Dossier Soil: carbon sequestration

Besides the ocean, soils are the largest carbon storehouse. With proper management, agricultural soils can even store significantly more carbon than they currently do. Capturing CO2 in the form of soil organic matter (carbon) not only contributes to the fight against climate change, this carbon also plays a key role in the proper functioning and fertility of the soil. Yet carbon storage in agricultural soils is not a simple story.

Hole in a field with a measuring tape

What does ILVO do?

  • Compost lying on a fallow field
    Using field trials, ILVO is investigating the carbon contribution of crops, fertilization, soil amendments, and tillage

Part 1: What and why

From CO2 to soil organic carbon

Plants need CO2 to grow. Through photosynthesis they convert it into glucose and into oxygen. Part of the carbon (C) they extract from the atmosphere in this way ends up in the soil directly via the roots, via substances secreted by roots (exudates), via crop residues left behind on the field or indirectly via external organic material such as animal manure, compost, wood chips, etc. Through soil life, this fresh organic material is converted partly into nutrients for the plants (mineralization) and partly into stable 'soil organic carbon' (humification). The fraction of carbon that remains is not the same for all organic materials but depends on the quality of the applied material ('effective organic carbon').

Carbon balance

The stable carbon, in turn, is subject to decomposition (estimated on average at 2% per year). Of this, part benefits the plant in the form of nutrients and part is lost in the form of CO2. The amount of organic matter degraded annually depends on several factors. Carbon storage can only occur if more carbon is supplied than is decomposed. In this way, the soil can act as a "carbon sponge" and thus a buffer against climate change. After all, if you can store 1 ton of stable carbon in the soil, you take 3.7 tons of CO2 out of the air.

Carbon balance

There is a limit to the amount of carbon in the soil

Carbon sequestration in soil is not uniform but is often highest immediately after a change in land use or land management. Subsequently, the soil evolves to a new equilibrium that is reached after a period of 20-100 years, after which the carbon stock remains more or less constant. The final carbon accumulation in soil depends on a number of factors:

  1. the capacity of a soil to sequester carbon (determined mainly by its clay content)
  2. climate: soil temperature and moisture content determine the rate of mineralizationhet klimaat: bodemtemperatuur en –vochtgehalte bepalen de mineralisatiesnelheid
  3. the quality of carbon added to the soilde kwaliteit van de toegevoegde koolstof aan de bodem
  4. the balance between carbon input and carbon lost from the soil through respirationde balans tussen koolstofinput en de hoeveelheid koolstof die verloren gaat uit de bodem door respiratie
  5. the initial carbon stockde initiële koolstofstock
  6. the management of the field or grassland

The role of carbon in the soil

In addition to being good for the climate, a soil rich in soil organic matter is good for the farmer. After all, a soil containing sufficient carbon will generally have:

  • better soil structure and consequently more resistance to compaction, erosion and compaction
  • improved soil fertility. Organic matter acts as a buffer against pH fluctuations and acts as a source of nutrients through mineralization
  • increased water permeability resulting in less runoff, a reduced risk of flooding and better replenishment of surface and groundwater
  • a higher water holding capacity which leaves more water available to the plant during the growing season which can prove crucial in periods of drought a more extensive and active soil food web which in turn contributes to a better soil structure, fertility and disease resistance
Earthworms are part of the complex soil food web

In other words, a soil rich in soil organic matter is better protected against the effects of climate change and will generate more stable crop yields, even in difficult conditions. Carbon storage is thus not only an important mitigation measure1 but also plays its role in climate adaptation2.

1Reducing the impact of agriculture on the climate (mitigation)

2Adaptations in agriculture to the already present effects of climate change

Impact of land use on carbon stocks

Grassland sequesters more carbon than arable land

The type of land use plays an important role in the amount of carbon present in a soil. It is well known that more carbon can be stored under grassland than under cropland. The explanation lies in the combination of a constant supply of organic matter in the form of roots, root exudates and grass residues and the absence of intensive tillage, which reduces the rate of decomposition of organic matter. Several French and Belgian studies have even shown that as much carbon can be stored in the soil under grassland as under forest. Therefore, the high carbon storage potential combined with a considerable acreage means that grassland in Flanders can play an important role in the fight against climate change.

French and Belgian studies have even shown that soil under grassland can sequester as much carbon as under a forest.

Carbon loss happens faster than carbon build-up

During the conversion of grassland to cropland, a significant part of the accumulated carbon is lost again. Based on European studies in which the carbon decomposition during the conversion of grassland to arable land was monitored at plot level, we can conclude that the decomposition is on average twice as fast as the build-up. The highest losses are observed in the first years after tillage and the amount of carbon finally lost will largely depend on the age of the grassland (initial carbon stock) and the soil texture (highest losses were observed on soils with low clay content).

Carbon loss – such as from destroying grassland – happens about twice as fast as carbon build-up.

This shows that maintaining carbon in soil is critical. Globally, there is three times more carbon in soils than in the atmosphere and four times more than in vegetation. Identifying and protecting carbon hotspots (e.g. old grasslands, soils with peat in the subsoil) can therefore play an important role in protecting our climate.

Part 2: how to store carbon in the soil

Various management measures can be used to increase the carbon stock under agricultural soils. This applies to both grassland and arable land.

Grassland

To optimize carbon sequestration under grasslands

  • choose for a medium-intensive grassland management. This results in good stubble and root development. Avoid too frequent renewal and mowing, as well as excessive grazing.
  • graze instead of only mowing. Grazing pastures possess a more extensive root system and a better-developed stubble which contributes to carbon buildup
  • leave the grassland as long as possible. The longer it is left on the same plot, the more carbon is stored underneath (until an equilibrium is reached)

To maximize the carbon stock under agricultural soils, the most efficient measure is to maintain existing grasslands and thus preserve existing carbon stocks.

Croplands

Compost or farmyard manure?

Increasing the organic carbon content on arable land can be done in several ways. Regular application of organic matter (e.g. crop residues or organic fertilizer) is the most logical method to increase the organic carbon content in a relatively short period of time. The increase in organic carbon content depends on the quality and stability of the administered organic material. For example, both compost and farmyard manure possess a higher carbon storage potential than slurry for example. Indeed, both products are rich in stable organic matter and, if applied at the same (carbon) dose in similar conditions (soil texture, climate, etc.), the carbon build-up in the soil will be similar. It should be noted that compost and farmyard manure are rich in nutrients and that the application of compost and farmyard manure should be done judiciously to limit additional nitrogen and phosphorus leaching.

Compost. This one is a mix of farmyard manure, green waste from nature management, and wood chips

Crop choice

Through a targeted crop choice and/or crop rotation, the supply of organic matter can be increased. Examples are perennial crops such as temporary grassland, deep-rooting crops such as alfalfa and red clover, and crops that leave a lot of structure-rich crop residues such as cereals. Within the group of crop residues, straw appears to have the greatest carbon storage potential. Increasing the proportion of straw left in the field after harvest can stimulate soil carbon accumulation but would immediately mean less straw is available for manure production. Also, the incorporation of straw remains an economic issue. For the farmer, the consideration of incorporation or sale is a choice between short term yield or a longer term investment in organic matter (soil quality).

Red clover - grass

Green cover crops

Green cover crops also add an amount of organic matter to the soil, and seeding them, where possible, is important to maintain at least the soil carbon content. The effect of a green cover crop depends greatly on biomass yield. A short growing period or reduced growth due to poor soil and weather conditions results in limited biomass yield and consequently a low amount of carbon added to the soil.

Tillage

In addition to adding fresh organic matter to the soil, less intensive tillage (e.g. no-till practices) of the soil is often put forward as a potential measure to increase soil carbon stock. However, several Flemish studies have shown that in our situation, the application of not-till usually results in a redistribution of carbon in the topsoil (0-30cm; more carbon in the 0-10cm) but not in higher carbon stocks in the soil (as compared to ploughing).

More info about how various farm management strategies affect the soil: KnowSoil

Agroforestry

Research in Flanders has shown that combining trees and/or shrubs with production crops on the same plot increases soil carbon content. Leaf fall, branch fall and root decomposition are put forward as the main explanations. Also wood edges and hedges at the edge of fields can contribute, especially if the wood chips from these small landscape elements are used, either as raw material in composting or by direct application in the soil.

More information about agroforestry

Rewetting

To make intensive agriculture possible many peat lands were drained in the past, causing the peat to break down, resulting in high CO2 emissions. By re-wetting these peat lands the decomposition is stopped and less CO2 will be emitted. An increased water level does require a different land use (wet cultivation or paludiculture). Crops that can be cultivated are, for example, bulrush, peat moss, duckweed, cranberries, reeds, willows and wild rice. Also this biomass harvested in these wetlands can potentially be used to sequester additional carbon, for example by using this biomass in the production of compost.

Part 3: the situation in Flanders

Monitoring the evolution in carbon stocks

Through the LULUCF (Land Use, Land use Change and Forestry ) regulation, the European Union wants to intervene in emissions induced by human activities and/or storage in carbon stocks. In doing so, it puts forward the "no-debit rule" for the period 2021-2030. Specifically, each member state must do everything possible so that existing carbon stocks are at least preserved at the end of that period. The figures currently used for LULUCF reporting to determine soil carbon (and its annual evolution) are based on historical trends and literature. These values do not sufficiently reflect reality. The figures need to be refined so that:

  • the real storage/emission for a certain land-use category (e.g. cropland, grassland) is better reflected in the emission inventory (currently a fixed, historical factor is used). These figures should also allow for an accurate estimation of the carbon stocks in Flemish agricultural soils.
  • the impact of modified soil management practices in agricultural soils and/or land use changes can be better estimated and better reflected in reporting.

In order to meet this objective, the Flemish Government aspires to start up a systematic monitoring of changes in organic carbon stocks in Flemish soils. To keep the start-up of a soil carbon monitoring network manageable and cost efficient, the present variability and number of sampling points that will be required to detect a certain change in organic carbon need to be estimated in advance. In a joint project of Ghent University, INBO and ILVO, the number of sampling locations was optimized on the basis of the observed variation from earlier sampling, and a methodology was developed for the systematic monitoring of carbon stocks in Flemish (agricultural) soils over a period of 10 years. ILVO is also working on mapping the carbon content of the top layer of cropland using satellite images.

Stimulating carbon build-up within Flemish agriculture

Although the majority of farmers are convinced of the importance of having sufficient organic carbon in the soil, they indicate that this is often not simple to do. Not only is building up organic matter a slow process, it requires a great deal of effort on the part of farmers and also involves costs that do not always translate immediately into yield increases. So the question is how to encourage farmers to maintain or increase the carbon stocks in their soils. In order to design a good policy, there is a need for insights into the relative importance of soil organic matter in the climate story, the most appropriate agronomic measures for (increasing) carbon storage under Flemish agricultural soils and how to stimulate its application at farm and Flemish level.

Mapping the potential

In order to map the carbon storage potential of various agricultural practices and/or land uses, existing data on common and relatively new measures (e.g. agroforestry, leguminous crops, green cover crops) will be brought together within EJP SOIL/Carboseq. These can then serve as input to refine and extend the (existing) C-models in Flanders. Based on all the collected insights and data it can be estimated what potential there still is for carbon storage under the current cropland and grassland in Flanders.

The financial picture

To maintain current carbon stocks and certainly to increase them substantially, compensation mechanisms are needed. In other words, carbon storage in the soil offers new opportunities for the Flemish farmer to contribute positively to climate change and to be compensated for it. ILVO therefore researches which measures are the most financially and technically feasible in the operational management, which financial instruments (private or public) can be used to stimulate and compensate farmers and which support exists for this, and how the impact of the measures and the evolution of carbon stocks can be monitored.

See also

In the media 31/03/2022

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