Can large herbivores enhance ecosystem carbon persistence? Kristensen et al. 2021

This article considers the overlooked role of grasslands and large herbivores in carbon storage. The principal question the authors pose is: what is the impact of large wild and domestic herbivores on the ability of ecosystems to absorb and store carbon over the long term? Their answer is that the activity of species like cattle, bison, boars, elephants, and rhinoceros, can significantly enhance ecosystem retention of carbon.

Questioning the assumption that fast-growing aboveground vegetation, especially trees, is the primary nature-based terrestrial sink for carbon, the authors argue for a whole-ecosystem carbon storage perspective. One problem with focusing on carbon storage in aboveground vegetation rather than that in the soil, they note, is that vegetation is more transient and vulnerable to disturbances, such as fire, while soil carbon tends to be stable, at least under natural and well-managed grasslands. Furthermore, the authors argue that the conventional focus on aboveground carbon storage has led to the “simplistic” generalization that large herbivores can be expected to damage vegetative ecosystems, and therefore have a negative impact on ecosystem carbon storage.

Yet this view misses the overall ecological impact of large herbivores, such as contributing to the soil through their wastes, and their bioturbation (churning of the soil by animals) activity. On the surface, large animals trample, forage, wallow and dig; just below the surface, the burrowing and digging of tunnels by soil-dwelling mammals like gophers, moles, voles, and shrews further loosens the soil; still deeper, there is the casting, burrowing and mining by macrofauna like insects, worms and dung beetles. Together, the multi-levelled bioturbation of these different species facilitates the vertical mixing of the organic material, putting it into contact with mineral soil particles for longer-term storage. Large grazing herbivores participate in vertical soil mixing (along with the smaller animals at lower levels in the soil), and therefore play an essential role in the long-term buildup of mineral-associated organic matter.

In addition to disturbing and mixing the soil, and enriching it through their body wastes, large herbivores clear pyrogenic (combustible) material on the ground and low shrubbery, thus increasing fire resistance. Their grazing also increases fine root growth and root exudation, which leads to increased microbial biomass. In turn, “microbial residues and plant exudates are effective substrates for persistent soil organic matter formation in the mineral-associated organic matter” [Kristensen 2021: 4].

In their conclusion, the authors emphasize the ecological value of natural grasslands, and the importance of preserving them:

Understanding the role that large herbivores may play in enhancing ecosystem carbon persistence, by reducing the flammability of aboveground carbon and shifting carbon storage from vulnerable pools towards more persistent soil pools at the biome scale, is crucial to balancing the ecosystem services provided by semi-open herbivore-rich systems against potential services from alternative land-uses, such as afforestation [Kristensen 2021: 9].