A new study led by the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of Zurich has discovered that the organic compounds that play a major role in carbon sequestration in deep soil are highly vulnerable to decomposition due to climate change. These findings have important implications for a key strategy of carbon management which relies on soils and forests – known as “natural carbon sinks” – to mitigate global warming.
During the process of photosynthesis, plants store carbon in their cell walls and soils, significantly contributing to carbon sequestration worldwide. In fact, nearly 25 percent of global carbon emissions are captured by forests, grasslands, and rangelands.
Since these processes have extended over several decades, soils now contain twice as much carbon as the atmosphere does, and deeper subsoils (over 20 centimeters or eight inches) account for about half of the soil carbon. However, with rising demands for croplands and timber, emissions from deforestation and agricultures are rising and currently account for a fifth of global greenhouse gases.
“Our study shows that climate change will affect all aspects of soil carbon and nutrient cycling. It also shows that in terms of carbon sequestration, there’s no silver bullet. If we want soil to sustain carbon sequestration in a warming world, we will need better soil management practices, which can mean minimal disturbance of soils during forest management and agriculture,” said co-author Margaret Torn, an ecologist and biochemist at the Berkeley Lab.
In previous research, Torn and her colleagues shown that warmer temperatures cause a significant decline – nearly 33 percent during a period of five years – in the carbon stocks stored in deep forest soils. In the current study, they provided new evidence that warmer conditions lead to a major drop in the soil organic carbon compounds created by plants during photosynthesis.
In an experiment at the UC Berkeley’s Blodgett Forest Research Station in the foothills of California’s Sierra Nevada mountains, the scientists used vertical heating rods to warm one-meter-deep plots of soil by four degrees Celsius (the amount of warming projected by the end of this century under a high greenhouse emission scenario). The experiment revealed that in only 4.5 years of warming at such a temperature, significant changes in carbon stocks at a half-a-meter depth below the soil surface occurred.
Moreover, spectroscopic analyses conducted at the University of Zurich revealed a 17 percent loss in lignin (a compound which give plants rigidity), and a nearly 30 percent loss in cutin and suberin (compounds in leaves, stems, and roots which protect plants from pathogens).
Finally, the experts were surprised to discover a major difference in the amounts of “pyrogenic carbon” – a type of soil organic carbon emerging from charred vegetation and other remnants from wildfires that is considered to have the highest potential to serve as a very stable form of sequestered carbon – in the artificially heated soil samples.
“We found much less pyrogenic carbon in the deep soils when they were heated. Pyrogenic carbon can stay in the soil for decades or even centuries, but we need to understand its vulnerability to warming or to changes in land management. Our study suggests that this material decomposed just as fast as anything else would when the soil was warmed,” Torn explained. “This shows that when you put material deep into soil where it’s in contact with minerals and microbes, those natural systems will decompose the material over time.”
In future research, the scientists plan to resample the soils in order to determine how nine years of warming have affected soil composition and health, and to conduct a new grassland warming experiment at the Point Reyes National Seashore in Northern California.
“We are also organizing all the world’s deep-soil warming (or whole-soil warming) experiments to share data and know-how and conducting synthesis of the data to see what we can learn,” Torn concluded. The study is published in the journal Nature Geoscience.
Soil carbon is a key component of the Earth’s carbon cycle and is critical for maintaining soil health and productivity. It exists in various forms, such as living organisms, decomposing organic matter, and stable soil organic matter.
It is often used as an indicator of soil health because it provides numerous ecosystem services, including nutrient cycling, water holding capacity, and disease suppression. Soil carbon can be divided into three main types:
This is the portion that is readily available for microbial processes. It is the most sensitive to management practices and can change rapidly over seasons or years.
This portion decomposes more slowly than active carbon. It’s primarily made up of decaying plant material and partially decomposed organic matter.
This is the most stable form of soil carbon and can remain in the soil for hundreds to thousands of years. It is largely resistant to decay and is stored deep within the soil.
Soil carbon sequestration is a process by which CO2 is removed from the atmosphere and stored in the soil carbon pool. This is a critical process that can help mitigate climate change. It involves the input of organic material into the soil through plant growth and residue turnover, and the conversion of this organic material into soil organic matter that is resistant to decay.
Several practices can increase soil carbon sequestration, such as cover cropping, crop rotation, reduced tillage, and the application of organic amendments (e.g., compost, biochar). These practices can also improve soil health and productivity.
Changes in land management practices can have significant effects on soil carbon levels. Deforestation, for example, can cause large losses, while reforestation and sustainable agriculture practices can increase soil carbon levels. Understanding and managing soil carbon is thus crucial for addressing climate change, enhancing soil health, and promoting sustainable land use.