Introduced grazers can reduce ecosystem function and reduce potential soil carbon sequestration

Background/Question/Methods

Land-use practices determine whether global greenhouse-gas emissions can be offset by soil-C sequestration. Grazing is a predominant land-use occurring on nearly a third of the earth’s terrestrial surface, and influences the potential for 10⁹ Mg·y^-1 of soil-C storage. Historically, nearly all natural grazing ecosystems have undergone conversion for livestock production by displacing native herbivores. This often leads to loss of soil-C, primarily through overgrazing. Current policies attempt to reduce stocking densities to arrest soil-C loss in these systems. However, it remains unknown whether maintaining diverse livestock species at appropriate stocking densities can match soil-C sequestration levels under the displaced native herbivores. The Trans-Himalayas provide replicate watersheds with comparable edaphic conditions that sustain equivalent densities of either native herbivores, or of livestock managed by traditional pastoralists, where we tested the effects of these alternative grazing practices on potential soil-C sequestration.

Results/Conclusions

We found that, despite similar grazing intensities, the pastoralist-managed watersheds had 49% lower soil-C than watersheds with native herbivores, and this represents a relative-loss of 40-100 Mg·km^-2·y^-1 of soil-C spread over the last 3-4 decades. Using a manipulative 4-year herbivore-exclusion experiment, we found that unlike the native herbivores, grazing by the livestock drives directional shifts in vegetation composition, and accompanying declines in plant production that reduce C-inputs to soil. These impacts appear to result from lower levels of diet-differentiation and feeding complementarity among the livestock species, compared to native herbivores. These data suggest that while maintaining sustainable stocking-densities of livestock is a major step, this alone may not be enough to realize the full potential1 of soil-C sequestration by grazing ecosystems of the world.