Tradeoffs in water and carbon services with land-use changse in grasslands

Background/Question/Methods

Increasing pressures for food, fiber, and fuel continue to drive global land-use changes, yet much of our understanding of how ecosystem services respond to land-use changes across abiotic drivers remain qualitative. Despite our growing reliance on ecosystem services for sustainability, efforts to optimize services are hampered by complex interactions among the services. Carbon sequestration and water provisioning are important services that can mitigate global environmental issues such as climate change and freshwater scarcity. Due to vegetation’s opposing influences on the carbon and water cycles, however, land-use changes may instigate tradeoffs between the two services. We examined the effects of land-use changes on ecosystem carbon storage and groundwater recharge in grasslands of Argentina and the United States to 1) understand the relationships between both services, 2) predict their responses to vegetation shifts across environmental gradients, and 3) explore how ecosystem-service markets could affect further land-use changes. Our comparisons focused on widespread vegetation shifts from natural grassland to woody encroached, rain-fed and irrigated cultivation plots across climate gradients and soil types.

Results/Conclusions

A tradeoff between ecosystem carbon storage and groundwater recharge was evident in most cases, with woody encroachment increasing carbon storage (29 Mg C/ha) but decreasing groundwater recharge (-7.3 mm/yr) and conversions to rain-fed cultivation driving the opposite (-32 Mg C/ha vs. 13 mm/yr). In contrast, crops irrigated with ground water tended to reduce both carbon storage and groundwater recharge compared to the natural grasslands they replaced. Ecosystem responses to land-use change also varied across environmental gradients, with grassland conversions having stronger effects on carbon storage under high-clay soils and stronger effects on groundwater recharge in humid climates. Thirty-year net present values that consider agricultural products together with carbon storage and groundwater recharge were predominantly losses (-$7300 to $6 /ha) after grassland conversions in the U.S., indicating that carbon and water markets may abate and reverse land-use changes. Carbon had a greater influence on the net value of the land-use changes than recharge had, highlighting the potential for regional carbon markets to drive land-use decisions. We draw from our findings and the literature to identify key opportunities for win-win and caveats for lose-lose land-use changes.