Potential of Different Practices for Carbon Sequestration in Soils

by Stephen Shafer on February 25, 2019

                           Potential of  Different Practices for Carbon Sequestration in Soil   

           This essay starts with  the current loci of carbon sequestration, geological hydrological and biological.  I’ll  then review  some “natural solutions”  for near-term biological C-sequestration, which are mostly through photosynthesis in living organisms.  I will look at  the potential for various of these to contribute to essential carbon dioxide draw-down and  to restoration  of carbon-depleted soils. I point at the end  to  the extraordinary potential for global CO2 draw-down and soil improvement that is  in  low-cost  organic  soil amendments  made from compost  that mobilize soil biota to sequester carbon in ag soils.  One such “inoculant”  leads to sequestration to a 30 cm depth of  10  or more tonnes of carbon /ha/yr .  Treating all 150 Mha of  U.S. arable land with 450 kg/ha (~400 lb/ac) of that inoculant  could,  if small field trials are valid,  sequester 1.6 Gt C ( = 5.86 Gt CO2)  nationally/yr.  This represents more than total yearly US emissions of  CO2 and 16% of global emissions.  It is time to start a healthy low-tech Manhattan Project that puts us in symbiosis with  soil life  instead of thrall to nuclear fission.  The stakes for human survival are just as high or higher  as in 1942.

           Carbon  sequestration  means long-term storage  away from the atmosphere.   What’s   long term is fuzzy; let’s say at least five decades, preferably centuries or eons.  Most of  Earth’s carbon  (see graph) is securely sequestered  in  marine sediments and deep terrestrial rock (lithosphere) or in deep ocean waters.  Three per cent  is  insecurely sequestered as  fossil fuels now being de-sequestered as if there is no tomorrow.

Graph 1 Distribution of carbon in the world including atmosphere.  Hover cursor over a sector to see value in Gt C. Source https://worldoceanreview.com/en/wor-1/ocean-chemistry/co2-reservoir/     

            Carbon  in the atmosphere is not sequestered, nor is that  in ocean surface.  In both of these relatively small compartments there is now too much carbon as CO2.    Some carbon in terrestrial plants (e.g. the wood of   long-lived  trees or in permanent structures)  is sequestered above ground at least for decades ,  while that in leaves, grasses, forbs and other annual plants is not.  Carbon in soil can be sequestered long-term if the soil is not turned over and   there is year-round plant cover on it to prevent erosion by wind and water abetted by heat.


                                  Acronyms and abbreviations

BEAM            Biologically Enhanced Agricultural Management

EtOH              Ethanol

MF                  Mycorrhizal fungi

SOC                Soil organic carbon content as % of soil

SOM               Soil organic matter as % of soil

E+ 6                10 to the power of 6 ( = 1 million) = * 10^6

F:B ratio          ratio of fungi to bacteria

M ha                million hectares = 2.47 million acres

Gt                    billion metric tons (tonnes)   = Petagram  (Pg)

M mt               million metric tons ( tonnes) = Teragram (Tg)

mt                    metric ton ( = tonne)  =  Megagram (Mg)        


            To brake  global heating, the world must stop de-sequestering fossil fuels and bring to  far above today’s  natural background the sequestration of carbon  from  the atmosphere, most of which is in CO2 .  This essay looks at different methods to increase total C-seq, skipping over the high-tech ones  to focus on low-tech  “natural solutions”  that  sequester carbon in soil,  at the same time improving it.  Amongst  the  low-tech ones  the  highlight will be the potential for  greatly enhancing C-seq in soil by spreading  compost rich in fungi that once interacting with plants in soil stimulate proliferation of mycorrhizal fungi  (MF)    To start, below   is my layout of  most methods for  C-seq  now usable.   It’s not all-inclusive.

High tech Carbon Capture and Storage

            *from fossil fuel combustion

            *from biofuel  combustion

            *directly from air

Low-tech  not using photosynthesis

            Biochar, terra preta

Low-tech  using direct photosynthesis                      

            Trees  (including bushes, shrubs)

            No-till + cover crops  (“conservation agriculture”)

            Short-stay long rest grazing by ruminants   (“holistic or adaptive multi-paddock”)

            Unmanaged vegetation not forests  or  range grasslands (e.g. marshes, peatlands)

            Natural soil amendments 

                        Traditional composts

                        Mycorrhizal fungi


            Another schema is nicely shown by Mary Hoff (2017)   in “Eight Ways to Sequester Carbon to Avoid Climate Catastrophe.” 

                        Title                                          Potential for C-seq in  Gt  CO2/yr

            Afforest/reforest                                             1-14

            Carbon farming                                               1-13

            Other vegetation                                             unknown

            *Bioenergy carbon capture and storage          1-20

            Biochar                                                            1-4

            *Fertilize oceans                                             1-4

            *Rock Solution                                               1-18

            *Direct capture  from air                                 3-16


            An important 2017 paper by Griscom et al  reviewed “Natural Climate Solutions.”  The many authors assessed twenty natural pathways, none being  any of  the methods  marked  in the lists above  with  an asterisk.  Those must be  considered to be too high-tech or to push  nature too  hard.   The graph below from that paper. shows the twenty pathways  and the climate mitigation potential as of 2030 for each under three scenarios – (a ) maximum under  constraints of land and supply (b) achievable at cost of  $100/t  CO2  (c) achievable at low cost of $ 10/t CO2

             The  seven   categories on my list don’t pour  neatly into  the twenty  in  the  Griscom et al  paper,   but fit the  four in Hoff’s  article that I thought are closest to  natural processes. Those four fairly well comprehend  the twenty in Griscom et al as follows:

  Griscom et al             Max     Cost eff                                   Hoff                Max     mid-range

Forests (6 subcat)        16.2        7.3                            Afforest/reforest         14        7.5

Ag and grasslands      

 (10 subcat)                  4.8       2.5                             Carbon farming           13        7

Wetlands (4 subcat)      2.7       1.5                             Other vegetation         unkn    unkn

Total                            23.7     11.8                                                                 27        14.5

Table 1.   Maximum potential and cost effective potential (<$100/t) in Griscom et al and,  for the corresponding categories in Hoff,  the top of range and the midpoint of range.  All values are in Pg  CO2/yr = Gt CO2/yr.  Global gross emissions in 2012 (Edgar database), for comparison, were 46.4 Gt CO2-e for big three GHG and 34.9 Gt CO2 for molecular CO2.

            Although Hoff gives more potential for mitigation  to carbon farming than Griscom et al do to their ten subcategories of Ag and grasslands, the two viewpoints agree reasonably on total potential for natural solutions and on potential for forest management.  Is Hoff over-estimating the potential of carbon farming or do  Griscom et al discount it too deeply?  The  answer to that depends on  what  I call “natural soil amendments.”          

            Set aside now   all the  “high tech” methods viz. CCS from combustion , direct capture from air,  ocean fertilization and “rock solution.”  None has the potential to both draw down CO2 and improve soil.  The first three are mightily expensive.  This leaves, from  my list, the following  “natural” solutions:

Low-tech  not using photosynthesis


Low-tech  using direct photosynthesis                      

            Trees  (including bushes, shrubs)

            No-till linked to cover crops

            Short-stay long-rest grazing by ruminants 

            Unmanaged vegetation not forests  or range grasslands

            Natural soil amendments 

                        Traditional composts

                        Mycorrhizal fungi

            Being interested in crop and animal agriculture, I now home in on methods applicable to cropland and grazing land. This means setting aside trees,  though they are in almost everyone’s view the  major  locus  of  biological C-seq anywhere that is at hand  today.  I will also set aside “unmanaged vegetation”  This leaves a smaller suite of methods, the names of which do not correspond neatly  to  the  ten subcategories in Griscom et al  though all fit well  into Hoff’s  “Carbon farming.”

Low-tech  not using photosynthesis


Low-tech direct photosynthesis                                 

            No-till + cover crops

            Short-stay long rest grazing by ruminants                   

            Natural soil amendments 

                        Traditional composts

                        Mycorrhizal fungi



Solution  and scope



cost effective


Tg CO2/yr

   Tg CO2/yr


Avoided grassland conversion 1.7 Mha





Cropland nutrient mgmt  44 Gt N/yr





Biochar  1670 Tg/yr crop residue

0.18 MgC/Mg dm




Grazing optimal on  712   Mha





Grazing legumes on 72 Mha





Grazing improved feed for 1.4 B head





Grazing animal mgmt for 1.4 B head





Conservation Ag  on 382 Mha





Trees in croplands 608 Mha





Improved rice  163 Mha










Table 2 .  The  ten natural solutions proffered under “Ag and Grasslands”  in Griscom et al. with the maximum mitigation potential and the cost-effective mitig. pot. given for each.  The scope (extent) is given for each in area or number head or (biochar) raw feedstock tonnage.  The intensity is given for some.  The value 0.18 Mg  C/Mg dm pertains to biochar.  Mg = metric ton  Tg = million metric tons  Gt = billion metric tons [= Pg ]  Mha = million hectares B head = billion head of cattle  C = carbon   N= nitrogen dm = dry matter


            Biochar on my highlighted list above corresponds to line 3 in table 2.  “No till plus cover crops” corresponds roughly to table 2  line 8 . “Short stay long rest grazing”  aligns roughly with lines 4 and 5, perhaps also with line 1.  Lines 6 and 7 of table 2 don’t  fit my schema; the descriptions look like recommendations for grain finishing of fewer cattle  in smaller spaces where I would call for  more land under  holistic grazing  management.  “Trees in croplands” [and silvopasture, not so designated in Griscom et al]  are sound solutions which  I  haven’t studied enough to have included at this point . Note that   Drawdown   gives “silvopasture” 9th place overall on its lineup  of solutions, on which  “tree intercropping” ranks 17th  place and “conservation agriculture”  16th .

          I  must   move past   biochar .  It  has ardent proponents as a technique for C-seq, especially if the gas generated in the low-oxygen heating can be combined-cycled to drive a machine.  Leaving biochar undisturbed guarantees sequestration  for decades, perhaps centuries.  It is not, however, well suited  for  boosting  carbon in  large areas of  soil.  To make a ton of biochar, more than five tons of feedstock (wood, crop residue) are  required
(inferred by me from  Supplementary Information for Griscom et al  p 9) ,  That ton of particulates has to then  be distributed throughout a large volume  of soil to be of  benefit.  Too much biochar can hurt soil.  Application rates are high; Lehmann et al  write  of 40 tonnes/ha.  In 2010 a review by the International Biochar Initiative gave a range of rates from 5 to  50  tonnes/ha, calling attention to problems in spreading crushed material on soil such as wash-away and blow-off.    I gather that a high proportion of carbon in biochar is recalcitrant, stable for decades or centuries against  oxidation  but  less interactive with soil than particulate carbon and humus carbon. Biochar stands  #72 in Drawdown.

            Eagle et al  (2012) tabulated four studies of  biochar, on three of which Prof. Lehmann was an author.  I could not tell how much of the sequestration potential in any of  these would translate into increased soil carbon elsewhere than at   the burial site.  In  Nature 2007, Lehmann writes that the biochar resulting from pyrolysis of 600 M mt of fast-growing vegetation could when put into soil sequester 1.6 Gt C (= 5.86 Gt CO2) per year.  I could not, however,  find  the land area that would be treated.  Eagle et al record in discussing that paper  an associated potential  seq rate of  19.57 t  CO2/ha/yr [though I don’t know how they came to that figure].   Even at that  high rate, 290 Mha  (716 E +6  acres) would have to  be treated  with  uniform dispersal throughout the topsoil of  those hectares to maximize benefit to soil as well as pure sequestration. 

            Other reports in the same table in Eagle et al have  much lower rates for C-seq..  I’m fuddled, but gather  that  (1)  biochar itself when sealed away can sequester perhaps a fifth of the carbon that was in the feedstock, which means much less C emission than burning that feedstock or letting it decay on soil surface  (2)  biochar in soil adds carbon but  may not add it by stimulating microbial activity, for  which it may be more  housing  than  a nursery (3)  no matter how  biochar is made, it would be difficult and expensive to spread  it into millions of hectares of soil.

            I am sure that modern biochar mixed with something like manure  is an excellent soil amendment for lawns, gardens and trees.  That said,   Griscom et al – and this is significant – do not regard it as a soil amendment for a global scale.  I guess this is because it would  be so  hard to  use on  millions of hectares.

             Griscom et al do not have  a category for what my list calls “natural soil amendments.”  This to me is an understandable but regrettable omission.  “Composting”  stands only  60th position in  Drawdown, below the median.  In my list, “traditional compost” includes manures and vegetal farm waste that have been stockpiled or more properly composted, but not raw manure from storage pits.    Eagle and colleagues did distinguish, among twenty-one  areas of possible intervention “Apply organic material e.g. manure.”   Their report tabulated eight studies of  “land application of organic material, USA.”  The potential for soil C-seq ranged from 0.70 to 3.5 t CO2/ha/yr with a median of 1.85.  The mass of organic material that must be applied to get rates like this, however,  is too much  make the practice  widely useful.  The Marin Carbon Project, for example, applied 70 tonnes/ha  finished compost to get C into soil at rates (per Toensmeir p.390) of 2.1-4.7  t C/ha  into soil.        

            Besides traditional  compost,   however, a little-known  innovation  for creating humus has shown results that could revolutionize soil management and potentially out-downdraw  all the ag and grasslands solutions presented by Griscom  et al.   The  innovation  is composting  that  bio-magnifies populations of desirable pathogen-fighting microbes.  It  reverses the usual ratio (<1) of desirable fungi to  bacteria.  This reversal  be done with or without turning. If  turning is not used, aeration requires a special design of the pile.  A high F:B humus must interact with living plants to sequester  carbon in soil.  I will showcase these highly bioactive preparations after a look at the C-seq potential of  “regenerative” solutions that do not use  them.


            There remain three “natural solutions” on Table 2 that have a counterpart on my list. 


Solution  and scope






Mg C/ha/yr

Tg  CO2/yr

  Tg  CO2/yr


Grazing optimal on   712   Mha





Grazing legumes on   72  Mha





Conservation Ag  on  382 Mha










Table 3.  The three natural solutions in Griscom et al that best fit the term

“Regenerative Agriculture” insofar as it is “carbon farming.” Excerpted from Table 2


Their total maximum potential (see table 3)  is 708 Tg CO2/yr = 0.708 Pg = 0.708 GtCO2/yr, about 15% of the Ag and grasslands total  in table 2.   This is disappointing for advocates  of  “Regenerative Agriculture,”  because these  three are  the  mainstays  of Regen Ag: no till, cover crops and holistic grazing.   Don’t despair.   The potential for optimal grazing may be  seriously underestimated.   Stanley et al reported, for instance, a  C-seq rate of 3.59 Mg C/ha/yr due to adaptive multi-paddock management, while Wang et al estimated  that converting from heavy continuous to multi-paddock grazing led to C-seq of  2 Mg C/ha/yr.  Machmuller et al (2015) reported 3.2 Mg C/acre.   Moreover, at  0.32 t/ha/yr, the estimated potential for “conservation agriculture” is also on the low side.  Toensmeier (p. 390) tabulates twelve figures ranging from 0.1 to  6.3 t C/ha/yr,  median 2

            At last we get to the central point  of this essay:  bioactive fertilizers with a high F:B ratio that enable a  thriving  population of mycorrhizal fungi are reported to promote extraordinary C-seq rates after  comparatively light application.  (see my companion piece “Biologically-Enhanced Agricultural Management”).  This potential has not been acknowledged in any wide-ranging authoritative review such as Griscom et al or Eagle et al  or Garnett et al.  Treating all 150 Mha of  U.S. arable land with 449 kg/ha of one such product –BEAM inoculant –could,  if small field trials are validated,  sequester 1.6 Gt C ( = 5.86 Gt CO2)  nationally/yr.  This represents more than total yearly US emissions of  CO2 and 16% of global emissions.  1.6 Gt  C  is the same figure that Lehmann projects  biochar could sequester.  It would come through BEAM, however, at much lower cost for processing, for transportation and application than  biochar would incur.

            On a global scale, high F:B bioactive fertilizer such as BEAM could bring back world cropland soils from the brink of collapse while sequestering carbon.  I don’t know its impact on grasslands but expect that it would work  well in conjunction with holistically  managed grazing to sequester C and make the soil more resilient and productive.


Graph 2. Global potential for C-seq in soil =  CO2 draw-down in 2030 for four categories of “natural solutions” in Griscom et al and for BEAM treatment of 1000 Mha (~2/3 of world’s) arable land.




Eagle, A., L. Olander, L.R. Henry, K. Haugen-Kozyra, N. Millar, and G.P. Robertson. 2012. Greenhouse Gas Mitigation Potential of Agricultural Land Management in the United States: A Synthesis of the Literature. Report NI R 10-04, Third Edition. Durham, NC: Nicholas Institute for Environmental Policy Solutions, Duke University

Garnett Tara et al “Grazed and Confused?” Food Climate Research Network 2017

Griscom BW, Adams J, Ellis PW et al Natural Climate Solutions pnas 114(44) 11645-11650 2017

Lehmann J . A Handful of Carbon Nature vol 447 10 May 2007 : 143-144

Lehmann, J., Gaunt, J. & Rondon, M. BIO-CHAR SEQUESTRATION IN TERRESTRIAL ECOSYSTEMS – A REVIEW  Mitig Adapt Strat Glob Change (2006) 11: 403

Drawdown The Most Comprehensive Plan Ever Proposed to Reverse Global Warming Edited by Paul Hawken.  New York, Penguin 2017

 Machmuller MB, Kramer MG, Cyles TK et al  Emerging land use practices rapidly increase soil organic matter. Nature Communications 30 April 2015  | DOI: 10.1038/ncomms 7995

 Roberts KG, Gloy BA, Joseph S et al [Lehmann senior author] Life Cycle Assessment of Biochar … Environ Sci Technol 44 (2) p. 827-833 2010

 Stanley P,  Rowntree JE et al  Impacts of soil carbon sequestration on life cycle greenhouse gas emissions in Midwestern USA beef finishing systems.  Agricultural Systems 162: 249-258 2018

 Want T, Teague RW et al GHG Mitigation Potential of Different Grazing Strategies in the United States Southern Great Plains. Sustainability 7, 13500-13521 2015,

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