Policy is usually focussed on reducing greenhouse gas (GHG) emissions from the electricity sector, transport and industry. There has, however, been a renewed interest in understanding how soils can serve as a sink for carbon dioxide since atmospheric concentrations of carbon dioxide have crossed 410 parts per million and oceans are already turning acidic.
Besides, increasing soil carbon offers a range of co-benefits and this would buy us time before other technologies can help us transition to a zero-carbon lifestyle.
What is soil organic matter?
Significant carbon pools on earth are found in the earth’s crust, oceans, atmosphere and land-based ecosystems. Soils contain roughly 2,344 Gt (1 gigatonne = 1 billion tonnes) of organic carbon, making this the largest terrestrial pool.
Organic matter makes up just 2–10% of the soils mass but has a critical role in the physical, chemical and biological function of agricultural soils. Carbon is a measureable component of soil organic matter.
Soil organic matter (SOM) is mainly composed of carbon, hydrogen and oxygen but also has small amounts of nutrients such as nitrogen, phosphorous, sulphur, potassium, calcium and magnesium contained within organic residues.
Organic matter contributes to nutrient turnover, soil structure, moisture retention and availability, degradation of pollutants, greenhouse gas emissions and soil buffering.
It is divided into ‘living’ and ‘dead’ components and can range from very recent inputs such as stubble to largely decayed materials that are thousands of years old.
Storing the carbon contained in organic matter within the soil is seen as one way to mitigate climate change by reducing greenhouse gas emissions (in this case carbon dioxide) but to do this an increase in the more stable or resistant fractions of organic matter is required.
What is soil organic carbon?
Soil organic carbon (SOC) comes from plants, animals, microbes, leaves and wood, mostly found in the first metre or so. There are many conditions and processes that determine changes to SOC content including temperature, rainfall, vegetation, soil management and land-use change.
In the presence of climate change, land degradation and biodiversity loss, soils have become one of the most vulnerable resources in the world. Soils are a major carbon reservoir containing more carbon than the atmosphere and terrestrial vegetation combined.
Soil organic carbon (SOC) is dynamic, however, and anthropogenic impacts on soil can turn it into either a net sink or a net source of GHGs.
After carbon enters the soil in the form of organic material from soil fauna and flora, it can persist in the soil for decades, centuries or even millennia. Eventually, SOC can be lost as CO2 or CH4 emitted back into the atmosphere, eroded soil material, or dissolved organic carbon washed into rivers and oceans.
The dynamics of these processes highlight the importance of quantifying global carbon fluxes to ensure maximum benefits of SOC to human well-being, food production, and water and climate regulation.
The Intergovernmental Panel on Climate Change (IPCC) provides guidelines for measuring, reporting and verifying national SOC stock inventories.
Role of SOC in Human Well-being:
- Achieving the SDGs:
As an indicator for soil health, SOC is important for its contributions to food production, mitigation and adaptation to climate change. Maintaining SOC storage at equilibrium or increasing SOC content towards the optimal level for the local environment can contribute to achieving the SDGs.
- SOC and biodiversity:
- SOC improves soil structural stability by promoting aggregate formation which, together with porosity, ensures sufficient aeration and water infiltration to support plant growth.
- The amount and quality of SOM (and consequently SOC) determines the number and activity of soil biota that interact with plant roots.
- SOC influences water holding capacity and porosity of the soil.
- With an optimal amount of SOC, the water filtration capacity of soils further supports the supply of clean water.
- Through accelerated SOC mineralization, soils can be a substantial source of greenhouse gas (GHG) emissions into the atmosphere.
Climate change effects on SOC
Temperature and precipitation are the most significant factors controlling SOC dynamics.
- Although the overall impact of climate change on SOC stocks is very variable according to the region and soil type, rising temperatures and increased frequency of extreme events are likely to lead to increased SOC losses.
- SOC hot-spots, which are respectively areas of high SOC, content (e.g. peatlands , black soils, permafrost lands, grasslands and forest soils) and large surface areas of low SOC content (e.g. drylands) constitute major zones of concern. With climate change and unsustainable management, these areas are likely to become net sources of GHG emissions.
However, if managed wisely, they have the potential to sequester large amounts of carbon in their soils, thus contributing to climate change mitigation and adaptation.
SOC management for Climate Change mitigation and adaptation
Climate change mitigation refers to efforts aimed at restraining, halting and/or reversing climate change through management strategies, behavioural changes and technological innovations that reduce the emission of GHGs.
Climate change adaptation, on the other hand, refers to efforts aimed at achieving higher resilience towards unprecedented climatic events and conditions. It implies the anticipation of climate change and its adverse effects, and strives to manage them through appropriate actions that minimize the associated risks and negative impacts.
Some management strategies:
It is estimated that an increase of just 1 tonne of soil carbon pool of degraded cropland soils can increase crop yield by several kilograms per hectare.
- Reforestation/afforestation of arable land
- Conservation/reduced tillage
- Crop rotations
- Cover cropping
- Organic farming
- Balanced combined applications of chemical fertilizer and manure
- Avoiding conversion and degradation of native ecosystem
- Restoring drained fields to wetlands
- Planting perennials in degraded/marginal land
- Adopting improved varieties of species with greater yield and/or biomass
- Irrigating water limited systems like Dry lands
- Adoption of genetically modified or naturally bred rice varieties with low root exudation
Soil and agriculture
After the changes undertaken as part of the Green Revolution, crop yields increased for several decades, but there has also been a dramatic increase in the use of chemicals — pesticides, herbicides and fertilizers. Still, agricultural yields have begun to drop in many places for a variety of reasons primarily related to degraded soils.
Industrial changes to agriculture have led to a range of adverse effects: loss of biodiversity, elimination of beneficial microbes and insects, reduction in yield, contamination of water bodies and soils, and increasing toxicity and deaths from chemical use in farm households.
India has a large number of successful sustainable agricultural practices that are consistent with ecological principles. These include natural farming permaculture and organic farming. The knowledge and innovations of farmers who have successfully experimented with these methods must be considered in research and policy.
The number of farmers in organic farming has been increasing steadily, but many are simply deploying regular agriculture with natural substitutes for chemicals.
Up to a third of rain fed farmers simply do not have the means to add chemicals, and are organic by default. Many States have some sustainable farming, with Madhya Pradesh reportedly having the highest acreage.
Lessons for India
Given that the Local techniques can contribute to relieving a range of challenges, State-level policy makers need to understand better the successes on the ground in India’s different agro-climatic zones.
They also need to identify what kinds of support are needed by farmers with small holdings to transition from existing practices.
Not paying attention to the successes of our own farmers has partly contributed to the agrarian crisis the country now faces.
India’s population will continue to increase through at least the middle of the century and we need to be able to grow more food, grown in less land and in more severe weather conditions. We should not ignore our own farmers’ successes at our own peril.
The Parliamentary Standing Committee on Agriculture in its 2016 report in fact recommended “revision of the existing fertiliser subsidy policy and promotion of organic fertilizers”.
The government has been promoting a Soil Health Card scheme to measure the health of the soils in different parts of the country and in each farm. There is little policy support for natural farming and the alternatives.
The ability of soils to sequester carbon is a win-win strategy for farmers, people and for climate change and it is time we stopped ignoring these at the policy levels.