Acidification may strip Indian soils of 3.3 billion tonnes of essential carbon, affecting crop growth, sequestration: Study

Over 30 per cent of cultivable land in India is said to carry acidic soil, impacting plant growth. Now, a new study highlighted another concern: Loss of soil inorganic carbon, a stable carbon pool. This depletion could hurt the health of soil and its ability to regulate nutrient levels, foster plant growth and store carbon.

Soils are turning acidic due to industrial activities and intense farming. In India, soil acidification might lead to loss of 3.3 billion tonnes of soil inorganic carbon (SIC) from the top 0.3 metres of its soil over the next 30 years, according to the study published in the journal Science.

Carbon in soil can be stored in the form of SIC or soil organic carbon (SOC). The former 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.

The latter, 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.

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, Yuanyuan Huang, Institute of Geographic Sciences and Natural Resources Research at the Chinese Academy of Science and the study’s author told Down To Earth.

Soil acidification is already a concern in the country, 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 per cent 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 SIC depletion. “Most of the soil inorganic carbon (by weight) is carbonate. The chemistry of carbonate makes it closely coupled with pH,” Huang explains.

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

So far, the focus has largely been on SOC, recording more than 96 per cent of publications and citations, while SIC is neglected, with only 4 per cent share in global soil carbon research, according to a 2024 study published in Geoderma journal.

The global stock of SOC is estimated to be 2,376-2,456 petagram (Pg = 10^15 g) at a depth of 2 metres. 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.

Using machine-learning models, the team estimated that 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 is more than five times the carbon found in all of the world’s vegetation combined. This hidden pool of soil carbon, the team noted, could be key to understanding how carbon moves around the globe.

Changes in the environment, especially soil acidification, could affect this pool. The team found that India and China are likely to be the most affected in future scenarios. 

This study, Huang added, only looks into the nitrogen fertilisation and deposition (example, acid rain)-induced soil acidification for the future period. 

“Because of the large addition of nitrogen fertilisation into croplands, China has experienced a decline in its soil pH over the past few decades. We did not specifically examine the trend of India’s soil over the past few decades,” Huang said, adding that India is also probably experiencing a similar trend due to substantial nitrogen additions. 

He called for a further assessment of a large number of samples that measure soil pH coherently through time and space in India.

Though the study estimated that India could lose around 3.3 billion tonnes of carbon from the top 0.3 metres due to soil acidification, Huang does not rule out the possibility that the lost SIC could be potentially relocating to deeper layers.

Globally, 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, respectively.

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.

Further, 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, the researchers warned.

As SIC is important for soil health, ecosystem services and functions along with carbon sequestration, the team hopes to see it incorporated into climate change mitigation strategies for maintaining and enhancing carbon storage. 

Going forward, the team hopes to find answers to questions such as what happens to the lost inorganic carbon — do they relocate into deeper soil layers, enter the atmosphere as carbon dioxide or find their way into rivers, groundwater and oceans?

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, Huang added.