Livestock and wild herbivores impact soil carbon differently

Domestic livestock differ from wild herbivores in their impacts on soil carbon, and ultimately on climate, a new study shows. And given the increasing use of veterinary antibiotics on domestic animals, which eventually enter soil and affect the soil microbes, antibiotics pollution in soil also could have a very large impact on climate, the study says.

The findings from the Indian Institute of Science (IISc), Bengaluru, published in Global Change Biology, build on previous research from IISc in Spiti in Himachal Pradesh, which highlighted the role of ‘herbivores’ or grass-eating mammals in stabilising soil carbon. Large herbivores such as yak and ibex play a crucial role in this process.

How herbivores impact soil carbon is of interest to scientists as these animals strongly influence the climate via their impacts on a large soil carbon stocks, with the world’s grasslands, steppes, and savannas storing an estimated 500 Petagrams (Pg) of carbon. A Petagram is equal to 10¹⁵ grams.

In the new study, IISc scientists examined whether livestock and native herbivores impact vegetation and soil carbon pools similarly. They found that the two groups differ in their impacts on vegetation composition, with herb-and-grass vegetation dominating in areas with native herbivores, and sedges in areas under livestock. Scientists attribute the differences to their diet selectivity.

On the whole, a mix of diverse livestock comprising many different species can be somewhat similar in their impact on soil carbon as native herbivores, “but livestock do not emerge as perfect substitutes since they store less soil carbon.”  In effect, livestock cannot entirely replace native grass-munching herbivores in their ability to store soil carbon, the study shows.

An added dimension to the native herbivores versus domesticated livestock issue is the increasing use of antibiotics on livestock, which has been found to affect soil microbes and ultimately soil carbon storage. Extensive use of antibiotics on livestock may restrict soil carbon by altering soil microbial ‘carbon use efficiency’ or the efficiency with which microorganisms convert absorbed carbon into their own biomass.

“Antibiotics enter the soil through livestock and there they affect the microbial communities,” Sumanta Bagchi, associate professor at the Centre for Ecological Sciences (CES) and Divecha Centre for Climate Change at IISc, and one of the study authors. “The altered microbial community is less efficient in storing carbon. Overall, livestock-soil stores 30% less carbon than wildlife-soil (soil impacted by dietary habits of wild herbivores). Some of this difference could be due to antibiotics, but we don’t know how much of this arises from antibiotics.”

The findings highlight the importance of continued conservation of native herbivores and new ideas to improve livestock management, the study concludes. They suggest that sequestering antibiotics, along with the restoration and rewilding of soil microbial communities may offer nature-based climate solutions by improving soil carbon storage in areas under livestock.

“This study is perhaps one of the first from India that proposes a link between lower soil carbon stock in livestock grazed systems with the presence of antibiotic residue,” says Abi Vanak, professor at the Ashoka Trust for Research in Ecology and the Environment (ATREE), Bengaluru. The antibiotic residue, presumably from livestock dung and urine interferes with soil microfauna, changes microbial community composition, and reduces their ability to use carbon compared to soils that had only native herbivores grazing on them, explains Vanak.

“Given how important savanna grasslands in India are for carbon sequestration, this has broader implications for how livestock is managed, and the overuse of routine antibiotics in veterinary care.”

In general, “the duration of the experiment of nearly two decades combined with the juxtaposed watersheds with heavy dominance of native and domestic herbivores, respectively, with a similar density and functional diversity, makes it quite unique,” says Jeppe Kristensen, 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.

Long-term experiments such as the recent study from IISc are needed to study the relatively slow-responding soil carbon pools, “as the long residence time of carbon in soils causes any signals due to management change to be blurred by legacy effects for many years.” Also, the high herbivore density and lack of diversity in many industrialised animal production systems makes comparison with effects of natural herbivore communities very difficult, says Kristensen.

More specifically, the IISc study is “quite unique” as it assesses 17 linked soil parameters simultaneously, “which gives quite extraordinary insights into different aspects of the organic matter transformations in soils,” says Kristensen. This helps gain a better understanding of some of the pathways through which the microbial community and processes mediates the differences seen in soil carbon pools and flows.

Such understanding of the underlying mechanisms is important to help predict what might happen under future scenarios, as global change factors do not affect microbial communities in the same way, or even same direction, he says. “Hence, there are most often both winners and losers, and if some of the microbes performing essential ecological functions are on the losing team, it is important to understand how to potentially mitigate it.”

Antibiotics and livestock

“Intensive livestock farming on an industrial scale has required the massive overuse (and abuse) of antibiotics, both as growth promoters, and to prevent diseases in overcrowded farms,” says Vanak. “On the other hand, extensive livestock rearing, such as nomadic pastoralism with native livestock varieties is less dependent on antibiotic use, and may, therefore, better mimic grazing by native herbivores.”

Kristensen, however, believes, based on both the IISc and other studies, that the impact of veterinary antibiotics is not as big as suggested. “The replacement of native herbivores with livestock is manifold more important than the introduction of antibiotics,” says Kristensen. “Veterinary antibiotics is certainly not the only way livestock systems change microbial communities, and I would think the effects are dwarfed by other herbivore impacts, for example, restructuring the vegetation, trampling,” he adds.

The use of veterinary antibiotics seems to have been part of the agricultural green revolution and “hence it is difficult to disentangle from the rapid “industrialisation” of livestock production in general during the post World War II period.” Also, animal products will continue to be used and, in fact, one could argue the reverse too that the use of drugs reduces the environmental footprint of animal products, as less area is required to produce feed for a more effective production, he says. “However, it is important to emphasise that high-input animal production is not environmentally sustainable overall.”

Even if the Spiti livestock system is not a perfect substitute of a native ecosystem and might be difficult to upscale directly, I think there are things to be learned from such mixed-livestock systems with close to natural densities and quite minimal external inputs when rethinking the way sustainable food production could look like in the future,” says Kristensen.

According to a 2019 report in the Indian Journal of Medical Research, India is an important producer of food animals for the global market in the form of meat, meat products and farmed seafood and a rise by 312 per cent in this market is expected by 2030. Antimicrobial agents are widely used to prevent diseases in these farmed animals and to increase productivity. India is the world’s fourth largest consumer of antimicrobials for animal use, after China, USA and Brazil. Projections show that at this pace, India will contribute to the largest relative increase in antimicrobial consumption for use in livestock between 2010 and 2030.

An August 2022 report of Future Markets Insights says that the global veterinary antibiotics market was worth USD 10904.3 million in 2022 and is projected to rise to USD 20,277 million in 2032 at a compound annual growth rate of 5.8%. The expanding animal healthcare industry, combined with the industry’s ongoing innovation, is driving the veterinary antibiotics market. Other contributing factors are prevalence of zoonotic diseases and people’s increased knowledge of medical care for animals.

Read more: Grazing and invasive species impact food availability for wild herbivores in MM Hills, finds study

Banner image: A yak grazing. Photo by Shauryankar Lingwal/Wikimedia Commons. A Himalayan ibex in the Spiti Valley. Photo by Madhumita Das/ Wikimedia Commons.