Agricultural Land Use and the Global Carbon Cycle

Abstract

Earth’s soils and the global carbon (C) cycle have been profoundly affected by agriculture. Agricultural land-use and land-cover change (LULCC) date back to the early Holocene ~12,000 years ago when hunter-gatherers in the Fertile Crescent region of the eastern Mediterranean began using agricultural practices to manage soils. Since then, agriculture has spread across the globe and is now the dominant global land use with about 40% of the ice-free land area covered by croplands and grasslands. Especially, clearing of natural vegetation to make room for cultivating crops has released up to one-third of the soil organic carbon (SOC) stock from the top meter of soil. However, effects on the soil inorganic carbon (SIC) stock continue to be overlooked. Further, methane (CH₄) is produced in anaerobic environments under agricultural practices, such as the sediments of wetlands, peatlands, and rice (Oryza sativa L.) paddies as well as by livestock production. Globally, up to 357 Pg (1 Pg = 1 Gt = 10¹⁵g) C pre-1850 and 168 Pg C post-1850 may have been released by agricultural land-use changes. To 2 m depth, about 133 Pg SOC may have been lost since the early Holocene. LULCC emissions from 1850 to 2015 have been estimated at 98.4 and 16.3 Pg C for crop and pasture land uses, respectively. This net C release together with emissions of CH₄ and nitrous oxide (N₂O) has contributed to increasing atmospheric greenhouse gas (GHG) concentrations and accelerating climate change . Climate change interferes with agriculture with severe negative effects. Thus, climate change adaptation and mitigation are necessary to sustainably intensify production amidst the increasing challenge of satisfying the demands for food, feed, fiber, and fuel of a growing, more affluent, and more animal products consuming population. Food-related GHG emissions are lower for plant-based diets. Improved soil management in agroecosystems can substantially reduce GHG emissions and sequester some of the atmospheric carbon dioxide (CO₂) as SOC and oxidize some of its CH₄. Additional benefits of agroecosystems with increased SOC stocks are more healthy and resilient soils. The hidden treasure of SOC has finally been recognized by policymakers. For example, to help address food security and climate change issues, the 4 per Thousand Initiative (4p1000) was proposed in December 2015 at COP21 in Paris to enhance the soil C stock on a large portion of the world’s managed soils by an average annual increase of 0.4% in 0–40 cm depth.