Refining Regional Soil C Stock Estimates of Temperate Highlands in Southern Brazil.

Agricultural expansion onto pristine wood- and grasslands in Brazil causes major changes in soil organic C (SOC) stocks. Assessment of these stocks, including variability and regional distribution in rural landscapes is a crucial component to fulfill the Brazilian commitment to sustainable agriculture. Our study aimed at refining existing SOC stocks estimates in the Campos de Cima da Serra (temperate highlands) of Rio Grande do Sul state. The adopted approach included soil sampling campaigns to obtain primary data, complemented by a compilation of existing studies (published surveys and research projects). The field study sampled surface soil layers (0-30 cm) as recommended in protocols for SOC inventory, stratified by predominant soil classes (Ultisols, Inceptisols and Histosols) and land uses (grasslands with and without annual burning, cultivated pastures, intensive annual horticultural crops, Pinus plantations). The largest SOC stocks (244.4 Mg C ha^-1) were measured in Ultisols and Inceptisols (191.1 Mg C ha^-1) under grasslands without burning, whereas lower stocks were found in Inceptisols under horticultural crops. Our study found SOC stocks that were on average higher than those estimated in the most recent SOC map of Brazil (2002) − 70% higher in Inceptisols and 120% higher in Ulisols. Additionionally estimated stocks were 75 % higher than those estimated in a SOC map of RS state from 2009. Our results highlighted that soils in this region store large C stocks, which have been generally underestimated in previous SOC inventory assessments. The SOC stock map obtained allowed more detailed visualization than previously possible of the spatial distribution of SOC in this region. These results can be used to prioritize policies supporting a "C-friendly" agriculture. Additionally, we contributed to a refinement of the Brazilian SOC inventory.