Large grass-munching herbivores can stabilise soil carbon

Large grass-eating mammals such as yak and ibex play a crucial role in stabilising the pool of soil carbon in grazing ecosystems that are a big part of the Earth’s land surface, new research shows. Scientists from the Indian Institute of Science (IISc), Bangalore, say protecting the herbivores that stabilise soil carbon should remain a key priority for mitigating climate change, as grazing ecosystems make up about 40% of the Earth’s land surface. Their study is published in the Proceedings of the National Academy of Sciences.

The scientists conclude that “herbivores exert strong influence on both the stability and the size of the soil carbon pool, and their persistence is essential for decarbonisation services derived from grazing ecosystems.”  The researchers caution that local extinctions of herbivores could lead to loss of soil organic carbon and, hence, nitrogen availability. The decomposition of soil organic carbon releases nutrients, including nitrogen, into the soil. Therefore, the higher the soil organic carbon, the higher the nitrogen content.

The IISc study recommends that “conservation and rewilding of large mammalian herbivores should remain a priority to protect the soil carbon pool and achieve natural climate solutions.” “Loss of herbivores and, hence, the decline in grazing, can affect soil carbon loss,” explains Sumanta Bagchi, associate professor at the Centre for Ecological Sciences (CES) and Divecha Centre for Climate Change at IISc, and a study author. The study provides an “understanding of the process, which can be converted into a potential solution.”

He adds that grazing by herbivores can emerge as a nature-based climate solution by conserving large mammalian herbivores and restoring their functional role in the soil carbon and nitrogen cycles.

The IISc scientists use a “very interesting and novel approach” to explore the development in soil organic matter (SOM) stocks over time to show that the variability is higher and, hence, persistence lower in areas without grazing says Jeppe Kristensen, a researcher at the Environmental Change Institute at the University of Oxford whose team had studied the link between herbivores and persistence of carbon at the ecosystem level.

An earlier February 2022 report by Kristensen’s team in Trends in Ecology and Evolution says that herbivores could increase the persistence of ecosystem carbon by redistributing carbon from above-ground vegetation pools vulnerable to disturbances into persistent soil pools. Kristensen says that the IISc study system has “a very interesting and rare diversity” of both domestic and wild herbivores.

Also, “their dataset’s relatively high temporal resolution shows that the assumptions on the slow change in soil organic matter stocks over a decade are wrong, at least in some highly dynamic grassland systems. This has implications for the way such studies of soil organic carbon (SOC) dynamics over time should be conducted in the future,” says Kristensen.

However, he does not see the effects on soil carbon as something specific to certain types of herbivores, despite differences between herbivores in terms of carbon storage and their body mass and metabolism. “So a diversity of both in the herbivore community is good,” he says. “To me, it’s a systems thing more than a specific grazing impact,” says Kristensen. “Grazing won’t do it alone, and not grazing in any form,” he explains.

In grasslands used mainly for livestock grazing, the intensity of grazing and the duration of rest periods influence the carbon storage processes. Continuous grazing reduces plant cover, diversity and productivity, increasing soil erosion and losses of soil carbon.

“If you have a system with a diversity of grazers at appropriate densities, you will often increase the diversity of organic matter inputs to the soil system, which will, in turn, increase the diversity of the soil community and the sub-pools in which soil organic matter is stored,” added Kristensen. This interplay between above- and below-ground biota spreads the risk of carbon loss due to the many different molecular forms and associations in which it is stored.

The role of small herbivores is not well-described by scientists, Kristensen says. “My feeling is that they play a much larger role in soil processes than we tend to think. Yet again, it is perhaps, in the interplay with larger animals they become particularly interesting, as there are many examples of associations between large grazers and smaller rodents around the world.” Smaller burrowing or digging animals such as earthworms, moles and shrews “must inevitably have a huge impact on soil processes,” he adds.

Kristensen also sees limited potential of grazing by large herbivores for mitigating greenhouse gas concentrations in the atmosphere. Instead, their significant role in stabilising ecosystems offers scope for a role in climate adaptation which is part of the nature-based solutions (NbS) framework.

However, Francesca Cotrufo, who published a review in Science in August 2022 on the role of grasslands as an important soil carbon sink, says there could be a potential nature-based solution “only if it comes as a deliberate land management/use change. You cannot claim NbS from a natural process already in place.”

Grasslands as a carbon sink

Grasslands, which store about one-third of the global terrestrial carbon stocks, are an important soil carbon sink. The Science review says that improved grazing management and biodiversity restoration can provide low-cost and/or high-carbon gains for natural climate solutions in global grasslands.

Grassland ecosystems cover an area of 52.5 million square km, accounting for about 40.5% of the Earth’s land surface, excluding Greenland and Antarctica. They store approximately one-third – 34% — of the global terrestrial carbon stocks and can act as an important soil carbon sink, with 90% of the carbon stored below ground as root biomass and soil organic carbon. The review says that climate change regulates the metabolic activity of microbes and, thereby, the storage of large-scale microbial dead plant matter and soil organic carbon.

With 67% of the world’s grasslands distributed in semiarid, arid, and cold climates, and only 23% occurring in humid climates, carbon sequestration in most grasslands is highly sensitive to climate change, it says. This can result in strong and diverse impacts on soil organic carbon accumulation and stability.

The impacts of climate change on soil carbon sequestration often vary with the grassland type, climate and soil conditions, the review adds. In semiarid steppes, for example, warming may enhance root-derived carbon but inhibit the decomposition of mineral-associated organic matter (MAOM) by suppressing fungal growth and soil respiration.  So, this results in an increase in the MAOM pool.

In humid, tall-grass prairies, on the other hand, warming may increase grasses that use a different pathway of photosynthesis to fix carbon into compounds with four carbon atoms, called the C4 cycle. Hence, the soil organic matter contains more compounds from the C4 cycle, which does not impact carbon sequestration much. In alpine grasslands, warming-induced permafrost degradation reduces the storage of organic carbon in the active layers of the soil by making soil microbes’ networks less stable and accelerating the decay of soil organic carbon.

The review says that natural grasslands are grazed by wild ungulates (hoofed animals), which can enhance soil organic carbon storage because they graze for short periods of time and move across the landscape. This maintains the plant cover, diversity, and productivity, promotes the growth of species with deep roots and helps in soil mixing by soil organisms. In addition, large herbivores create habitats for many mammals and other digging or burrowing animals which disturb the soil and loosen it up.

However, the effects of large wild herbivores on soil carbon storage can vary strongly with soil nutrient availability across grasslands and under different levels of herbivore density, the review says.

For example, grazing by large herbivores increases the storage of soil carbon in the upper layers of soil when there are high levels of nutrients or fertilisers in the soil, but it has no effect on soil carbon storage under normal surrounding conditions. All in all, herbivores strongly influence the stability and size of soil carbon pools, as Bagchi’s study concludes. Their conservation should remain a priority.

Banner image: Himalayan ibex in Spiti Valley. Photo by Madhumita Das/Wikimedia Commons.