Cows and ants team up to boost soil fertility

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Cows and ants work together to keep soil healthy. (Zhiwei Zhong)

Leaf litter decomposition rates were higher in pastures with both cattle and ants than on plots with either or both animal groups excluded, according to researchers who say these findings could inform better management of grassland ecosystems.

In a study, published April 22 in Functional Ecology, researchers examined how the two animal groups affected microbial decomposition of leaf litter, a fundamental process for recycling carbon and nitrogen, in Chinese grasslands.

"As the demands for meat and milk increase across the globe, cows are becoming the dominant large animal in global grasslands, especially in China. On the other hand, ants are one of the most common and important soil animals on land," said co-author Zhiwei Zhong, an associate professor of vegetation ecology at Northeast Normal University in China. "Although these animals often live together in grasslands and can potentially interact with each other, very few studies have explored their interactions, and how these interactions [affect] important processes like the decomposition of dead leaves."

In a previous study, Zhong and his colleagues found that cows and ants were good for each other. As the mammals munch down grass, they allow more light to warm the soil surface, which benefits the insects. In turn, cattle preferred to graze in areas with ants because they enhanced plant quality and quantity, likely by improving soil nutrient availability. This finding led the team to suspect that the different creatures would also collaborate to alter soil properties.

To test this hypothesis, the researchers established plots in grasslands of northeast China's Jilin Province for four years. The four treatments included plots where the researchers allowed both cattle grazing and ant colonies, excluded grazing but allowed ants, allowed grazing but eliminated ants with the use of ant bait, or excluded both ants and cattle.

At the end of the four-year period, the researchers found that plots with either cows or ants had greater soil nitrogen levels, more microbes and higher litter decomposition rates than the plots with both animal groups excluded. But in plots with both insects and cattle, these three measures were even higher than in plots with just one or the other.

According to Zhong, cows increase soil fertility by adding feces and urine. Likewise, ants improve soil fertility and moisture levels through burrowing and building of ant mounds.

"These positive feedbacks between cattle and ants thus led to an increase in nutrients in the soil (e.g. nitrogen), which benefited the soil microbes and thus led to faster decomposition of dead leaves when both occurred together," he explained.

Mammals and insects are important parts of many ecosystems, including grasslands. Although the size of an individual ant is vastly smaller than a cow, they can be so abundant that the overall mass of ants in an ecosystem can exceed that of mammals. Indeed, the researchers found that either ants or cows alone had similar impacts on soil nutrients and litter decomposition.

"In grasslands with livestock, people commonly think the large animals are the key drivers of ecosystem processes, given their large body sizes and energy requirements," Zhong said. "However, our results showed that the much smaller animals, ants, can in fact play a nearly equal important role as livestock in affecting decomposition."

According to Zhong, this finding supports renowned entomologist E.O. Wilson's idea that "the little things ... run the world."

But these little things are facing global declines due to climate change and human activities such as intensive grazing and mowing, land-use changes and insecticide use, highlighting a need for better management of ecosystems, Zhong said.

"If we can use more environmentally friendly management techniques (e.g. reduce the application of insecticides) and proper land-use strategies, like not having too many cattle grazing in one area, this will make our whole ecosystems healthier and more stable," Zhong said.

According to Zhong, one limitation of the study is that the researchers considered all ants as one group, so the team wasn't able to evaluate the contribution of different ant species. In addition, other soil fauna such as worms and springtails may have aided litter decomposition in their plots, but the researchers didn't monitor these animals.

In future work, the researchers plan to evaluate interactions between livestock and soil fauna on decomposition in different grassland ecosystems and across longer time spans.

Zhong notes that grasslands are important carbon pools, reservoirs that soak up carbon from the atmosphere and help mitigate global warming. By recycling carbon and nitrogen from dead plant material back into the soil and atmosphere, litter decomposition plays an important role in global carbon and nitrogen cycles.

"We want to know how cattle and ants can further interact on longer-term scales to affect the carbon emissions and storage in these grasslands," he explained. "This is important because long-term carbon changes might affect future climate change." 

The study, "A facilitation between large herbivores and ants accelerates litter decomposition by modifying soil micro-environmental conditions," published April 22 in Functional Ecology, was authored by Xiaofei Li, Jilin Agricultural University and Northeast Normal University; Anita C. Risch, Swiss Federal Institute for Forest, Snow and Landscape Research; Dirk Sanders and Guofang Liu, University of Exeter; Chelse Prather, Chinese Academy of Sciences; Zhongnan Wang, Nazim Hassan, Deli Wang and Zhiwei Zhong, Northeast Normal University; and Qiang Gao, Jilin Agricultural University.

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