The nationally determined contributions (NDCs) to the Paris Agreement typically include afforestation and reduced deforestation as additional carbon sinks. The first Biennial Transparency Reports for NDCs submitted in 2015 are due in 2022 for developed countries to 2024 for developing countries. But debates continue about the feasibility of both temperature-based and carbon-based climate change targets.
What has been missing in the Paris Agreement is the explicit mention of soil carbon as part of carbon management and carbon markets. Soil carbon has in general been perceived as difficult to govern and manage considering the largely individual ownership and the challenges of measuring soil carbon at scale. The drumbeat in support of soil carbon as an integral part of climate mitigation has however been growing recently.
Countries like India have got high marks for their overall commitment and climate actions thus far. However, afforestation efforts have led to conflicts, with some communities indicating the socioeconomic and political challenges climate actions will continue to face.
The Trillion Tree Campaign is receiving some support across the world despite some disagreements about the science behind it. More importantly, wildfires like the one in Australia remind us that trees can cough up the carbon they sequester very rapidly. Global warming itself drives extreme droughts which turn these forests into tinderboxes.
The degradation of soil due to human appropriation of land for agriculture, urbanisation and other uses, has resulted in a substantial loss of soil carbon. Restoring this carbon lost since the beginning of agriculture would have multiple co-benefits. The Intergovernmental Panel on Climate Change (IPCC) has argued that soil carbon sequestration is worth the cost independent of its impact on climate mitigation.
The co-benefits include improved soil fertility, climate resilience, reduced erosion, reduced loss of forests, benefits for wetlands, peatland and biodiversity. Up to threefold increase in crop yields are reported with increased soil carbon. Additional co-benefits include increased populations of insects, worms, birds and other pollinators.
Several studies have been advocating natural climate solutions which focus on land carbon management. These typically focus on forests, agricultural and grasslands, and wetlands. Researchers reported a comprehensive list of conservation and restoration actions in 2017.
This team also listed specific actions and their climate mitigation potential in terms of additional carbon sequestered. These included reforestation, biochar, agroforestry, management of nutrients and grazing, restoration of coasts and peatlands, with contributions also arising from avoided forest conversions and wildfires, coastal and peat impacts. Soils contain up to thrice the amount of carbon in the atmosphere, which should make them attractive to be included in NDCs.
A more recent study reported an update to the 2017 study. Improved estimates of the total potential of carbon sequestration by natural solutions are 24 billion tonnes of CO2-equivalent per year. About 25% of that is reported to be from organic carbon sequestration in soils. They further divide the soil organic carbon (SOC) pool into 40% requiring protection from being released and 60% needing to be restored. Forest soils and agricultural and grasslands have a share of 9% and 47% of total SOC, respectively. But a stunning 72% is argued to be the mitigation potential of wetlands!
The new study also reviews ways in which projects can be designed to accomplish these mitigation potentials of SOC. The pathways include avoiding forest and grassland conversion, peatland and coastal wetland impacts. Other actions are reforestation, biochar, cover crops, agroforestry, managing grazing and planting legumes in pastures, restoration of peatlands coastal wetlands.
The study suggests general and specific activities to realise these pathways. General activities are categorised into protection, management and restoration. Protection includes establishing and enforcing protected areas, improving land tenure and management by indigenous community. Management includes synergising agricultural and ecosystem services with climate mitigation and market forces. Restoration focuses on mitigation programmes and their evaluation and certification with the participation of indigenous communities.
Specific activities are identified under each of these general activities. They include sustainable intensification of agriculture, diet shifts, avoided loss of high-carbon forests, capacity building for biochar management, conservation agriculture practices, cover crops, restoration of native freshwater and saltwater wetland species, etc.
Natural climate solutions protect and increase soil fertility, flood control and climate resilience. They also reduce soil erosion, biodiversity and soil water loss and habitat conversion. These benefits contribute directly and indirectly to the UN Sustainable Development Goals, the UN Framework Convention on Climate Change (UNFCCC) and the UN Convention on Combating Desertification. These solutions will also enhance disaster resilience in service of the Sendai Framework for Disaster Risk Reduction.
There have been many other voices in support of SOCs as a tool for climate mitigation. In 2018, a team of scientists from France, Iran, the UK, New Zealand and the US listed eight steps for putting more carbon into the soil. They advocate preventing carbon loss and promoting uptake, verifying outcomes, deploying technologies, coordinating policies, involving communities, testing strategies, and providing incentives.
The Global Soil Partnership (GSP) established in 2012 under the UN Food and Agricultural Organisation (FAO) coordinates soil management and awareness. GSP also oversees research, data and harmonisation of efforts for voluntary sustainable soil management. FAO member countries serve as national focal points for all activities related to soil carbon, soil biodiversity, soil erosion, and soil pollution.
Another diverse team of scientists called for a global agenda for collective action on soil carbon in a comment published in Nature Sustainability in 2019. This team lists three priorities for such a global agenda. The first one underlines the need for a vision for action supported by a political case. The second calls for building a strong business case among public and private investors. The third argues for a much more convincing value proposition, for farmers and land managers.
Until complete decarbonisation becomes socially and technologically feasible, taking advantage of all carbon sinks is the only option. Soil carbon is a viable and significant carbon sink with enormous benefits. Sequestering carbon in soils offers a clear political and business case for being an integral part of the NDC portfolio. Considering that the next round of NDCs are due this year, soil carbon is an urgent issue to be addressed.
The Indian context
Several estimates of SOC exist for India but based on a small set of samples from a few regions. Climate – predominantly temperature and rain – plays a critical role in the leaching or emission of carbon from soils. India’s climate ranges from hot and cold arid to humid (sub-humid and per-humid) to coastal. Estimating carbon stocks is obviously a challenge considering soil heterogeneity and the large spatial variability of soil and vegetation types. Human perturbations that release soil carbon like agriculture, urbanisation and deforestation also vary from state to state.
A team from the National Soil Survey and Land Use Planning in Nagpur had estimated soil carbon stock for India in the top 150 cm was 69 picogram in 2000. But this valiant effort was based only on a limited number of data points. A more comprehensive tally of organic and inorganic carbon in all major soil types in India came from an ICRISAT team: its researchers found that Indian soils hold relatively less carbon due to the natural tendency of Indian soils to sequester less SOC. This is largely related to the large extent of the drier climatic zones of the country.
This also means that Indian soils have a higher potential for SOC build up. Different soil types, land uses and cropping systems determine the SOC sequestration potential. Selection of appropriate sites must consider all these factors in addition to regional climate.
A comprehensive SOC mapping is needed as a part of the Soil Health Card scheme that was launched in 2015. The sampling has been increased significantly under this scheme, and a dashboard provides relative distributions of various soil properties, including SOC. Quantifying standing SOC pools in detail and tracking their growth and loss will be essential for NDC protocols. And quantification of the impact of soil health on crop yields and food security may be needed for tracking returns on investment.
Improving soil health will also require concerted general and specific activities to prevent losses and enhance sequestration of SOC. For a country like India, including SOC as a separate item in its NDC can be a good incentive for sustained efforts to build and maintain the SOC pool. India can augment its afforestation efforts to generate additional carbon sinks via SOC while also fortifying its food security.
Raghu Murtugudde is a professor of atmospheric and oceanic science and Earth system science at the University of Maryland. He is currently a visiting professor at IIT Bombay.