COS 164-5 - Livestock grazing and climate change impacts on dryland biogeochemical dynamics across four continents

Abstract: Drylands hold up to 20% of the terrestrial global soil organic carbon (C) pool. Given their water limitation on net primary productivity (NPP), soil C accrual happens slowly in these ecosystems. However, dryland soil C is vulnerable to disturbance and shifts in climate, both of which could induce losses that are irrecoverable over decades to half centuries. There remains uncertainty over how land use practices, such as livestock grazing, influence biogeochemical dynamics when compounded with climate change. We investigated how grazing pressure and projected climate change impact biogeochemical dynamics on dryland sites across four continents. We ran a daily ecosystem simulation model, DAYCENT, for grazed semi-arid dryland sites in North America, South America, Asia, and Africa. We ran simulations under three grazing pressures (light, moderate, heavy) and three climate change scenarios (SSP 1-2.6, SSP 2-4.5, SSP 5-8.5) to answer questions about how these changes might affect C storage (i.e. soil C pools), inputs (i.e. NPP), and fluxes (i.e. greenhouse gas emissions) at each site through the end of the century. Our results suggest a site-dependent response to grazing, climate change, and their interactive effect, with the North and South American sites responding primarily to grazing while the Asian and African sites were more influenced by climate change. All climate change and grazing scenarios resulted in simulated decreases in soil C across sites, with the largest losses in the African dryland (over 500 g C m⁻²). In the North and South American sites, substantial losses occurred under heavy grazing. In the Asian and African site, heavy grazing decreased soil C, but the largest losses were under SSP 2-4.5 and SSP 5-8.5. In the North and South American site, light grazing led to increases in simulated NPP. In the Asian site, simulated belowground NPP increased, but only under SSP 1-2.6. In the African site, simulated NPP decreased regardless of grazing intensity, which was most pronounced under SSP 2-4.5 and SSP 5-8.5. Simulated carbon dioxide emissions increased in the North and South American site under light grazing. In the Asian site, there were small increases in emissions due to light grazing, though only under SSP 1-2.6. Emissions decreased by 20-60 g C m⁻² yr⁻¹ in the African site; this decrease was largest under SSP 5-8.5. Our simulation results show the importance of considering the site-specific impacts of climate change as these localized shifts in climate may overwhelm changes in grazing management.