Restoration maintains high soil carbon stocks in Asia's largest tropical grassland.

Land management changes threaten the ecological functionality of grassland ecosystems worldwide, including the ability of these ecosystems to store carbon in their soils. The Banni grasslands in India, Asia's largest tropical grassland, are no different in this regard. Despite being a highly biodiverse ecosystem with an extensive land use history, information on soil carbon stocks in this ecosystem remains conspicuously absent. In this study, we map soil organic carbon (SOC) stocks within the upper 30 cm depth across land use-land cover classes by combining high-resolution satellite imagery with field-based soil carbon data from a network of long-term monitoring sites. We find that Banni currently stores 27.69 million tons of carbon in its soils, with an average SOC density of 119.61 ± 3.50 tons of carbon per hectare (tC/ha, mean ± SE). These estimates compare favorably with arid and semi-arid grasslands as well as seasonally flooded savannas across the tropics. The highest SOC densities are found in restored grasslands (142.72 ± 5.72 tC/ha), reaffirming the potential of carbon recovery from ecological restoration. Tracing 10 years of land change in the region indicates the further expansion of the invasive tree Prosopis juliflora across the landscape, but its impacts on changes in SOC concentration estimates remain varied. Our results indicate the large SOC storage associated with woody encroachment in Banni and point toward the need for the active management of Banni so that carbon benefits can accrue at meaningful timescales for reversing land degradation and contributing to land-based climate action.