2026-05-05
Many things wrong about using soil capture of carbon to excuse ongoing emissions of greenhouse gases
By Chris Johansen, Green Issue Co-editor
In 2023 I published a Green Issue article entitled “Upset about Offsets” concluding that various projects claiming to sequester carbon are being used as an excuse to continue emitting greenhouse gases. Here I would like to dig deeper into the schemes that claim credit for sequestering carbon in the soil through changes in land management practices.
The Australian Government provides incentives to sequester carbon called Australian Carbon Credit Units (ACCUs). One ACCU represents one tonne of carbon dioxide equivalent (tCO2-e) that would have otherwise been released into the atmosphere. Thus, if projects can demonstrate that they can increase soil carbon levels, by changes in land management practices, they can earn ACCUs, which can be sold to fossil fuel carbon emitters to offset their emissions. A very convenient way for emitters to avoid reducing their own emissions.
The methodology of accruing ACCUs through changes in land management essentially involves sampling the soil of a designated area at the beginning and end of a period during which the changed management is implemented. These soil samples are analyzed for soil carbon and thus the carbon stock, the total carbon in a designated amount of topsoil (usually the top 30 cm of the soil profile), is calculated. Of course, the extra emissions involved in implementing that management change, such as nitrous oxide through extra additions of nitrogenous fertilizer, need to be deducted from the amount of carbon calculated as sequestered.
Eligible changed agricultural land management strategies could include application of fertilizers or amendments to alleviate nutrient deficiencies, improved pasture establishment or rejuvenation, stubble retention, minimum tillage, using legumes or cover crops, introduction of irrigation, etc.
However, there are a number of problems with the measurements and their interpretation in calculations of soil carbon stock. The first problem is in getting a representative sampling of soil carbon for a given area of land. Soil organic matter, and hence soil carbon, is notoriously spatially variable, depending on where a clump of vegetation fell or dung was deposited.
Having been involved in sampling soils for carbon and various nutrients in several countries for over half a century, I have some worries about the soil sampling methods recommended. My experience has been with relatively small plots, ranging from 25 to 500 m2. These plots have been on relatively uniform soil areas, such as soils of the Indo-Gangetic Plain. Even for small plots on apparently spatially uniform soil, I wouldn’t be happy with any less than 10 cores per plot. Of course, cores can be combined for analysis – e.g. 4 cores per analyzed sample. Even then variability remains high making it difficult to obtain statistically significant differences between plots with different treatments.
To make it worthwhile to chase ACCUs, large carbon estimation areas comprising many hectares are needed. For me, this would present a nightmare of how to obtain a representative estimate of soil carbon stock. The officially recommended methodology concedes that if soil characteristics vary across a carbon estimation area then it can be stratified according to soil type, and separate sampling be conducted for each stratum. But they suggest 9 soil cores to be composited into 3 composite samples for analysis for each stratum, and the results averaged across strata. For me, without going into the statistical details, this would fall far short of being able to obtain a reasonable estimate of carbon stock for a multi-hectare area. Certainly inadequate to detect a significant change over time.
But, assuming that realistic estimates of carbon stock over a large area can be made, there are complications in interpreting what those data mean. Soil organic matter, and thus soil carbon, is ephemeral, changing with varying ambient conditions of temperature and rainfall. High temperatures can encourage breakdown of soil organic matter and loss of carbon. Periods of high rainfall can stimulate vegetative growth and thus increase soil organic matter when it decomposes in the soil. And Australia is notorious for its intermittent periods of “droughts and flooding rains”.
A recent study with different grazing management practices has shown that changes in soil carbon over an 11 year period could be better attributed to changes in weather over that period rather than particular management practices designed to increase soil carbon. This complicates interpretation of whether an increase in soil carbon (indeed assuming it can be accurately measured!) is actually due to the changed management conditions or weather patterns.
To really quantify whether a changed management practice can increase soil carbon, a control plot is needed. That is, one where the original management practice is continued, something that most farmers would not be willing to do if they were convinced about moving to an improved management practice.
But even if a genuine increase in soil carbon due to management change over time can be reasonably measured, it can subsequently be lost depending on future weather conditions. Another general problem with offsets being subject to negation, like a tree plantation used for offsetting being lost in a wildfire.
Furthermore, even if the above-mentioned hurdles can be overcome, it remains that the accumulated ACCUs are still available as an excuse for greenhouse gas emitters to carry on emitting, rather than concentrating on reducing their emissions. So, sorry, soil carbon ACCUs are not for me (even though being an aficionado of soil carbon since the 1960s).
Nevertheless, soil carbon remains a crucial component of soil fertility and most agricultural soils would benefit from its increase. Thus, I’m all for comprehensive measurements of soil carbon, but within the context of improving agricultural productivity rather than providing offsets for polluters. Reducing greenhouse gas emissions should be considered separately from sequestration of soil carbon.
Header photo: Dry Australian farm land. Credit: Michael Coghlan, Adelaide. Creative Commons Attribution-Share Alike 2.0 Generic
[Opinions expressed are those of the author and not official policy of Greens WA]