Declining biodiversity reduces carbon sequestration in grasslands

A Swedish study shows that the soil carbon content of global grasslands increases with plant diversity and that this relationship is particularly strong in warm, arid climates

by Matteo Cavallito

 

The carbon stock in global grassland soils decreases as plant biodiversity declines. A phenomenon particularly evident in warmer and drier areas. That is the conclusion of a study by the Swedish University of Agricultural Sciences in Uppsala, the results of which were published in the journal Nature Communications. The researchers’ explanation is that species-poor vegetation appears to decompose faster in the soil.

The research assumes particularly important significance for at least two reasons. On the one hand, the magnitude of the phenomenon: one-third of the Earth’s global carbon stock is in fact stored in grasslands. On the other is the importance of understanding the carbon sequestration mechanism in light of efforts to mitigate climate change.

A link between carbon and plant diversity

The study examined 84 grasslands spread over a total of six continents. A group of 29 researchers involved, led by Marie Spohn, a professor in the Department of Soil and Environment and Forest Soil Biogeochemistry at the same Swedish university, thus had the opportunity to examine plains as diverse as North American steppes, the savanna of the Serengeti, the tundra of Svalbard and the natural grasslands of the Alps among others.

Their findings, statement from the Swedish university says, show that soil carbon content increases with plant diversity in global grasslands and that this relationship is particularly strong in hot, arid climates.

The explanation that emerged differs from initial hypotheses. The researchers, in fact, had identified the decisive factor as the tendency of species-rich grasslands to produce more aboveground plant biomass. What turned out to be decisive, however, was the “relationship between species richness and the chemical composition of the vegetation.”

Factors influencing decomposition

In species-rich grasslands, researchers explain, plant biomass experiences a higher carbon-to-nitrogen ratio. To put it simply: it contains more carbon. The result is that plants in turn contain less protein relative to the amount of fiber. And such plant material is decomposed more slowly in the soil. “Organic matter with a high C:N ratio decomposes slowly due to its low nutritional value for microorganisms,” the study states.

Consequently, “the increase in C:N ratio of plant biomass with higher plant diversity likely decreases the decomposition rate, and hence positively affects soil organic carbon content.”

Moreover, “Plant diversity can affect not only the C:N ratio and the composition but also the diversity of organic compounds in plant litter and soils, which might also influence the decomposition rate, and hence the SOC content”, the research finds. Finally, the positive relationship between diversity and carbon sequestration may also be related in part to other mechanisms.“For instance, high plant diversity can lead to high soil microbial biomass and soil aggregation, both of which can promote soil organic carbon sequestration.”

The importance of grasslands

Slower decomposition of plant parts in species-rich grasslands thus appears to be crucial. “The study has far-reaching implications as it suggests that ecosystem management that restores plant diversity likely enhances soil carbon sequestration, particularly in warm and arid climates,” says Marie Spohn. The research also highlights the central role of grasslands globally.

A recent FAO paper estimated that the top 30 centimeters of soil in these environments absorb 63.5 million tons of carbon each year. Application of best management practices, such as incorporation of animal manure, agroforestry and rotational grazing, would increase storage by between 0.18 and 0.41 tons of carbon per hectare each year.