What effect does planting biomass crops have on soil carbon?

Key messages:

• Biomass crops accumulate relatively large quantities of above and below ground biomass which is made of carbon sequestered from the atmosphere.
• The frequent harvest of above ground biomass enhances vigorous shoot regeneration and root turnover, which may enhance soil carbon stock.

• Biomass crops require no tillage during the lifetime of the plant following establishment, which can prevent soil carbon loss via disturbance when compared with annual crop soils which are regularly ploughed.

Introduction

Biomass crops play a key role in the UK’s Net Zero Strategy which outlines the country’s plan to achieve net zero emissions by 2050. Biomass crops have the potential to contribute to this target by producing renewable energy, reducing greenhouse gas emissions, and supporting soil organic carbon (SOC) sequestration; the long-term storage of carbon (C) in soil. Greater levels of SOC provide the benefits of enhanced soil quality, a more resilient supply of water, improved quality of water, improved biodiversity, and reduced atmospheric CO₂. Unfortunately, much of our soil is depleted of SOC and degraded, largely affected by land use, soil management, and farming systems. Intensive agriculture has caused UK’s arable soils to lose about 40 – 60% carbon. There is the need to restore soil carbon and increase SOC stock in the soil by adopting management practices that enhance soil carbon concentration in the soil. 

Perennial biomass crops compared to annual crops have a higher SOC accumulation potential, and therefore planting on arable, or degraded, land can enhance SOC stock. However, because of the potential to lose SOC during establishment (i.e. ploughing) care should be taken to avoid sites with high SOC stocks (e.g. forests). There is some uncertainty in land use change from agricultural grasslands due to differences in pasture management practices, and initial SOC stocks. Conversions from agricultural grasslands generally show a neutral to negative change in SOC, with the potential for losses to be recovered over the perennial biomass crop lifetime.

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Fundamentals of soil carbon

Soils contain 2-3 times more carbon than the atmosphere, and a slight increase in the stocks could play a crucial role in lowering atmospheric CO₂. A challenge in the management of soil carbon is to conserve existing soil carbon stocks and remove carbon from the atmosphere by adding to stocks retained in the soil. Soil carbon sequestration is one way of transferring atmospheric CO₂  into soil and increasing the pool of carbon stored in the soil, a process primarily mediated through plants.

Soil carbon is the solid carbon stored in soils. There are inorganic and organic soil carbon forms. Inorganic soil carbon is predominantly found in carbonate minerals derived from weathering of rocks and minerals, whereas soil organic carbon (SOC) is the carbon found in soil organic matter (SOM). SOM is organic material (e.g., plant tissues, microorganisms, animals) in various stages of decomposition in the soil. SOC is an important component of the functioning of the ecosystem. It helps to improve soil structure and provides resilience to physical degradation. This reduces risks of soil erosion and nutrient leaching from the soil. Keeping SOC in the soil helps to reduce emissions of atmospheric CO₂, increases microbial activity, improves soil aeration and increases water storage and availability to plants. 

Figure 1. shows a simplified diagram on how carbon is transferred between the atmosphere, plants, and soils of our ecosystems. SOC is formed from the interaction of ecosystem processes such as photosynthesis, respiration and decomposition of SOM. During photosynthesis, light energy is captured by plants and used to convert CO₂ absorbed by the plants from the air, and water from the soil, to build carbohydrates which act as a source of food for plant growth. Atmospheric carbon fixed in the plant leaves and branches are transferred down through the roots to the soil. Plants exude carbon through their roots, feeding soil microorganisms, which in turn decompose organic residues such as fallen leaves, branches, and roots in the soil, releasing carbon into the soil. During this process, the soil microorganisms release CO₂ into the atmosphere through respiration. Thus, there is an inflow of CO₂ into the soil (carbon inflow) and CO₂ moving out of the soil (carbon outflow). Soil can therefore act as a source and sink for atmospheric CO₂. The amount of carbon present in soil depends largely on the rate of decomposition of soil organic carbon to CO₂ by microorganisms and the rate of SOM input into the soil. GreaterSOM input and longer residence times locks more carbon out of the atmosphere.

Fig. 1 A simplified diagram on carbon transfer

• During photosynthesis, light energy is captured by plants and used to convert CO₂ absorbed by the plants from the air, and water from the soil, to build carbohydrates which acts as a source of food for plant growth.
• Atmospheric carbon fixed in the plant leaves and branches are transferred down through the roots to the soil.
• Plants exude carbon through their roots to feed soil microorganisms.
• The microorganism in the soil decomposes organic residue such as fallen leaves, branches and roots in the soil releasing carbon deep into the soil. During this process, the soil microorganisms release CO₂ into the atmosphere through respiration.

Thus, there is an inflow of CO₂ into the soil (carbon inflow) and CO₂ moving out of the soil (carbon outflow). Soil therefore acts as a source or as a sink for atmospheric CO2. The amount of carbon present in soil depends largely on the rate of decomposition of soil organic carbon to CO₂ by microorganisms and the rate of SOM input into the soil. The more SOM input, the greater the residence time, and the more carbon that is locked out of the atmosphere.

Current state of soil carbon in UK soils

A survey of  soil carbon stock estimates and sequestration potentials from 20 regions globally reported average SOC densities of 133 t C ha−1 and 164 t C ha−1 in England and Wales, respectively. According to the 2019 UK state of the environment report, UK soils store about 10 billion tons of carbon, equivalent to around 80 years of the country’s annual greenhouse gas emissions. Soil degradation leads to increased carbon emissions and contributes to climate change. 

Restoring soil carbon is essential for improving soil quality, which is necessary for sustaining and improving food production, increasing supply and quality of water, enhancing biodiversity, and reducing atmospheric CO₂ , among other benefits. Restoring soil carbon in degraded lands requires increasing SOC concentration in the soil by adopting best management practices. One option could be to plant perennial biomass crops that live for 10-20 years and accumulate biomass which is made of carbon. Several UK government policies have called for actions to sustainably manage UK soils to prevent degradation, advocating for measures aimed at protecting and improving soil carbon stocks. In managing UK soils, biomass has a key role to play in improving soil carbon and resilience to climate change. 

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Biomass crops and Soil Carbon

Biomass crops are non-food crops where the above ground biomass is harvested to produce bioenergy or other products. They are usually perennial woody or herbaceous plants. Some commonly grown woody biomass crops are short rotation coppice willow (Salix spp), poplar (Populus spp) and short rotation forestry. Examples of herbaceous biomass crops are Miscanthus (Miscanthus x giganteus) and switch grass (Panicum spp). The advantages of using these crops for biomass production include rapid growth rate and high biomass production, low nutrient requirement, and ability to re-sprout after multiple harvests. They are considered to have the potential to sequester large volumes of CO₂ captured from the atmosphere during plant growth through photosynthesis and store carbon in the soil. The benefit of biomass crops sequestering carbon makes them important in contributing to reducing greenhouse gas (GHG) emissions.

The effect of biomass crops on soil carbon

Biomass crops accumulate relatively large quantities of below and above ground biomass which is made of carbon sequestered from the atmosphere. The fallen leaves and branches also add organic matter to the soil. Studies have shown that leaving Miscanthus crops standing over winter increases litter fall at around 30-45% which contributes to the buildup of biomass on the soil surface. Miscanthus organic material is shown to have a slow decomposition rate and is predicted to have the potential to store between 2-3 metric tonnes of CO₂ per hectare depending on the crop yield and the initial organic carbon level.

Short rotation coppice (SRC) biomass crops such as willow and poplar are harvested on a 2–5-year cycle, a process termed coppicing.  Coppicing involves cutting down the tree or above ground biomass to its base and allowing it to reshoot multiple stems. After a few years, the stem or above ground biomass can be harvested, and the cycle begins again. The frequent harvest of above ground biomass enhances vigorous shoot regeneration and root turnover, which may enhance soil carbon stock. 

The root structure of biomass crops after establishment continuously grows throughout the life cycle of the plant, storing and transferring carbon to the soil. After coppicing the above ground biomass, the roots are left in the soil and continue growing. Perennial rhizomatous grasses such as Miscanthus allocate a large proportion of the aboveground carbon to the roots and rhizomes, further increasing soil organic carbon stocks. Miscanthus can contribute 0.98 ± 0.14 Mg C4-C ha−1 yr−1 through litter drop and root turnover. 

Biomass crops require no tillage during the lifetime of the plant following establishment, which can prevent soil carbon losses via disturbance to the soil. Tilling (ploughing) soils reduces SOC stocks in the soil through enhancing soil aeration and reducing the physical protection of SOM, leading to increased decomposition rates and release of CO₂ into the atmosphere. After establishment of biomass crops on the soil, the soil is not tilled, thus, there is less disruption of soil aggregates and exposure of SOM to microbial activities. For example, a study found that, SRC willow planted in South England had lower soil respiration (912 ± 42 g C m-2 yr-1 ) and was a net sink for carbon (221 ± 66 g C m2 yr-1). Substantial amount of carbon can be stored in the soil when the land is ploughed less frequently.

The amount of carbon stored in the soil largely depends on the initial soil carbon content and prior land use. Planting biomass crops on lower carbon soils, such as arable lands, minimises soil carbon losses and promote soil carbon sequestration in the long term. Studies have shown, that soils with high carbon stock such as grasslands, forest and peatlands have high carbon stock and conversion to planting of biomass crops is likely to result in soil carbon loss. It is recommended that landowners and managers should sample their soils and estimate carbon stocks prior to planting biomass crops to better predict soil carbon stock change after planting biomass crops. 

Conclusion

Soil Organic Carbon (SOC) is formed from the interaction of ecosystem processes such as photosynthesis, respiration and decomposition of SOM. Soil carbon provides the benefit of enhancing soil quality which is essential to sustain and improve food production, increase supply and quality of water, enhance biodiversity, and reduce atmospheric CO2. Planting biomass crops could provide a mechanism to enhance soil carbon. Biomass crops have high above ground and below ground biomass which stores a significant amount of carbon in the plant. Coppicing above ground biomass enhances vigorous shoot regeneration and root turnover which enhances soil carbon stocks. Furthermore, biomass crops after establishment, requires no tillage during the lifetime of the plant, which facilitates better accumulation of soil carbon. For these reasons, planting biomass crops help to improve soil quality, provide resilience to physical soil degradation and help mitigate climate change.

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