Are grassland soils of the pampas sequestering carbon?

A belief that grassland soils act as carbon sinks and mitigate greenhouse gas emissions generated by livestock production is common among some farming organizations. A recent theoretical analysis proposed that grazing lands in South America sequester carbon as soil organic matter but this result is controversial. Here, we search for empirical evidence of changes in the carbon stock in the grassland soils of the Pampean region of Argentina. For this purpose, 22 natural grassland sites were selected, distributed across the region. The sites were first sampled in 2007 as part of a regional survey of soil carbon stocks; they were re-sampled in 2019. The sites represent a wide range of soil and climate conditions. Samples were taken to a depth of 50 cm. Bulk density, and inorganic and organic carbon, were determined, and the carbon stocks were estimated on an equivalent mass basis. The results were analyzed using ANOVA. No statistically significant differences were found in either inorganic or organic carbon stocks between the sampling times. Across all the sites, the average organic carbon stock was 59.0 t ha^-1 in 2007 and 56.2 t ha^-1 in 2019. These results suggest that the Pampas grasslands are not sequestering carbon.