Pathway
Soil
Carbon removal potential: 2.3-5.3 GtCO₂/yr¹
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The soil is the largest terrestrial carbon sink, holding both organic and inorganic carbon, and storing more CO₂ than the atmosphere and plant life combined. The primary carbon exchange between the atmosphere and the land is through plant biomass, where photosynthesis incorporates ~440 Gt CO₂ each year2. Fixed CO₂ slowly leaks into the soil nourishing the microbial community. Most of the absorbed CO₂ is released back into the atmosphere through plant and microbial respiration, however some carbon is retained in the soils. Terrestrial ecosystems are a net sink of ~13 Gt CO₂ a year2, one third of anthropogenic emissions. Manipulation of the soil carbon budget, even by a few percent, offers a significant opportunity to mitigate climate change.
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The natural carbon cycle has been unbalanced by human activity. Naturally, above ground biomass absorbs around 440 GtCO₂ each year through photosynthesis and releases the same amount via respiration and decomposition. Humans emit around 40 GtCO₂ from fossil use and land-use change each year. The excess of CO₂ has increased the drawdown capacity of aboveground biomass, sequestering an additional 13 GtCO₂ a year . A GtCO₂ is a billion tonnes of CO₂. Adapted from Global Carbon Budget²
The primary strategy for increasing the soil carbon budget is through “regenerative agriculture” practices. Modern intensive agricultural practices are heavily reliant on agrochemicals and soil tillage. Churning soils exposes soil organic carbon (SOC), which oxidises and returns to the atmosphere. The aim of regenerative agriculture is to reverse land-degradation practices by reducing soil disturbance and enabling the build up of SOC. Beyond helping mitigate climate change, regenerative agriculture provides an array of ecological co-benefits. Practices should result in more productive soils with improved environmental resilience and less reliance on agrochemicals.
Although simple in practice, soil carbon dynamics are difficult to quantify and the full potential of these practices is not thoroughly understood. SOC can vary in different climates, soils types, or depending on the crop. Accurately measuring SOC is time-intensive and measurements can vary from within the same field, at different depths, soil densities and between seasons. Reliable measurement and verification is a key barrier limiting the adoption of soil carbon removal strategies.
Positive Practices:
Planting diverse cover crops
Treating soils with microbes
In-farm fertility (no external nutrients)
Rotating crops
Adopting agroforestry practices
Planting perennial crops
Improving grazing management
Bioengineering plants/microbes for increased carbon drawdown
Co-Benefits:
Increased soil water retention capacity
Increased resilience to droughts, floods
Reduced chemical use resulting in less pollution through production and runoff
Increased biodiversity and natural pest resistance
Improved crop productivity
Reduced inputs and expenses for growers
Better nutrient density in crops
Issues We Care About:
Forming a robust system for measurement, reporting and verification of soil organic carbon content.
Soil carbon Is distribution is variable across the whole root zone, which can span 1m deep'. Measurements should encompass the whole root zone to obtain accurate true soil carbon representation.
Providing growers with access to the right technology, data and education to help them transition to carbon farming practices.
Ensuring the switch to regenerative practices is not reversed and soil carbon is stored permanently.
Understanding how microbes can optimise the soil carbon sequestration potential.
Sources
1. https://oceanvision.org/roadmaps/
2. IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, 2019
3. Ocean Acidification, 2012
4. Global Carbon Budget, Friedlingstein et al. 2020
5. Project Drawdown, solutions
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