By Jakub Olewski

When this article was written in 2012, Jakub was in the final year of his PhD research at the Scottish Agricultural College and University of Aberdeen. He was working on the synchrony between nitrogen supply from green manures, and crop demand.

Soil organic matter (SOM) comprises all the organic compounds found in living and dead matter in the soil. Chemically they range from relatively simple molecules such as amino acids, carboxylic acids and carbohydrates, through to more complex lipids, peptides and polysaccharides, to complex proteins and polymers of barely known composition such as humic acids. SOM is often referred to as soil organic carbon (SOC) because C is the core of each organic molecule and on average SOM contains 58% C. Soils are estimated to contain 1500 Pg (1 petagram = 1015 gram) C up to 1 m depth, about twice the amount of C in the atmosphere, which highlights their importance in climate change mitigation efforts.

Vegan organic growers and farmers do not need to be convinced of the importance of keeping levels of SOM high. SOM is a huge reservoir of plant nutrients, especially nitrogen (N), hence it needs to be continually replenished, protected and even gradually built up, if possible, to increase the potential of the soil to provide steady high yields (other nutrients are sourced mostly from minerals present in the soil, nevertheless SOM plays an important role in their turnover and availability to plants).

SOM, living soil organisms and soil structure are entwined and affect one another in many different ways through various physicochemical mechanisms and feedback loops. Starting with any of the elements will inevitably lead us to the others because they cannot function separately. Soil structure, like the structure of aboveground ecosystems, creates a variety of microhabitats for a tremendous variety of soil creatures that require different types of ‘homes’, ‘food’ and ‘infrastructure’.

Soil structure matters for soil organic matter

It is not only soil aggregation that makes soil more than just a mix of clay, silt and sand with a pinch of organic material, but aggregates in themselves are not such a random mix. There is an aggregate hierarchy: macroaggregates, microaggregates and primary particles.

Macroaggregates are over 250 µm in diameter and are built of particles 20-250 µm in diameter (1 µm = 1 micrometre = the size of a typical small bacterium). They are held together by temporary binding agents: roots and fungal hyphae which typically last a growing season or longer in perennial systems. Macroaggregates are not resistant to rapid wetting and can be destroyed by agricultural practices.

There are two main groups of fungi living in soil that, together with roots, stabilise macroaggregates: saprotrophic fungi and arbuscular mycorrhiza (AM fungi). The former feed on dead organic matter, mostly of plant origin, the latter live in symbiosis with plants and form a structure extending from their roots to the surrounding soil. AM fungi help plants to obtain nutrients, mostly phosphorus, sulphur and N, and in return they receive organic C compounds, mostly carbohydrates and lipids; they live in symbiosis with most plants except the goosefoot and cabbage families. Hyphae of AM fungi can stabilise aggregates longer than hyphae of saprotrophic fungi because they are typically stronger structurally, and can last for a few months after the host plant dies. Tillage is damaging to both groups of fungi.

Microaggregates (particles smaller than 250 µm) are formed within macroaggregates and ultimately become bonded by persistent organic materials such as humic material, other organic polymers of biological origin, and mineral compounds. If given sufficient time during their formation they can become very stable due to the additive effect of many binding agents and simply because they are small (see illustration). Organic matter in these aggregates is usually intimately associated with clay or surrounded by clay particles that protect it against decomposition; hence microaggregates play a crucial role in the long-term accumulation of organic matter.

It’s about equilibrium

SOM content of soils is determined by the equilibrium between inputs of organic materials and loss by respiration, leaching and erosion. All soil C ultimately returns to the atmosphere, mostly as carbon dioxide (CO2). It just has different residence times depending on its chemical composition and placement within the soil structure. SOM decomposition is vital because it closes the C cycle by releasing CO2 assimilated by plants; it also releases plant nutrients from organic compounds, for example N from proteins; and it provides energy for soil microorganisms and fauna. However, it needs to be balanced with inputs of new organic matter otherwise SOM will gradually become depleted and too small to effectively perform its function. Here are the main groups of practices that can promote C sequestration in soils:

No-till soil management and modification of tillage practices (conservation tillage) can lead to C sequestration. C losses through respiration are reduced by not promoting increased SOM oxidation during tillage and the physical destruction of aggregates. Conservation tillage may not be enough in some circumstances and additions of organic matter in the form of composts or crop residues may be needed; sometimes it may be too dry and too hot for organic matter to accumulate. There is also much uncertainty with regard to the effects of no-till on nitrous oxide (N2O) emissions. Some studies show that the emissions may be higher; however, there are considerable C gains from reduced fuel needs for cultivation.

Another way of increasing SOM content is the incorporation of crop residues and the modification of rotations so that they contain a high proportion of ley and plant species that sequester more C (especially below ground, in the roots). The inclusion of grasses is particularly important due to their deep fine roots that promote the creation of macroaggregates, by holding them together, and by providing food for saprotrophic fungi, as well as supporting AM fungi living in symbiosis with the grass roots. It should also be mentioned that no-till soil management, additions of fresh plant materials, and rotations that incorporate leys, are all conducive to greater earthworm populations whose profound macro- and microaggregate forming and stabilising role cannot be underestimated.

Conversion of arable land to woodland or an agroforestry system can also be an effective way to increase SOM content. However, it has to be taken into account that the increase is usually in the labile (easily decomposable, unprotected within the soil structure) C fraction first and that it can easily be lost if the management is not maintained. Losses of SOM can happen much faster in warmer and drier climates than in temperate zones, but this also means that the fertility benefits of organic matter additions are more immediate due to the quick release of nutrients during decomposition. Conversion of arable land to grassland can also be beneficial in terms of SOC stocks because of high plant productivity and lack of cultivation. At the same time, trade-offs in the form of emissions of non-CO2 gases such as methane and N2O have to be taken into account. From our stockfree point of view the development of grassland systems should not of course be used for animal production. It is the demand for animal products that creates the need for typical pastures and grasslands in the first place. Yet alternative grassland-like systems could be used in many productive ways, for example for biogas, biochar and green manure production as well as extensive orchards, to name but a few options already well known to GGI readers.

Last but not least, on the topic of C sequestration in soil, biochar is currently gaining much attention. Biochar is organic matter that has been converted by pyrolysis (heating in oxygen-poor atmosphere) to a form that is highly resistant to decomposition. Biochar can be used as a soil amendment potentially providing many other benefits apart from C sequestration. There are also potential negative effects and sustainability issues that need to be addressed before large- scale deployment of biochar is advocated. Interested readers can find up-to-date news and research reports on the International Biochar Initiative website: www.biochar-international.org.

Theoretical and practical limits to carbon sequestration in the soil

Increase in SOM has its limits and the law of diminishing returns applies: after raising it to a high content it is more difficult and costly to raise it further. If soil is disturbed, for example when land use is changed, then after some time a new equilibrium between the organic matter supply and loss will be reached and no further increase in SOM will be observed; however, SOM content could drop quickly if practices are not maintained. Each soil in the given climatic conditions has got its own maximum C concentration which is regulated by silt and clay content.

We also need to consider potential conflicts between methods of increasing SOM content and other demands, for example an increase in food production. In some cases, especially in the tropics, these aims converge because the fertility of soils is greatly limited by a low SOM content. In general, however, the immediate expanding demand for food and biomass for purposes other than maintenance of SOM may jeopardise efforts to increase organic matter inputs to soil.

Here veganism comes into play: we may argue about the exact area of land and crops needed per capita if the world was vegan, but it is always less than that needed for typical omnivore diets, as argued in Growing Green International magazine and elsewhere. The very real conflicts between increasing food demand and the need to sequester more C in soil can easily and very cost effectively be avoided by a dietary change. In turn, the land not needed for arable crops could be converted to woodland, agroforestry and other perennial systems conducive to greater SOM accumulation, whilst a greater proportion of non-food and non-fodder biomass could be used to replenish SOM stocks.

Author’s notes

For a review of research and current knowledge on soil structure and SOM nexus, see:

Six, J., Bossuyt, H., Degryze, S. and Denef, K. (2004) A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics. Soil and Tillage Research 79, 7-31.

For fascinating photos of microscopic soil life and structure see www.microped.uni-bremen.de/SEM_index.htm (University of Bremen)

This article appeared in Growing Green International magazine Num 29 (Summer 2012), p10.