a table of existing models (you’ll hear more about those in articles 3,4 and 5).  And also a discussion of different programs offering credit for soil C conservation.  You can go to Australia where they have the Emissions Reduction Fund.  How much credit you get depends on where in the country you are and what you practice.  

soil c

Compost and biosolids are mentioned here (non-synthetic organic- based fertilizers) as Australia has plenty of each.  They are included only to say they don’t count against you.  Two examples from Canada are discussed- both centered on conversion to no-till.  

For a real discussion of the role of composts/ biosolids and other residuals based amendments we have to focus on articles #3 and 4.  The third article is the home run for this month.  The authors look at data from multiple long-term field sites in Europe where a wide range of amendments have been applied.  They get that it isn’t stealing cow pies and composts, digestates and biosolids are among the amendments included.  The general category here is exogenous organic matter or EOM.  Remember those initials EOM.  

They use the data to inform one of the carbon storage models : AMG.  The data is used to calculate approximate factors for the different types of organic matter.  

c sequestration

As I understand ‘h’ it refers to the fraction of carbon in the EOM that is likely to remain in the soil, either by partitioning in the active SOM pool or the stable organic fraction.  There is a lot of text on working with the model that has lots of equations.  The take home is that the efforts gave an r2 of >0.6 or pretty good.  The other take home is that a lot of the C added with EOM stays put.  The h values for green waste, compost, sludge and sludge compost range from 43- 100 with most of the values well over 50.  

We make good stuff.

The 4th paper I found because it was cited by the authors of the 3rd paper and is an earlier version.  Again- data from multiple long-term field sites are here used to enhance the RothC model (one of the models referred to in article #2).  The name of this model comes from the long -term field trial in Rothamsted in the UK.  In the RothC model each EOM type is split into fractions of labile(DPM), resistant(RPM) and humified(HUM).  There is a built -in transfer of C from the resistant to the labile and humified pools.  The speed of this transfer is determined in part by the clay content of the soil.  For this study the authors also collected each of the EOMs and analyzed them.  They then plugged the data into the RothC model.  Here again our amendments had relatively high resistant organic matter, ranging from 38% of total C for a MSW compost to 85% for the biosolids.  The model worked well for these materials with some exceptions.  For example, the authors clearly didn’t understand the power of the biosolids:

The sewage sludge in the Ultuna experiment was also discarded from the correlation because it led to an unexpectedly large C accumulation in soil that may be ascribed to an inhibition of soil microbial activity, possibly due to increased soil heavy metals content or low soil pH (Witter et al., 1993; Kätterer et al., 2011). 

Studies have actually shown that biosolids enhance microbial activity.  Maybe since the article was written, the authors have seen the biosolids light.  Lucky for us, at least one other of the biosolids modeled was left in for analysis.  The rate of C accumulation (tons of C ha-1 yr-1) for the EOM modeled based on an application of 2 tons ha-1 for 20 years (20 tons ha-1 cumulative loading) is:

•    Biosolids 0.38  
•    MSW 0.18
•    Green waste + biosolids compost 0.39
•    Biowaste compost 0.42
Or pretty damn high.

That takes us to the final paper in the library.  Here one of the authors is David Powlson, one of the speakers at the WSU SoilCon meeting.  The focus of the paper is on long-term sites in the UK and whether the ‘4p1000’ initiative is attainable.  Remember that is an increase of SOM in the surface soil of 4% per year.  Here are the % increases for regular application rates of the different amendments.  Each % is for a 10 year period

Farm yard manure        51    8    10.5
Vegetable compost     68    21    3.4
Biosolids                       114    11
Biosolids compost       80    20