Context: Recently, a new study has highlighted the loss of soil inorganic carbon, a stable carbon pool, which could hurt the health of the soil and its ability to regulate nutrient levels, foster plant growth, and store carbon in Indian soil.

Key highlights of the study

• Globally, soils contain more than three times the carbon found in vegetation and twice that in the atmosphere.

• By 2100, changes in global warming and soil pH could reduce SIC by 1.35 to 5.83 gigatonnes under various warming scenarios.

• India and China are likely to experience the most significant SIC losses, primarily due to vast SIC reserves and widespread soil acidification from nitrogen additions.

• Annually, 1.13 billion tonnes of inorganic carbon are transferred from soils to inland waters, highlighting a significant cycle of carbon among land, atmosphere, freshwater, and oceans.

• The study underscores the importance of SIC in maintaining soil health, supporting ecosystem services, and aiding in climate change mitigation, recommending its consideration in carbon storage enhancement strategies.

About Soil acidification in India

• Over 30% of cultivable land in India is said to carry acidic soil, impacting plant growth.

• Soil acidification is affecting about 48 million hectares (mha) out of 142 mha of arable land. 

• Acidic soils in India are widespread in the humid southwestern, northeastern, and Himalayan regions. The northeastern region, in particular, has recorded acidity in approximately 95% of the soils.

• This is worrying because acidic soils affect crop growth and productivity by reducing the availability of plant nutrients. It also predisposes plants to other biotic and abiotic stress factors.

• Soil acidification creates an environment ripe for soil inorganic carbon depletion. 

• Most of the soil inorganic carbon (by weight) is carbonate. The chemistry of carbonate makes it closely coupled with pH.

• Low pH levels (acidic soils) dissolves solid carbonate and removes it either as carbon dioxide gas or releases them directly into the water.

• Soils are turning acidic due to industrial activities and intensive farming. 

About Soil acidification

• Soil acidification is a process where the soil pH decreases over time. 

• This process is accelerated by agricultural production and industrial activities and can affect both the surface soil and subsoil.

Contributing factors

Some contributing factors to soil acidification include:

• the application of high levels of ammonium-based nitrogen fertilizers to naturally acidic soils

• leaching of nitrate nitrogen, originally applied as ammonium-based fertilizers

• harvesting plant materials (plant material is alkaline so when it is removed the soil is more acidic than if the plant material had been returned to the soil).

Effects

• Excessively acidic soils may lead to a dramatic decline in crop and pasture production because the pH of the soil changes the availability of soil nutrients.

Acidic soils may have some or all of the following problems

• helpful soil microorganisms may be prevented from recycling nutrients (e.g. nitrogen supply may be reduced)

• phosphorus in the soil may become less available to plants

• deficiencies of calcium, magnesium, and molybdenum may occur

• the ability of plants to use subsoil moisture may be limited

• aluminum, which is toxic to plants and micro-organisms, may be released from the soil

• levels of manganese may reach toxic levels  

• uptake by crops and pastures of the heavy metal contaminant, cadmium may increase.

Carbon in soil

• Carbon in soil can be stored in the form of Soil Inorganic Carbon or Soil Organic Carbon (SOC). 

• Soil Inorganic Carbon includes mineral forms of carbon like calcium carbonate produced by weathering parent material in soil or from the reaction of soil minerals with atmospheric carbon dioxide.

• Soil Organic Carbon, which plays a role in nutrient cycling, is the main component of soil organic matter such as plant and animal waste, microbes, and microbial byproducts.

• Together, soils store more than thrice the quantity of carbon in vegetation or double the quantity of carbon in the atmosphere.

• Global soils store 2,305 (± 636) billion tonnes (1 petagram is a billion tonnes) of carbon as SIC over the top 2 metre depth. 

• This hidden pool of soil carbon could be the key to understanding how carbon moves around the globe.

• Every year, approximately 1.13 billion tonnes of inorganic carbon are lost from soils to inland waters. 

• This loss could have overlooked implications for carbon transport between the land, atmosphere, freshwater, and ocean.

Soil Inorganic Carbon (SIC) loss in india

• India is likely to be most affected by SIC losses due to relatively large stocks of SIC and the magnitude of soil acidification associated with nitrogen additions.

• Soil acidification creates an environment ripe for SIC depletion. “Most of the soil inorganic carbon (by weight) is carbonate. The chemistry of carbonate makes it closely coupled with pH,

• Low pH levels (acidic soils) dissolve solid carbonate and remove it either as carbon dioxide gas or release it directly into the water.

• In India, soil acidification might lead to the loss of 3.3 billion tonnes of soil inorganic carbon (SIC) from the top 0.3 meters of its soil over the next 30 years.

• There is a possibility that the lost SIC could be potentially relocating to deeper layers.

Documentation of soil carbon stock

• The global stock of SOC is estimated to be 2,376-2,456 petagram (Pg = 10^15 g) at a depth of 2 meters. SIC estimates at a national or global scale have been poorly documented in comparison to SOC pools due to a lack of research.

• So the team from the United States, Australia, and France created a global SIC database containing 223,593 measurements of 55,077 soil profiles from site studies.

Impact of soil carbon loss on Climate

• Future global warming and soil pH changes will deplete SIC in the top 0.3 m of soil by 1.35, 3.45, and 5.83 gigatonnes of carbon (GtC) under different scenarios, where temperatures could likely reach around 1.8°C, 2.7°C and 4.4°C warming by 2100.

• The large SIC pool revealed through this study and its high vulnerability to acidification-induced losses may pose a risk to limiting net carbon dioxide emissions to the atmosphere in line with the Paris Agreement goals.

Road Ahead

• SIC is important for soil health, ecosystem services, and functions along with carbon sequestration; it should be incorporated into climate change mitigation strategies for maintaining and enhancing carbon storage. 

• We also need to study practices that can mitigate the negative impacts of agriculture practices on soil inorganic carbon while enhancing crop yields to support the growing population in the future.