What to consider when increasing soil carbon stocks

Soil organic matter underpins soil health, providing numerous benefits to its physical and chemical properties. But what is it, and how can farmers effectively increase levels to get the most from their land?

Organic matter is plant and animal tissue in various stages of decomposition, a large proportion of which is dead micro-organisms.

Microbial populations and the availability of carbon greatly affect the functions, benefits and ecosystem services that soils provide.

A number of techniques can be used to improve soil carbon stocks, such as cover cropping, applying organic manures and using root exudates.

But how does the organic material applied become the stable soil carbon that many of us talk about, and what can be done to help promote this?

University of Reading researcher Tom Sizmur tells us more.

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The currency of soils

“Organic matter and soil carbon are effectively the currencies of soils,” explains Dr Sizmur.

Soil micro-organisms use carbon to build biomass and perform a range of key soil functions – including water and nutrient retention, improving soil structure, stabilising aggregates or water infiltration – which bring a number of benefits to crops.

“Up until 10 years ago, soil scientists believed organic matter entered soils and slowly decomposed. Any plant material that wasn’t fully decomposed then became stable soil carbon,” Dr Sizmur says.

“However, we now have a far greater understanding of what makes stable soil organic matter.”

First, living micro-organisms begin to shred up and decompose organic material such as crop residues, manure and straw.

They release extracellular enzymes to break organic matter into a dissolvable form that they can absorb into their tissues.

Stable carbon

Once absorbed, the microbes use carbon derived from the organic matter for one of two things:

  1.  They release it from the system as carbon dioxide via the respiration process. This carbon does not become stable soil organic matter.
  2.  They use carbon to gain biomass and, when these microbes die, a process known as microbial necromass, a large proportion of this becomes stable organic matter.

This stable carbon is protected in the soil both physically and chemically. Soil aggregates provide physical protection, which is why reducing tillage reduces the release of carbon because the aggregates are not disturbed by mechanical cultivations.

Carbon is also protected through chemical sorption, which binds organic matter to the soil surface.

This is why building carbon levels in clay is significantly easier than sandy soils, as small clay particles have a larger overall surface area to which dead microbes can bind.

Each of these natural soil processes can be manipulated, some more easily than others, says Dr Sizmur.

“Not much can be done about chemical sorption, as this is defined by soil type. But what is key is manipulating the amount of carbon used to generate microbial biomass rather than being released as carbon dioxide.”

Cover crop trial in a field

The cover crop trial at the University of Reading © Tom Sizmur

Carbon use efficiency

Carbon use efficiency (CUE) is a measure of the carbon consumed by microorganisms that is used to make biomass.

All microbes have varying CUEs, as a result of building biomass at different rates. It is expressed as a percentage and is calculated as

carbon used to make biomass divided by total carbon consumed.

A microbial population with a high CUE builds biomass more efficiently and releases less carbon dioxide.

Finding ways to encourage microbes to build more biomass will, therefore, help to build stable soil carbon.

What influences soil CUE?

Soils have a large range of CUEs, far greater than most other ecosystems, explains Dr Sizmur.

This is because many factors can affect how easy it is for micro-organisms to access carbon. It all depends how decomposable the material is and how much energy is required to break it down.

This is influenced by:

  • Substrate quality
  • Nutrient availability (to aid the breakdown process)
  • Microbial traits.

When it comes to substrate quality, larger, more recalcitrant molecules such as those found in straw or woody materials have a lower CUE.

This is because a lot of energy is used by the microbes to break down the lignin present in order to acquire carbon. Efficiency is reduced further in the presence of low soil available nitrogen.

On the other hand, simple and low-molecular-weight compounds such as simple sugars have a higher CUE.

Therefore roots and green biomass, such as those from cover cropping, have a higher CUE due to their N components, sugars and root exudates which are more readily available to soil microbes.

How can we use cover crops to build soil carbon?

Cover crops can increase the carbon use efficiency of soils, so more carbon enters the soil to become microbial biomass and ultimately stable carbon.

Analysis of a number of published studies investigating the impact of cover crops and soil organic carbon levels discovered that, on average, cover cropping can increase soil carbon stocks by 12% when compared with a control.

When breaking down the data further, University of Reading researcher Tom Sizmur found that a continuous cover crop (providing continuous cover throughout the year) increased organic matter the greatest (36%), while an overwinter cover crop increased organic levels by 8%, followed by a summer catch crop by 7%.

Crop biomass was key when it came to increasing carbon levels, with higher biomass crops resulting in larger increases in soil carbon.

The method of cover crop establishment also played a significant part, with greater jumps in organic matter found in no-till systems (16%), whereas reduced tillage and conventional establishment both increased carbon by 9%.

Cover mixes

A recent study by the university into the effects of cover crop species on carbon levels suggests a mix can promote stable organic matter levels to a greater extent than a single species.

The theory is that with greater botanical cover crop residues entering the soil and becoming available to microorganisms, a greater proportion becomes microbial biomass.

Buckwheat, berseem clover, radish and sunflower were put to the test in a pot trial where microbial biomass levels were recorded.

The mixed crop, with 25% of each species, generated the greatest microbial biomass, followed by radish, then clover, sunflower and buckwheat.

The same species were used in a field trial assessing the yield of a following winter wheat crop. A similar pattern emerged, with the mix offering the greatest wheat yield increase, followed by the radish and clover.

However, following the sunflower and buckwheat cover crops, the yield of the winter wheat fell compared with the control, but there was likely to be no statistical significance between this.

“The key outcome is that, on average, the wheat yield after individual cover crops was very similar to the control, but the wheat yield after the mixture was greater than the control,” Dr Sizmur says.

The University of Reading project was funded by BBSRC in collaboration with Kings Crops, a division of Frontier Agriculture.

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